1. Where have we been? Statements, (to specify instructions):
calling methods on objects, specifying arguments
text output, text input, graphical output.
declaring variables,
assigning values to variables (including new objects)
Expressions and values
literal values
constants
variables
combining values with operations (+, -, *, etc)
Defining methods
giving methods parameters
passing values to methods when you call the method
using parameter values in the method

2. Programs that make decisions
Programs that perform the same action every time are boring!
You can vary the action in a program
By clicking different buttons
By getting input from the user:
String name = UI.askString("name:"); :
UI.printf("Hello %s, how are you?", name);

3. Programs that make decisions
But this just changes the values, not the action itself.
To vary the action inside a method:
Need a conditional, or choice, statement:
IF some condition is true
THEN do this action
ELSE do that action
We do this in English instructions all the time:
IF you solved the mouse maze
THEN raise your hand
IF your name starts with “A” or your name starts with “J”
THEN draw a small circle in the top left corner of your notes
ELSE draw a small square in the bottom right corner of your notes.

4. Decisions in Java Java has an if … else … statement:
Can do an action only in some circumstances:
if ( countTimes > 10 ) {
UI.clearGraphics();
this.drawBoard(10, 10, 100); }
Can choose between different actions:
if ( userChoice.equals(“Yes" ) ){
UI.drawImage("Nod.png", left, top);
else {
UI.drawImage(“Shake.png“, left, top);

5. Java: if and if … else LDC 4.2
Two forms of the if statement:
if (〈condition 〉) {
〈actions to perform if condition is true 〉 }
⇒ just skip the actions when the condition is not true !
and
if (〈condition 〉 ) {
else {
〈actions to perform if condition is false 〉
Note: the { … } represent a "Block" – a sequence of
actions that are wrapped up together into a single statement.

6. boolean valued expression
if … vs if … else … if (
boolean valued expression ) {
statements }
else {
statements }

7. Method with a condition
/** Ask for amount and currency; print note if –ve, print value.*/
public void convertMoney( ) { ;
double amount = UI.askDouble("Enter amount $NZ");
if ( amount < 0 ) {
UI.println("Note: you have entered a debt!"); }
String currency = UI.askString ("Enter currency (US or Aus)");
if ( currency.equals("US") ) {
UI.printf("$NZ%.2f = $US%.2f\n", amount, (amount * 0.668));
else {
UI.printf("$NZ%.2f = $AUS%.2f\n", amountt, (amount * 0.893));
Like println, but can control the format:
%.2f  floating point, 2dp
%d  integer
%s  string
\n  new line
What is printf?

8. Multiway choice: if … else if … else if …
Can put another if statement in the else part:
if (〈condition1 〉 ) {
〈actions to perform if condition1 is true〉 : }
else if (〈condition2 〉 ) {
〈actions to perform if condition 2 is true (but not condition 1) 〉
else if (〈condition3 〉 ) {
〈actions to perform if condition 3 is true (but not conditions 1, 2)〉
else {
〈actions to perform if other conditions are false〉

9. Example with multiway choice
public void convertMoney( ) {
double amount = UI.askDouble("Enter amount");
if (amount < 0 ) {
UI.println("Note: you have entered a debt!"); }
String currency = UI.askString("Enter currency (US or Aus)");
if (currency.equals("US") ) {
UI.printf("$NZ%.2f = $US%.2f%n", amount , amount * 0.668);
else if ( currency.equals("Aus") ) {
UI.printf("$NZ%.2f = $AUS%.2f\n", amount , amount * 0.893);
else {
UI.printf("I cannot convert to %s currency%n", currency);

10. Example 2 with multi way choice
public void printPay( int day, int hours ) {
double rate = 13.45;
double pay = rate * hours;
if ( day > 7 ) {
UI.println(" Day must be between 1 and 7 "); }
else if ( day < 6 ) {
UI.printf("Pay = $ %.2f %n", pay);
else if ( day == 6 ) {
pay = pay * 1.5;
UI.printf("Pay = $ %.2f ( time-and-a-half) %n", pay);
else {
pay = pay * 2;
UI.printf("Pay = $ %.2f ( double-time) %n", pay);

11. Boolean expressions LDC 4.1
What can go in the condition of an if statement?
A Boolean value – a value that is either true or false.
Boolean expressions:
constant values: true, false
numeric comparisons: (x > 0) (day <= 7), (x == y), (day != 7)
boolean method calls: month.equals("July") word.contains("th")
boolean variables: outlineOnly [ if declared boolean outlineOnly; ]
logical operators: !, &&, || (not, and, or)
( x > 0 && x < 7 && outlineOnly )
( month.startsWith("Ju") || month.equals("May") )
( ! fileModified || ! (cmd.equals("exit")) )
more methods on String
equalsIgnoreCase("John”)
startsWith(“Ab”)
endsWith(“ies”)

12. Writing Boolean expressions
Mostly, boolean expressions are straightforward, There are just a few traps:
== is the "equals" operator for simple values, = is assignment
(age == 15) vs (age = 15 );
But only use == for numbers (or characters, or references)
Use the equals method for Strings, not == (occasionally == will give the right answer by chance!)
cur.equals("US") vs cur == "US"
String equality is case sensitive:
“NZ".equals(“nz") → false
“NZ".equalsIgnoreCase(“nz") → true

13. Boolean Variables A boolean value is a value!
⇒ it can be stored in a variable.
Useful if the program needs to remember some option.
Must declare the variable, and assign to it, before using it
boolean printSteps = UI.askBoolean("Print all steps?"); :
if ( printSteps )
UI.println("Processed input");
UI.println("Computed Statistics");

14. Compound Boolean expressions: operators
Using logical operators:
Not: ! eg ( ! currency.equalsIgnoreCase(“US") )
And: && eg ( x > 0 && x < 7 && outlineOnly )
Evaluates each conjunct in turn. If any conjunct false, then value of whole expression is false If all conjuncts true, then value of whole expression is true
Or: || eg ( month.startsWith("Ju") || month.equals("May") )
Evaluates each disjunct in turn. If any disjunct true, then value of whole expression is true If all disjuncts false, then value of whole expression is false
Can combine into complicated expressions:
( ! fileModified || ( cmd.equals("exit") && lastSaveTime > 5000) )
safest to use lots of (…)

15. Traps with Boolean expressions
When combining with && and ||, which binds tighter?
if ( x > 5 && y <= z || day == 0 ) { ….
Use ( and ) whenever you are not sure!
if ( ( x > 5 && y <= z ) || day == 0 ) { …
if ( x > 5 && ( y <= z || day == 0 ) ) { …
The not operator ! goes in front of expressions:
if ( !(x > 5 && y <= z ) { … NOT if ( (x !> 5 && y !<= z )
if ( ! cur.equals("US") ) { … NOT if ( cur.!equals("US") ) { …
exception: if ( ! (count == 0) ) { … OR if ( count != 0 ) { …

16. Object oriented programming
Key idea of OO programming
program structured into classes of objects.
each class specifies a kind of object – eg, the actions it can perform.
Calling methods in OO languages like java
tell an object to perform a method, passing arguments
Making objects
Some objects are predefined.
Create objects with bluej:
Right-click on class, and select new ……
This is how we run programs with BlueJ.
not standard, and not a general solution

17. Objects Question: How can a program make new objects?
More Questions: What is an object anyway? Why do we need them?
An object is typically a collection of data with a set of actions it can perform.
The objects we have made so far are a bit strange – no data; just actions. (TemperatureConverter, Drawer)

18. Examples of objects Butterfly program CartoonFigure program
Each butterfly is represented by an object which stores the state of the butterfly (position, wing state, direction)
Butterflies have methods
move(double dist) and
land()
CartoonFigure program
Each cartoon figure is represented by an object which stores the state of the cartoon figure (image, position, direction facing, smile/frown).
CartoonFigure objects have methods
walk(double dist)
smile() frown()
lookLeft() lookRight()
speak(String words) think(String words)

19. Using objects
If the variable bf1 and bf2 contained Butterfly objects, you could do:
public void showButterflies(){
Butterfly bf1 = ?????
Butterfly bf2 = ?????
bf1.move(10);
bf2.move(20);
bf1.land();
bf1.move(5); }
Problem: How do you get a Butterfly object into the variables?
Nothing new here:
Just standard method calls!

20. Creating Objects Need to construct new objects:
New kind of expression: new
Butterfly bf1 = new Butterfly( … … )
Constructor calls are like method calls that return a value.
have ( )
may need to pass arguments
returns a value – the new object that was constructed.
Constructor calls are NOT method calls
there is no object to call a method on.
must have the keyword new
name must be the name of the class
Calling the constructor
100, 300
Creates a new object, which is put into bf1

21. Creating Objects: new Calling a constructor:
Butterfly b1 = new Butterfly(100, 300);
UI.setColor( new Color(255, 190, 0) );
Calling a constructor:
new ( a keyword)
Butterfly ( the type of object to construct )
( … ) (arguments: specifying information needed to construct the new object)
This is an expression: it returns the new object
can put in a variable
can use in an enclosing expression or method call
new
〈Class name〉 (
〈arguments〉 )

22. Reading Documentation
Documentation of a class:
Specifies the methods:
name
type of the return value (or void if no value returned)
number and types of the parameters.
void move (double dist)
moves the butterfly by dist, in its current direction.
Specifies the constructors:
number and types of the parameters
(name is always the name of the class,
return type is always the class)
Butterfly(double x, double y)
requires the initial position of the butterfly
Bluej lets you see the documentation of your classes

23. Example: Butterfly Grove program
public class ButterflyGrove{ /** A grove of Butterflies which fly around and land */ public void oneButterfly(){ Butterfly b1 = new Butterfly(50, 20); b1.move(5); b1.move(10); b1.move(15); b1.move(11); b1.move(12); b1.move(13); b1.move(14); b1.move(16); b1.land(); }
public void twoButterflies(){
Butterfly b1 = new Butterfly(100, 20);
b1.move(5);
b1.move(10);
b1.move(15);
double x = 400*Math.random();
Butterfly b2 = new Butterfly(x, 40);
b2.move(10);
b1.move(12);
b1.move(11);
b1.move(7);
b1.land();
b2.move(20); b2.move(25); b2.land(); }

24. Objects are values too:
Objects can be passed to methods, just like other values.
public void Butterflies(){
Butterfly b1 = new Butterfly(100, 20);
Butterfly b2 = new Butterfly(x, 40);
this.upAndDown(b1);
this.upAndDown(b2); }
public void upAndDown(Butterfly b){
b.move(10);
b.move(15);
b.land();
b.move(20);

25. Menu More defining methods with parameters Methods that return values
Administration:

26. Another Java Program
Design a Java program to measure reaction time of users responding to true and false "facts".
Ask the user about a fact: "Is it true that the BE is a 4 Year degree?"
Measure the time they took
Print out how much time.
Need a class
what name?
Need a method
what parameters?
what actions?

27. ReactionTimeMeasurer
/** Measures reaction times for responding to true-false statements */
public class ReactionTimeMeasurer {
public ReactionTimeMeasurer(){
UI.addButton("Measure Time", this::measureReactionTime); }
/** Measure and report the time taken to react to a question */
public void measureReactionTime() {
// find out the current time and remember it
// ask the question and wait for answer
// find out (and remember) the current time
// print the difference between the two times
Write the method body in comments first,
(to plan the method without worrying about syntax)
Work out what information needs to be stored (ie, variables)

28. ReactionTimeMeasurer
Returns a very big integer
⇒ long
(milliseconds since 1/1/1970
/** Measure and report the time taken to react to a question */
public void measureReactionTime() {
long startTime = System.currentTimeMillis();
UI.askString("Is it true that the sky is blue?");
long endTime = System.currentTimeMillis();
UI.printf("Reaction time = %d milliseconds \n", (endTime - startTime) ); }
Just asking one question is not enough for an experiment.
 need to ask a sequence of questions.
only got to here in lecture 6.

29. Multiple questions, the bad way
/** Measure and report the time taken to react to a question */ public void measureReactionTime(){ long startTime = System.currentTimeMillis(); UI.askString( "Is it true that John Quay is the Prime Minister"); long endTime = System.currentTimeMillis(); UI.printf("You took %d milliseconds \n", (endTime - startTime) ); startTime = System.currentTimeMillis(); UI.askString( "Is it true that 6 x 4 = 23"); endTime = System.currentTimeMillis(); UI.askString( "Is it true that summer is warmer than winter"); UI.askString( "Is it true that Wellington’s population > 1,000,000"); }
Lots of repetition.
But not exact repetition.
How can we improve it?

30. Good design with methods
Key design principle:
Wrap up repeated sections of code into a separate method,
Call the method several times:
public void measureReactionTime ( ) {
this.measureQuestion( ); }
public void measureQuestion ( …… ) {
long startTime = System.currentTimeMillis();
UI.askString("Is it true that " ……… );
long endTime = System.currentTimeMillis();
UI.printf("You took %d milliseconds \n", (endTime - startTime) );
"John Quay is the Prime Minister");
"6 x 4 = 23");
“Summer is warmer than winter");
"Wellington’s population > 1,000,000 ");
We need to
parameterise
the method
String fact
fact

31. Improving ReactionTimeMeasurer (1)
public void measureReactionTime() {
this.measureQuestion("John Quay is the Prime Minister");
this.measureQuestion(“6 x 4 = 23");
this.measureQuestion(“Summer is warmer than Winter");
this.measureQuestion("Wellington’s population > 1,000,000 "); }
public void measureQuestion(String fact) {
long startTime = System.currentTimeMillis();
UI.askString("Is it true that" + fact);
long endTime = System.currentTimeMillis();
UI.printf("You took %d milliseconds \n", (endTime - startTime) );

32. Understanding ReactionTimeMeasurer
What happens if we call the method on the object RTM1:
RTM1 . measureTime();
public void measureReactionTime(){
this.measureQuestion("John Quay is the Prime Minister");
this.measureQuestion("6 x 4 = 23");
this.measureQuestion(“summer is warmer than Winter");
this.measureQuestion("Wellington’s population >1,000,000");
The object the method was called on is copied to "this" place
this:
RTM- 1

33. Understanding method calls
public void measureQuestion(String fact){
long startTime = System.currentTimeMillis();
UI.askString("Is it true that " + fact);
long endTime = System.currentTimeMillis();
UI.printf("You took %d milliseconds \n", (endTime - startTime) ); }
"John Quay is…"
this:
RTM- 1    

34. Understanding ReactionTimeMeasurer
public void measureReactionTime(){
this.measureQuestion("John Quay is the Prime Minister");
this.measureQuestion("6 x 4 = 23");
this.measureQuestion(“summer is warmer than Winter");
this.measureQuestion("Wellington’s population > 1,000,000");
this:
RTM-1 

35. Understanding ReactionTimeMeasurer
New measureQuestion worksheet:
public void measureQuestion(String fact){
long startTime = System.currentTimeMillis();
UI.askString("Is it true that " + fact);
long endTime = System.currentTimeMillis();
UI.printf("You took %d milliseconds \n", (endTime - startTime) ); }
Each time you call a method, it makes a fresh copy of the worksheet!
" 6 x 9 = 54 "
this:
RTM- 1    

36. Understanding ReactionTimeMeasurer
public void MeasureReactionTime(){
this.measureQn("John Quay is the Prime Minister");
this.measureQn("6 x 4 = 23");
this.measureQn(“summer is warmer than Winter");
this.measureQn(" Wellington’s population > 1,000,000");
this:
RTM-1  

37. Problem
A good experiment would measure the average time over a series of trials
Our program measures and reports for each trial.
Need to add up all the times, and compute average:
problem:
MeasureReactionTime needs to add up the times
MeasureQuestion actually measures the time, but prints it out.
How do we get the time back from MeasureQuestion to MeasureTime?

38. Methods that return values
Some methods just have "effects":
UI.println("Hello there!");
UI.printf("%4.2f miles is the same as %4.2f km\n", mile, km);
UI.fillRect(100, 100, wd, ht);
UI.sleep(1000);
Some methods just return a value:
long now = System.currentTimeMillis();
double distance = 20 * Math.random();
double ans = Math.pow(3.5, 17.3);
Some methods do both:
double height = UI.askDouble("How tall are you");
Color col =JColorChooser.showDialog(UI.getFrame(), "paintbrush", Color.red);

39. Defining methods to return values
Improving ReactionTimeMeasurer:
public void measureReactionTime() {
long time = 0;
time = time + this.measureQuestion("John Quay is the Prime Minister");
time = time + this.measureQuestion("11 x 13 = 143");
time = time + this.measureQuestion(“Summer is warmer than Winter");
time = time + this.measureQuestion(" Wellington’s pop > 1,000,000 ");
UI.printf("Average reaction time = %d milliseconds\n", (time / 4)); }
public void measureQuestion(String fact) {
long startTime = System.currentTimeMillis(); ……
make measureQuestion return a value
instead of just printing it out. .
Specifies the type of value returned.
void means "no value returned"
long

40. Syntax: Method Definitions (v3)
/** Measure time taken to answer a question*/
public long measureQuestion ( String fact ){
long startTime = System.currentTimeMillis(); :
〈Comment〉
〈Header〉
〈Body〉 { }
public
〈type〉
〈name〉 (
〈parameters〉 )
〈type〉
〈name〉 ,

41. Defining methods to return values
If you declare that a method returns a value,
then the method body must return one!
public long measureQuestion(String fact) {
long startTime = System.currentTimeMillis();
String ans = UI.askString("Is it true that " + fact);
long endTime = System.currentTimeMillis();
UI.printf("You took %d milliseconds\n" , (endTime - startTime) ); }
return (endTime - startTime) ;
New kind of statement
Means: exit the method and return the value
The value must be of the right type

42. Returning values.  What happens if we call the method:
RTM-1 . askQuestions();
public void measureReactionTime(){
long time = 0;
time = time + this.measureQuestion("John Quay is the Prime Minister");
time = time + this.measureQuestion("6 x 4 = 23");
time = time + this.measureQuestion(“summer is warmer than Winter");
time = time + this.measureQuestion(“Wellington’s pop > 1,000,000");
this:
RTM-1 

43. Returning values return value: public long measureQn(String fact){
long startTime = System.currentTimeMillis();
UI.askString("Is it true that " + fact);
long endTime = System.currentTimeMillis();
return (endTime - startTime) ; }
"John Quay is…"
this:
RTM- 1
" "

44. Returning values.   What happens if we call the method:
RTM-1 . askQuestions();
public void measureReactionTime(){
long time = 0;
time = time + this.measureQuestion("John Quay is the Prime Minister");
time = time + this.measureQuestion("6 x 4 = 23");
time = time + this.measureQuestion(“summer is warmer than Winter");
time = time + this.measureQuestion(" Wellington’s pop > 1,000,000");
this:
RTM-1  

45. Aside: Random numbers
Math.random() computes and returns a random double
between 0.0 and 1.0
To get a random number between min and max:
min + random number * (max-min)
( Math.random() * 70.0)
gives a value between and 120.0
This is an expression:
can assign it to a variable to remember it
can use it inside a larger expression
can pass it directly to a method

46. Menu Repetition/Iteration Admin: Test Submission
When the assignments are marked, marks and comments are available via the link on the Assignments page

47. Repetition / Iteration
Doing some action repeatedly:
“Polish each of the cups on the shelf”
“Put every chair on top of its desk”
“Give a ticket to everyone who passes you”
“Keep patrolling around the building until midnight”
“Practice the music until you can play it perfectly”
Two patterns:
Do something to each thing in a collection
Do something until some condition changes

48. Repetion/Iteration in Java LDC 4.5
Several different ways of specifying repetition.
For statement:
Do something to each element of a list
for ( type value : listOfValues ) {
do something to value }
While statement:
Repeat some action until some condition becomes false
while (condition-to-do-it-again ) {
actions to perform each time round

49. For statement Three components a list of values
a variable that is assigned each value of the list in turn.
actions to perform for each value in the list
// print each number in a list of numbers:
for ( Double num : listOfNumbers ) {
UI.println(num); }
// print each string in a list of numbers that starts with "A":
for ( String str : listOfStrings ) {
if ( str.startsWith("A") ) {
UI.println(str);
listOfNumbers: , 32.2, 6.9, 49.5, 83.4, , 1.0
num:
listOfStrings: "Jamie", "Andie", "Jules", "Amy", "Mark"
str: " "

50. For statement ("foreach" version)
for ( Double num : listOfNumbers ) {
UI.println(num); }
Meaning:
Repeatedly (for each value in the list)
put the next value of the list into the variable
do the actions.
for (
type
variable :
list of values of type ) {
action }

51. Lists of values What type is a list of values?
How do we get a list of values?
ArrayList <Double> numberList = UI.askNumbers("Enter numbers");
for (double num : numberList) {
UI.println(num); }
UI.setColor(Color.red);
UI.setLineWidth(5);
for (double radius : numberList) {
if (radius> 20 && radius < 200) {
UI.drawOval( 300 – radius, 250 – radius, radius * 2.0, radius * 2.0);
Have to use Double, not double
Double is the "wrapped-up" version of double,
for putting into a list
List of doubles
Asks for a list of numbers, ending with 'done'

52. Lists of values What type is a list of values?
How do we get a list of values?
ArrayList <String> nameList = UI.askStrings("Enter names");
for (String name : nameList) {
UI.println("Hello " + name); }
UI.println("=========== Long names ============");
if (name.length() > 6 ) { UI.println(name); }
UI.println("=========== Short names ============");
if (name.length() <= 6 ) { UI.print(name + ", "); }
UI.println();
List of String values
Asks for a list of strings, ending with empty line
print without a new line
print just a new line

53. Doing more with the loops: using Variables
Add up all the numbers in a list:
ArrayList <Double> numberList = UI.askNumbers("Enter numbers");
double total = 0.0;
for (double num : numberList) {
total = total + num; }
UI.println("Total of numbers = " + total );
numberList: , 32.2, 6.9, 49.5, 83.4, , 1.0
Declare and initialise variable
Add each number into the total:
- Uses current value in total
- Adds the next number to it
- Puts result back into total

54. Doing more with the loops: using Variables
Count the number of long names in a list.
ArrayList <String> nameList = UI.askStrings("Enter names");
int count = 0;
for (String name : nameList) {
if (name.length() > 6 ) {
count = count + 1; }
UI.printf("There were %d long names out of %d names \n", count, nameList.size() );
Declare and initialise variable
Add 1 to the count
Number of values in a list

55. Lists are values too: passing lists around
public void analyseNames() { ArrayList <String> nameList = UI.askStrings("Enter names"); UI.println("Total characters: " + this.totalChars (nameList) ); UI.println("Starts with A: " + this.wordStartingWith(nameList, "A") ); } public int totalChars(ArrayList <String> strings ){ int count = 0; for (String str : strings) { count = count + str.length(); return count; public String wordStartingWith(ArrayList <String> strings, String pattern ){ if ( str.startsWith(pattern) ) { return str; } // returns first word starting with the pattern return "<none>";

56. Menu Repetition/Iteration with while Admin: Test
Monday 5-6pm: Monday 6-7pm:
ABBISS to BROWN  KK301 LI to MORTON  KK301
BUCK to EVANGELISTA  MC101 MULLER to RUSSELL  MC101
EVANS to GULLIVER  MC102 RYAN to STOECKLEIN  MC102
GUNNING to HYDE  MC104 STRYDOM to VOGELS  MC104
IRVINE to LEUYHOLD  CO122 VOSS to VAN'T WOUT  CO122

57. While statements: repeating with a condition
For statements: repetition over a list of values.
While statements : general repetition, subject to a condition.
while (condition-to-do-it-again ) {
actions to perform each time round }
while ( true ) {
UI.println("this repeats forever!");
int n = 1;
while ( n <= 100) {
UI.println(n) ;
n = n + 1;
Similar structure to the if statement

58. While statement Meaning: Similar to if, but NOT THE SAME! Repeatedly
If the condition is still true, do the actions another time
If the condition is false, stop and go on to the next statement.
Note: don’t do actions at all if the condition is initially false
Similar to if, but NOT THE SAME!
keeps repeating the actions,
as long as the condition is still true each time round
no else — just skips to next statement when condition is false
while (
condition ) {
action }

59. While with numbers #1 Print a table of numbers and their squares:
public void printTable(int max){
int num = 1;
while ( num <= max ) {
UI.printf(“ %3d %6d %n”, num, (num*num));
num = num + 1; }
Repetition with while generally involves
initialisation: get ready for the loop
test: whether to repeat
body: what to repeat
“increment”: get ready for the next iteration
Initialise
Test
Body
Increment

60. While with numbers #2 Draw a row of squares:
public static final double SIZE = 20; ⋮
/** Draws n squares in a horizontal row, starting at (left,top) */
public void drawSquares (int left, int top, int n){
int count = 0;
while ( count < n ) {
double x = left + count * SIZE;
UI.drawRect(x, top, SIZE, SIZE);
count = count + 1; }
Initialise
Test
Body
Increment
++;
Shorthand for
count = count + 1

61. While with numbers #3 Counting down: public void countDown(int start){
int count = start;
while ( count >= 1) {
UI.println( count );
count = count – 1; }
UI.println(“ GO”); :
this.countDown(5);

62. Nested while loops with numbers
Draw a grid of circles
public void drawCircles(int rows, int cols, int diam ) {
int row = 0;
while (row < rows) {
int col = 0;
while ( col < cols ) {
int x = LEFT + row*diam;
int y = TOP +col*diam;
UI.fillOval(x, y, diam, diam);
col++; }
row++;
Outside loop:
do each row
Inside loop:
do each column within the
current row

63. Menu
More while loops
Admin:
Test

64. Designing loops with numbers
When the number of steps is known at the beginning of the loop:
int count = 0; int num = 1;
while ( count < number) { OR while ( num <= number) {
do actions 〉 do actions 〉
count = count + 1; num = num + 1;
} }
Can count from 0 or from 1
If counting from 0, loop while count is less than target:
(count is the number of iterations that have been completed)
If counting from 1, loop while num is less than or equal to target:
(num is the iteration it is about to do)

65. Designing nested loops with numbers
2D structures, eg table of rows and columns:
Can do rows in the outside loop and columns in the inside loop, or vice versa
int row = 0; int col = 0;
while ( row < numberOfRows ) { while ( col < numberOfCols ) {
int col = 0; int row = 0;
while ( col < numberOfCols ) { while ( row < numberOfRows ) {
do actions for row, col 〉 do actions for row, col 〉
col++; row++;
} }
row++; col++
} }

66. General while loops
/** Practice times-tables until got 5 answers correct */
public void playArithmeticGame (){
int score = 0;
while ( score < 5) {
// ask an arithmetic question
int a = this.randomInteger(10);
int b = this.randomInteger(10);
int ans = UI.askInteger("What is " + a + " times " + b + "?" );
if ( ans == a * b ) {
score = score +1; }
UI.println("You got 5 right answers" );
public int randomInteger(int max) {
return (int) (Math.random() * max ) + 1; }

67. General while loops This seems unnecessarily complex!!
/** Ask a multiplication problem until got it right */
public void practiceArithmetic (){
int a = this.randomInteger(10);
int b = this.randomInteger(10);
String question = "What is " + a + " times " + b + "?";
boolean correct = false;
while ( ! correct) {
int ans = UI.askInteger(question);
if ( ans == a * b ) {
correct = true; }
UI.println("You got it right!" );
This seems unnecessarily complex!!

68. Loops with the test "in the middle"
If the condition for exiting the loop depends on the actions, need to exit in the middle!
Common with loops asking for user input.
break allows you to exit a loop (while, or for)
Must be inside a loop
Ignores any if 's
Does not exit the method ( return does that )
while ( true ) {
actions to set up for the test
if ( exit-test ) {
break; }
additional actions

69. General while loops with break
/** Ask a multiplication problem until got it right */
public void practiceArithmetic (){
int a = this.randomInteger(10);
int b = this.randomInteger(10);
String question = "What is " + a + " times " + b + "?";
boolean correct = false;
while ( ! correct) {
int ans = UI.askInteger(question);
if ( ans == a * b ) {
correct = true; }
UI.println("You got it right!" );
Only use break when the exit is not at the beginning of the loop.
true
Setting up for test
Test and break
break;
no additional actions

70. More loops with user input
Make user guess a magic word:
public void playGuessingGame(String magicWord){
UI.println("Guess the magic word:");
while (true) {
String guess = UI.askString("your guess: ");
if ( guess.equalsIgnoreCase(magicWord) ) {
UI.println("You guessed it!");
break; }
UI.println("No, that wasn't right. Try again!");
Setting up for test
Test and break
Additional actions

71. Testing your program
A) Need to try out your program on sample input while removing the "easy" bugs.
Can be a pain if need lots of input (eg TemperatureAnalyser)
UI window has a menu item – "set input" – to get input from a text file instead of user typing it.
 don't have to type lots of data each time
Create the text file, eg in Notepad
Select file using menu before the program has started asking for input.
File can contain multiple sequences of data.
B) Need to test your program on a range of inputs
Easy, "ordinary", inputs
Boundary cases — values that are only just in range, or just out of range Need to check that your if conditions are right
Invalid data—does your program handle invalid input correctly?
Creating test cases involves creativity – have to try to come up with ways to break your program.

72. Menu
Files
Admin
assignments
you are important to people!

73. Files The UI text pane window is transient:
Typing large amounts of input into the text pane is a pain!
It would be nice to be able to save the output of the program easily.
Large amounts of text belong in files
How can your program read from a file and write to a file?
Writing to files is like writing to the UI text pane!
Use print, println, printf methods
But, need extra objects: File and PrintStream objects
Reading from files is a bit different
Doesn't use "ask…" methods
Need to use "next…" methods
And need extra objects: File and Scanner objects

74. Text with the text pane red: 40 green: 60 blue: 30 all done
UI Window
red: 40
green: 60
blue: 30
all done
UI.askInteger();
UI.println();
My Program :
int r =UI.askInteger("red");
int g =UI.askInteger("green");
int b =UI.askInteger("blue");
UI.setColor(new Color(r,g,b);
UI.println("all done");

75. Text with Files Needs several objects:
Need File object to talk to the actual file on the disk.
Need Scanner/PrintStream object to talk to the File object
Program talks to the Scanner or PrintStream object.
A real file: “myfile.txt”
nextInt();
My Program :
int r =scan.nextInt();
int g =scan.nextInt();
int b =scan.nextInt();
UI.setColor(new Color(r,g,b);
outFile.println("all done");
Scanner
object
File object
PrintStream object
println();

76. Using a Scanner
Scanner: a class in Java that allows a program to read input from a file (or any other source of characters such as a String, a socket, …)
File: a class in Java that connects to an actual file on disk and get characters in and out of the file
Program needs to make a File object an get the next token, or the next line
String fileName = "My File.txt";
File inFile = new File(fileName);
Scanner scan = new Scanner(inFile); ⁞
int r = scan.nextInt();
My File.txt
25 53

77. Scanner
A Scanner breaks up the source into a sequence of chunks that the program can get, one at a time.
lines, (separated by the end-of-line characters)
tokens (separated by spaces, tabs, or end-of-line's)
Program can read the next token (or the next line)
Scanner scan = new Scanner ( new File("My File.txt") );
while ( scan.hasNext() ){
double radius = scan.nextDouble();
UI.drawOval(X-radius, Y-radius, radius*2, radius*2); }
My File.txt
25 53

78. Scanner "next" methods Method What it does Returns next()
Read and return next token
String
nextInt() nextDouble()
Read the next token.
Return it as a number, if it is a number.  Throws an exception if it is not a number.
int
double
nextBoolean()
Return true if it is "true"; return false if it is "false". 
Throws an exception if it is anything else.
boolean
hasNext()
Returns true if there is another token
hasNextInt() hasNextDouble()
hasNextBoolean()
Returns true if there is another token AND
the next token is an int / double / Boolean
nextLine()
Read characters up to the next end-of-line and return them as a string.
Reads and throws away the end-of-line character.  If the first character is an end-of-line, then it returns an empty string ("").
close()
close the file

79. Scanner methods.
Scanner has a cursor that keeps track of where it is up to in the file.
ACCY308 Lecture Tue GBLT2
ACCY308 Lecture Fri GBLT3
ACCY308 Lecture Tue GBLT3
ACCY330 Lecture Fri RHLT2
ACCY330 Lecture Wed RHLT2
ACCY401 Comp-Lab Mon RWW402
ACCY401 Lecture Mon RWW220
ACCY402 Lecture Wed RWW311
ACCY412 Lecture Wed RWW311
ACCY421 Lecture Thu RWW311
ALIN201 Lecture Mon KK204
ALIN201 Lecture Wed KK204
ALIN201 Tutorial Wed AM102
ALIN301 Lecture Tue KK105
ALIN301 Lecture Thu KK105
ALIN301 Tutorial Thu MY103
ANTH101 Lecture Mon KKLT303
ANTH101 Lecture Tue KKLT303

80. Reading lines using Scanner:
/** Read lines from a file and print them to UI text pane. */
public void readFile(){
File myfile = new File(“input.txt”);
Scanner scan = new Scanner(myfile);
UI.println(“ input.txt ”);
while (scan.hasNext()){
String line= scan.nextLine();
UI.println(line); }
UI.println(“ end of input.txt ”);
Almost right, but compiler complains!!!
Dealing with files may “raise exceptions”
Missing bits to handle exceptions !!
12am up to here

81. Files: handling exceptions
If a piece of code might raise an exception:
Have to enclose it in a
try { … }
catch (IOException e) { … }
public void readFile(){
File myfile = new File(“input.txt”);
try {
Scanner scan = new Scanner(myfile);
while (scan.hasNext()){
String line = scan.nextLine();
UI.println(line);
UI.println(“ end of input.txt ”);
catch (IOException e) { UI.println(“File failure: ” + e); }
what to do
what to do if it goes wrong
10am up to here

82. Reading from files: example
/** Finds oldest person in file of ages and names. */ public void printOldest(String filename){ try { Scanner scan = new Scanner(new File(filename)); String oldest = ""; int maxAge = 0; while (scan.hasNext()){ int age = scan.nextInt(); String name = scan.nextLine(); if (age > maxAge) { maxAge = age; oldest = name; } UI.printf(“Oldest is %s (%d)%n”, oldest, maxAge); } catch (IOException e) { UI.println("File failure: " + e); }
66 Marie Curie
48 James Clerk Maxwell
84 Isaac Newton
62 Aristotle
Read a token,
then read rest of line

83. Reading data from a file
public void drawShapes(String filename){
try {
Scanner scan = new Scanner( new File(fileName) );
while ( scan.hasNext() ){
double left = scan.nextDouble();
double top = scan.nextDouble();
String shape = scan.next();
int r = scan.nextInt();
int g = scan.nextInt();
int b = scan.nextInt();
UI.setColor( new Color (r, g, b) );
if (shape.equals("Oval") ){ UI.fillOval(left, top, WIDTH, HEIGHT); }
else { UI.fillRect(left, top, WIDTH, HEIGHT); } }
catch (IOException e) { UI.println(“File failure: ” + e); }
Oval
Rect
Oval
Stop at end of file
Reading all the values on the line
Do something with all the values

84. A common simple pattern
File with one entity per line, described by multiple values:
while (sc.hasNext() ){
String type = sc.next();
double cost = sc.nextDouble();
int wheels = sc.nextInt();
String colour = sc.next();
String make = sc.next()
if (wheels > 4) { …. }
else { … …
bicycle green Giant
truck black Isuzu
car red Toyota
Read all the values
into variables
process the values in
the variables

85. Reading files line by line
If items have a varying number of values:
May need to read a line at a time, then process:
/**Adds up sales of item on each line of a file */
public void addCounts(){
try {
Scanner scan = new Scanner(new File(“data.txt"));
while (scan.hasNext()){
String line = scan.nextLine();
Scanner lineSc = new Scanner(line);
int code = lineSc.nextInt();
String item = lineSc.next();
int lineTot = 0;
while (lineSc.hasNextInt()) {
lineTot = lineTot + lineSc.nextInt(); }
UI.printf("%s (%d): %d\n", item, code, lineTot);
} catch (IOException e) { UI.println("File failure: ” + e); }
973 biscuits
731 cake 3 5 2
189 fruit
446 beans
Wrapping a Scanner
around a String,
Lets you “read” values
from the String

86. Files that specify how big they are.
Sometimes a data file may specify how many values it contains
Can then use a "counted" loop to read the values:
try {
Scanner scan = new Scanner( new File ( orderFileName ) ); while ( scan.hasNext( ) ){
String model = scan.nextLine();
int count = scan.nextInt();
int totalOrders = 0;
int i = 0;
while (i < count){
totalOrders = totalOrders + scan.nextInt();
i++; }
UI.println( model + " had a total of " + totalOrders + " orders.");
scan.close();
} catch (IOException e) { UI.println("File error: " + e); }
Honda EV Orders.txt
Fit EV 5 35
270 15
380 89
Clarity 6 28 18 9 17 29

87. Files that specify how big they are.
Image files: ppm format
"Magic number" – code for ppm files
read into
variables:
int cols
int rows P3
12 5
255
width (number of columns of pixels) and height (the number of rows of pixels)
maximum colour value
red-green-blue of each pixel, in turn
nested while loops to
read colour of each pixel
set colour of UI
draw pixel.

88. Menu
Finishing files
Defining Objects
Admin
Test
Assignment …

89. Writing to a File Open a File object
Wrap it in a new PrintStream object.
Call print, print ln, or printf on it.
Close the file
try {
PrintStream out = new PrintStream(new File("powers-table.txt"));
int n=1;
out.println("Number\tSquare\tCube");
while ( n <= 1000 ) {
out.printf("%4d \t%7d \t%10\n", n, n*n, n*n*n);
n = n+1; }
out.close()
catch (IOException e) { UI.println("File error: " + e); }
Just like printing to UI
PrintStream Object
File Object

90. Checking if files exist
Can check that file exists before trying to read:
public void lineNumber(String fname){ /** Make a copy of a file with line numbers */
File infile = new File(fname);
if ( ! infile.exists()) { UI.println("The file " + fname + " doesn't exist"); return; }
File outfile = new File("numbered-” +fname);
try {
Scanner sc = new Scanner ( infile );
PrintStream out = new PrintStream(outfile);
int lineNum = 0;
while (sc.hasNext()) {
out.println(lineNum + ": " + sc.nextLine() );
lineNum++; }
out.close();
sc.close();
} catch (IOException e) { UI.printf(“File failure %s\n”, e);}

91. Passing an open scanner
First method: Just opens and closes the file
public void countTokensInFile(String fname){
try {
Scanner scan = new Scanner (new File(fname));
int numTokens = this.countTokens(scan);
UI.printf(“%s has %d tokens\n”, fname, numTokens);
sc.close();
} catch (Exception e) {UI.printf(“File failure %s\n”, e);} }
Second Method: Just reads from the scanner and counts
public int countTokens (Scanner sc){
int count = 0;
while (sc.hasNext()) {
sc.next(); // throws result away !
count = count+1;
return count;
scan:
Scanner-2543
File-872
973 biscuits 27
731 cake 3
189 fruit 54
446 beans 1 I
sc:

92. UIFileChooser
So far, we’ve specified which file to open and read or write with a String.
eg: File myfile = new File("input.txt");
How can we allow the user to choose a file?
UIFileChooser class (part of ecs100 library, like UI)
Method
What it does
Returns
open()
Opens dialog box;
User can select an existing file to open.
Returns name of file or null if user cancelled.
String
open(String title)
Same as open(), but with specified title;
save()
User can select file (possibly new) to save to.
Returns name of file, or null if the user cancelled.
save(String title)
Same as save(), but with specified title.

93. Using UIFileChooser methods: open
/** allow user to choose and open an existing file*/
String filename = UIFileChooser.open();
File myfile = new File(filename);
Scanner scan = new Scanner(myfile); OR
Scanner scan = new Scanner(new File(UIFileChooser.open()));
/** allow user to choose and open an existing file, specifies a title for dialog box*/
File myfile = new File(UIFileChooser.open(“Choose a file to copy”));
Two “open” methods in one class? Overloading : two methods in the same class can have the same name as long as they have different parameters.

94. Using UIFileChooser methods: save
/** allow user to choose and save to a (new/existing) file*/ String filename = UIFileChooser.save(); File myfile = new File(filename); PrintStream ps = new PrintStream(myfile); OR PrintStream ps = new PrintStream(new File(UIFileChooser.save())); /** allow user to choose and save to a (new/existing) file, Specifies a title for dialog box */ File myfile = new File(UIFileChooser.save(“File to save data in”));

95. Why objects? A program has a collection of classes
Each class has a collection of methods
FlagDrawer class had several methods:
public void doJapanFlag ()
public void doFrenchFlag()
Why do you have to create a FlagDrawer object before you can call these methods on it?
Why do you have to call the method on an object?
What is the object for? ?

96. Classes and Objects A class is a description of a type of object.
includes descriptions of methods you can call on this kind of object
Some kinds of objects we have used:
UI Scanner
println… ask… next… next, nextInt, hasNext,…
draw… fill… clear…
String File
length( ), substring… exists…
CartoonFigure Flower
boilWater, toast, bake … grow, bloom, pick …
What else did the objects need?
Information/Data, specifying the state of the object.
Stored in fields of the object

97. What is an Object An object is
A collection of data wrapped up together
plus
A collection of actions to operate on the collection of data
All specified in a class:
Fields where data is stored
Methods describing the actions
Constructor to make new objects
Constants
Some objects (top level program objects) may have no data.

98. CartoonStory program Java Program with 2D cartoon objects
Uses CartoonCharacter objects:
Methods:
public void lookLeft( )
public void lookRight( )
public void smile( )
public void frown( )
public void walk(double distance)
public void speak(String msg)
public void think(String msg)
Information a CartoonCharacter object must store:
its images
its size
its state (position, direction, emotion)

99. CartoonStory Program public class CartoonStory{
public void animate( ){
CartoonCharacter cf1 = new CartoonCharacter(150, 100, “green”);
cf1.lookRight;
cf1.lookLeft( );
cf1.frown( )
cf1.speak("Is anyone here?");
CartoonCharacter cf2 = new CartoonCharacter(300, 100, “blue”);
cf2.smile( ); cf2.lookLeft( ) ; cf2.speak("Hello");
cf1.lookRight( ); cf1.smile( );
cf1.speak("Hi there, I'm Jim");
cf2.speak("I'm Jan"); }
Two different objects of the same type
Two different objects of the same type

100. Defining a class of objects
CartoonCharacter is not part of the Java libraries ⇒ have to define the class
Need to define:
methods:
specify the actions the objects can do
constructor:
specifies how to make a new CartoonCharacter object
fields:
for storing the information about the state of each object

101. CartoonCharacter: methods
public class CartoonCharacter {
public void lookLeft( ) { public void lookRight( ) {
// erase figure // erase figure
// change direction // change direction
// redraw figure // redraw figure
} }
public void frown( ) { public void smile( ) {
// erase figure // erase figure
// change emotion // change emotion
// redraw figure // redraw figure
} }
public void walk(double dist) { public void speak(String msg) {
// erase figure // draw msg in bubble
// change position // wait
// redraw figure // erase msg

102. CartoonCharacter: wishful methods
public class CartoonCharacter {
public void lookLeft( ) { public void lookRight( ) {
this.erase( ); this.erase( );
// change direction // change direction
this.draw( ); this.draw( );
} }
public void frown( ) { public void smile( ) {
this.erase( ); this.erase( );
// change emotion // change emotion
this.draw( ); this.draw( );
} }
public void walk(double dist) { public void speak(String msg) {
this.erase( ); // draw msg in bubble
// change position // wait
this.draw( ); // erase msg
} }
public void erase( ) { public void draw( ) {
??? ???

103. CartoonCharacter: draw
public void draw( ) {
// work out which image to use (eg, “green/right-smile.png”)
// draw the image on the graphics pane
// wait a bit }
String filename = imageFolder+"/"+direction+"-"+emotion+".png" ;
UI.drawImage(filename, figX, figY, wd, ht);
UI.sleep(500); // wait 500 mS
But where are those variables defined?
Where do they get their values?

104. Menu Objects and fields Admin Test: Beta-tester opportunity –
pickup today, or
at school office (3rd floor Cotton)
distribution of grades
suspected errors in marking?
Beta-tester opportunity –
startup company working with the School of Design
get the app free, and

105. CartoonCharacter Objects
Objects need places to store values – called “Fields”
Objects are like entries in your Contacts
CartoonCharacter-24
figX:
imageFolder: “ ”
figY:
wd:
ht:
emotion: “ ”
direction: “ ”
CartoonCharacter-27
figX:
imageFolder: “ ”
figY:
wd:
lecture to here
ht:
emotion: “ ”
direction: “ ”

106. Using fields:
A method can refer to a field of the object it was called on:
this . fieldname
eg:
public void lookLeft( ) {
this.erase( ) ;
this.direction = “left”;
this.draw( ) ; }
public void draw( ) {
String filename = this.imageFolder + ”/” + this.direction + “-” +
this.emotion + “.png” ;
UI.drawImage(filename, this.figX, this.figY, this.wd, this.ht);
UIsleep(500); // wait 500 mS
note: fields have no ( )
Object the method
was called on

107. Using fields: CartoonCharacter-24 Object figX: wd: figY: ht: 150 40
emotion: imageFolder:
direction:
Object :
cf1. lookLeft( );
cf1. walk(20);
public void lookLeft( ) {
this.erase( ) ;
this.direction = “left”;
this.draw( ) ; }
150 40
300 80
“smile”
“green”
“right”
“left”
cf1: CartoonCharacter-24
ID of Object
Method worksheet
this: CartoonCharacter-

108. Using fields: CartoonCharacter-24 figX: wd: figY: ht:
emotion: imageFolder:
direction:
Object
public void draw( ) {
String filename = this. imageFolder + ”/” + this.direction + “-” +
this.emotion + “.png” ;
UI.drawImage(filename, this.figX, this.figY, this.wd, this.ht);
UI.sleep(500); }
150 40
300 80
“smile”
“green”
“left”
Method Worksheet
this: CartoonCharacter-
“ ”

109. Using fields: CartoonCharacter-24 figX: wd: figY: ht:
emotion: imageFolder:
direction:
Object :
cfg1. lookLeft( );
cfg1. walk(20);
public void lookLeft( ) {
this.erase( ) ;
this.direction = “left”;
this.draw( ) ; }
150 40
300 80
“smile”
“green”
“left”
cfg1: CartoonCharacter-24
ID of Object
this: CartoonCharacter-

110. Using fields: CartoonCharacter-24 figX: wd: figY: ht: 150 40
emotion: imageFolder:
direction:
cfg1.lookLeft( );
cfg1.walk(20); :
public void walk (double dist) {
this.erase( ) ;
if ( this.direction.equals(“right”) { this.figX = this.figX + dist ; }
else { this.figX = this.figX – dist ; }
this.draw( ) ; }
150 40
300 80
“smile”
“green”
“left”
cfg1: CartoonCharacter-24
this: CartoonCharacter-

111. Objects and Classes Classes define objects:
Fields: places in an object that store the information associated with the object
methods can refer to fields of the object they were called on: this.fieldname
How do you set up the fields?
Methods: can be called on any object of the class
Constructors: specify how to set up an object when it is first created.
Constants: specify names for values

112. Setting up an object Must declare the Fields of an object?
Declared in the class
(not inside a method)
Must specify the type and the name
(just like local variables in methods)
Can specify an initial value (but you don’t have to!)
if not, automatically initialised with 0 or null
(unlike local variables)
Have a visibility specifier (“private”)
Fields remain indefinitely
The set of field declarations is a template for the object
(just like a method is a template for a worksheet).
Just as local variables
must be declared

113. Syntax of Field declarations:
public class CartoonCharacter {
// fields
private double figX; // current position of figure
private double figY;
private String direction = "right"; // current direction it is facing
private String emotion = "smiling"; // current emotion
private String imageFolder; // base name of images
private double wd = 40; // dimensions of figure
private double ht=80;
// methods …….
private
type
field name ; =
expression
Like variables, BUT
(a) NOT inside a method
(b) have private in front

114. Setting up an object How do you initialise the values in the fields?
Can specify an initial value in the field declaration
but only if every object should start with the same value!!!
Must have a way of setting up different objects when you create them:
Constructor:
specifies what happens when you make a new object
(eg, evaluate the expression
new CartoonCharacter(150, 100, “green”)
We have seen constructors with no parameters.
Can have parameters that can be used to set up the new object.

115. CartoonCharacter class
Shorthand for declaring two fields (or variables) of the same type
public class CartoonCharacter {
// fields
private double figX, figY; // current position of figure
private String direction = "right"; // current direction it is facing
private String emotion = "smile"; // current emotion
private String imageFolder; // folder where images stored
private double wd = 40, ht=80; // dimensions
// constructor
public CartoonCharacter(double x, double y, String base){
this.imageFolder = base;
this.figX = x;
this.figY = y;
this.draw(); }
// methods …….
public void lookLeft() {
this.erase(); …..
Got to here in lect 16, 2014T1

116. Syntax of Constructor Definitions (2)
public
class name (
type
parameter name ) { ,
statement }
public CartoonCharacter(String base, double x, double y){
this.imageFolder = base;
this.figX = x;
this.figY = y;
this.draw(); }

117. Constructors Defining a Constructor Constructor typically
Part of the class
Like a method, but called with new
Does not have a return type
(new always returns an object of the given type)
this will hold the new object that is being constructed
Constructor typically
fills in initial values of fields
may call other methods on the object,
can do anything an ordinary method can do.
The constructor of the “top level” class may set up the user interface.

118. What happens with new ? When an object is created CartoonCharacter-24
figX:
figY:
emotion:
direction:
imageFolder:
wd:
ht:
When an object is created
eg new CartoonCharacter(100, 200 , "yellow");
New chunk of memory is allocated (new filing card).
Reference (ID) to object is constructed CartoonCharacter-24
Any initial values specified in the field declarations are assigned to the fields.
If no initial value, default values:
0 for fields of a number type (int, double, etc)
false for for boolean fields
null for fields of an object type (String, Scanner, Car, …)
The arguments are passed to the constructor
The actions specified in the constructor are performed on the object.
The reference is returned as the value of the constructor.
100.
200
“ smile ”
“ right ”
“ yellow ”
null
40.
80.

119. The whole Program Simple class: - no fields - constructor for UI
- methods
public class CartoonStory{
public CartoonStory(){
UI.addButton(“go”, this::playStory); }
public void playStory(){
CartoonCharacter cf1 = new CartoonCharacter(150, 100, “green”);
cf1.lookLeft();
cf1.lookRight();
cf1.frown()
cf1.speak("Is anyone here?");
CartoonCharacter cf2 = new CartoonCharacter(300, 100, “blue”);
cf2.speak("Hello");
cf2.lookLeft() ;
cf1.smile();
cf1.speak("Hi there, I'm Jim");
cf2.speak("I'm Jan");
public static void main(String[ ] args){
CartoonStory cs = new CartoonStory();
Note the main method
⇒ don't need BlueJ

120. CartoonCharacter: fields & constructor
public class CartoonCharacter {
// fields
private double figX; // current position of figure
private double figY;
private String direction = "right"; // current direction it is facing
private String emotion = "smile"; // current emotion
private String imageFolder; // base name of image set
private double wd = 40; // dimensions
private double ht=80;
// constructor
public CartoonCharacter(double x, double y, String base){
this.imageFolder = base;
this.figX = x;
this.figY = y;
this.draw(); }

121. CartoonCharacter: methods
public void lookLeft() { public void lookRight() {
this.erase(); this.erase();
this.direction = "left"; this.direction = "right";
this.draw(); this.draw();
} }
public void frown() { public void smile() {
this.erase(); this.erase();
this.emotion = "frown"; this.emotion = "smile";
this.draw(); this.draw();
} }
public void walk(double dist) {
this.erase();
if ( this.direction.equals(“right”) {
this.figX = this.figX + dist ; }
else {
this.figX = this.figX – dist ;

122. CartoonCharacter: methods
public void speak(String msg) {
double bubX = this.figX - …; // and bubY, bubWd, bubHt
UI.drawOval(bubX, bubY, bubWd, bubHt);
UI.drawString(msg, bubX+9, bubY+bubHt/2+3);
UI.sleep(500);
UI.eraseRect(bubX, bubY, bubWd, bubHt); }
public void erase() {
UI.eraseRect(this.figX, this.figY, this.wd, this.ht);
public void draw() {
String filename = this. imageFolder +"/"+this.direction+"-"+
this.emotion+“.png” ;
UI.drawImage(filename, this.figX, this.figY, this.wd, this.ht);

123. Running the program: main
> java CartoonStory or call main on the class from BlueJ
public static void main(String[ ] args){
CartoonStory cs = new CartoonStory(); }
cs: CartoonStory-3
CartoonStory-3
Very simple object!
- no fields
- no constructor

124. CartoonStory Program: playStory
public void playStory(){
CartoonCharacter cf1 = new CartoonCharacter(150, 100, “green”);
cf1.lookLeft();
cf1.lookRight();
cf1.frown()
cf1.speak("Is anyone here?");
CartoonCharacter cf2 = new CartoonCharacter(300, 100, “blue”);
cf2.speak("Hello");
cf2.lookLeft() ;
cf1.smile();
cf1.speak("Hi there, I'm Jim");
cf2.speak("I'm Jan");
this: CartoonStory-3
CartoonCharacter-24
figX: wd:
figY: ht:
emotion:
direction:
imageFolder :
150.
100.
40.
80.
“ smile ”
“ right ”
“ green ”
cf2: CartoonCharacter-
cf1: CartoonCharacter-24
Is anyone here?

125. CartoonStory Program: playStory
public void playStory(){
CartoonCharacter cf1 = new CartoonCharacter(150, 100, “green”);
cf1.lookLeft();
cf1.lookRight();
cf1.frown()
cf1.speak("Is anyone here?");
CartoonCharacter cf2 = new CartoonCharacter(300, 100, “blue”);
cf2.speak("Hello");
cf2.lookLeft() ;
cf1.smile();
cf1.speak("Hi there, I'm Jim");
cf2.speak("I'm Jan");
this: CartoonStory-3
cf1: CartoonCharacter-24
cf2: CartoonCharacter-27
CartoonCharacter-27
figX: wd:
figY: ht:
emotion:
direction:
imageFolder :
300.
100.
40.
80.
“ smile ”
“ right ”
“ blue ”
Hello

126. Keeping track of Multiple objects
CartoonCharacter-24
figX: wd:
figY: ht:
emotion:
direction:
imageFolder :
150.
100.
40.
80.
“ frown ”
“ right ”
“ blue ”
CartoonCharacter-27
figX: wd:
figY: ht:
emotion:
direction:
imageFolder :
300.
100.
40.
80.
“ smile ”
“ right ”
“ blue ” :
cf2.lookLeft() ;
cf1.smile();
public void lookLeft() {
this.erase() ;
this.direction = “left”;
this.draw() ; }
cf1: CartoonCharacter-24
cf2: CartoonCharacter-27
this: CartoonCharacter-

127. Keeping track of Multiple objects
CartoonCharacter-24
figX: wd:
figY: ht:
emotion:
direction:
imageFolder :
150.
100.
40.
80.
“ frown ”
“ right ”
“ blue ”
CartoonCharacter-27
figX: wd:
figY: ht:
emotion:
direction:
imageFolder :
300.
100.
40.
80.
“ smile ”
“ left ”
“ blue ” :
cf2.lookLeft() ;
cf1.smile();
public void smile() {
this.erase() ;
this.emotion = “smile”;
this.draw() ; }
cf1: CartoonCharacter-24
cf2: CartoonCharacter-27
this: CartoonCharacter-

128. Menu Another example of defining objects Scope, Extent, Visibility
Event-Driven Input
Admin
Test marks
Beijing Summer School on mobile apps development
July 3 -14
Two students will be sent
See the forum message
Ian Welch if you are interested

129. Bouncing Balls
Two classes: Bouncer and BouncingBall

130. Designing Bouncer (“top level” class)
How does the user interaction work?
buttons,
constructor
What are the methods?

131. Designing BouncingBall class
What fields does it need?
What methods should it have?
What should happen when it is first created?

132. BouncingBall: fields & constructor
public class BouncingBall {
// fields
private double xPos;
private double height;
private double xSpeed;
private double ySpeed;
private Color col;
// constructor
public BouncingBall(double x, double y, double sp ){ }

133. BouncingBall: methods
public void draw () { }
public void move() {
public double getX() {

134. Places: variables vs fields
Two kinds of places to store information:
Variables (including parameters)
defined inside a method
specify places on a worksheet
temporary – information is lost when worksheet is finished
new place created every time method is called (each worksheet)
only accessible from inside the method.
Fields
defined inside a class, but not inside a method
specify places in an object
long term – information lasts as long as the object
new place created for each object
accessible from all methods in the class, and from constructor.

135. Extent and scope
A place with a value must be accessible to some code at some time.
Extent: how long it will be accessible
local variables (and parameters) in methods have a limited extent ⇒ only until the end of the current invocation of the method
fields have indefinite extent ⇒ as long as the object exists
Scope: what parts of the code can access it
Full scope rules are complicated!!!
local variables: accessible only to statements
inside the block { … } containing the declaration
after the declaration
fields: at least visible to the containing class; maybe further.

136. Scope of variables
//read info from file and display while (scan.hasNext() ){ String ans = scan.next(); if ( ans.equals("flower") ) { Color center = Color.red; int diam = 30; } else if (ans.equals("bud") ) { Color center = Color.green; int diam = 15; : UI.setColor(center); UI.fillOval(x, y, diam, diam);
while (scan.hasNext() ){
String ans = scan.next();
Color center = null;
int diam = 0;
if ( ans.equals("flower") ) {
center = Color.red;
diam = 15; }
else if (ans.equals("bud") ) {
center = Color.blue;
diam = 30; :
UI.setColor(center);
UI.fillOval(x, y, diam, diam); ; ;
different variables!
different variables!
Out of scope
Out of scope
may not be intialised
may not be intialised
How do you fix it?

137. Fields: scope, visibility, encapsulation
Fields are accessible to all code in all the (ordinary) methods in the class.
Should they be accessible to methods in other classes?
⇒ visibility: public or private
public means that methods in other classes can access the fields
cfg1.figX = in the CartoonStory class would be OK
private means that methods in other classes cannot access the fields
cfg1.figX = in the CartoonStory class would be an error.
The principle of encapsulation says
Keep fields private.
Provide methods to access and modify the fields, if necessary
⇒ LDC 5.3

138. GUI’s and Event driven input
In a GUI, the interaction is controlled by the user, not by the program
User initiates "events"
buttons
menus
mouse press/release/drag
text fields
sliders
keys
Program responds

139. PuppetMaster How does Java respond to buttons etc?
Smile
Frown
Right
Walk
Speak
Distance
Left
How does Java respond to buttons etc?
When a button pressed / text entered in box / slider changed / mouse clicked:
Java looks up the object & method attached to the button/box/etc
Calls the method
passing the value for box or slider.
passing kind of action and position (x and y) for mouse.

140. Setting up event-driven input
Setting up the GUI:
To add a button to the UI:
specify name of button and method to call (object ::method or class ::method) (must be a method with no parameters)
eg: UI.addButton("go", this::startGame); UI.addButton("end", UI::quit);
To add a textfield to the UI:
Specify name of textfield and method to call
(must be a method with one String parameter)
eg UI.addTextField("name", this::setName);
To add a slider to the UI:
Specify name of slider, min, max, initial values, and method to call
(must be a method with one double parameter)
eg UI.addSlider("speed", 10, 50, 20, this::setSpeed);

141. Event driven input and fields
Each event will make a new method call.
 can't remember anything between events in local variables in the methods.
Typically, need fields in the main object to remember information between events.
eg: PuppetMaster has to remember the CartoonCharacter object in a field

142. PuppetMaster: Design Structure of the PuppetMaster class:
public class PuppetMaster … {
// fields to store values between events/method calls
private ….
// Constructor
public PuppetMaster(){
// set up the buttons, slider, textField
// initialise fields }
// methods to respond to the buttons, slider, textField
public void …

143. PuppetMaster: setting up Buttons etc
public class PuppetMaster … {
// fields
// constructor
public PuppetMaster(){
UI.addButton( "Smile", this::doSmile);
UI.addButton( "Frown", this::doFrown);
UI.addButton( "Left", this::doLeft);
UI.addButton( "Right", this::doRight);
UI.addTextField( "Say", this::doSpeak);
UI.addButton( "Walk", this::doWalk);
UI.addSlider( "Distance", 1, 100, 20, this::setDist); … }
// methods to respond
Smile
Frown
Say
Left
Right
Walk
Distance 1
100

144. Responding to buttons and textFields
public class PuppetMaster {
// fields
// constructor
public PuppetMaster(){
UI.addButton("Smile", this::doSmile);
UI.addButton("Frown", this::doFrown); ⋮
UI.addTextField(“Say", this::doSpeak); }
public void doSmile(){
// tell the CartoonCharacter to smile
public void doFrown(){
// tell the CartoonCharacter to frown
public void doSpeak(String words){
// tell the CartoonCharacter to say the words
Methods called by buttons
must have no parameters
A method called by a button
must have no parameters
Methods called by a textField
must have one String parameter

145. PuppetMaster: Using Fields
Actions on the CartoonCharacter happen in response to different events
⇒ will be in different method calls
⇒ need to store character in a field, not a local variable.
public class PuppetMaster{
// fields
private CartoonCharacter cc = new CartoonCharacter(200, 100, "blue");
// constructor
public PuppetMaster(){
UI.addButton("Smile", this::doSmile); // call doSmile on this
UI.addButton("Frown", this::doFrown); : }
public void doSmile(){
this.cc.smile();
public void doFrown(){
this.cc.frown();

146. PuppetMaster: TextFields (boxes)
public class PuppetMaster{
private CartoonCharacter cc = new CartoonCharacter(200, 100, "blue");
public PuppetMaster(){
UI.addButton("Smile", this::doSmile); // call doSmile on this
UI.addButton("Frown", this::doFrown);
UI.addTextField(“Say", this::doSpeak); : }
public void doSmile(){
this.cc.smile();
public void doSpeak(String words){
this.cc.speak(words);

147. PuppetMaster: Sliders
public class PuppetMaster {
private CartoonCharacter cc = new CartoonCharacter(200, 100, "blue");
private double walkDist = 20 ;
public PuppetMaster(){
UI.addButton("Smile", this::doSmile); :
UI.addButton(“Walk", this::doWalk);
UI.addSlider( "Distance", 1, 100, 20, this::setDist); }
public void doWalk() {
this.cc.walk(this.walkDist);
public void setDist(double value){
this.walkDist = value;
Typical design:
field to store value
from one event,
for use by another event
A method called by
a slider must have
one double parameter

148. GUI: Mouse input
Just like buttons, except don’t have to put anything on screen
Each press / release / click on the graphics pane will be an event
Must tell UI object::method to call when a mouse event occurs
UI.setMouseListener(this :: doMouse);
Must define method to say how to respond to the mouse parameters: kind of mouse event and position of mouse event
public void doMouse(String action, double x, double y) {
if (action.equals("pressed") ) {
// what to do if mouse button is pressed }
else if (action.equals("released") ) {
// what to do if mouse button is released
else if (action.equals("clicked") ) {
// what to do if mouse button is clicked
where action
occurred
press-release
in same place

149. Using the mouse. Want to let user specify input with the mouse,
eg: drawing lines
Typical pattern:
On "pressed",
just remember the position
On "released",
do something with remembered position and new position 1
(260,90)
(100,80) 2

150. Mouse Input /**Let user draw lines on graphics pane with the mouse. */
public class LineDrawer {
private double startX, startY; // fields to remember “pressed” position
public LineDrawer(){
UI.setLineWidth(10);
UI.setMouseListener(this::doMouse); }
public void doMouse(String action, double x, double y) {
if (action.equals("pressed") ) {
this.startX = x;
this.startY = y;
else if (action.equals("released") ) {
UI.drawLine(this.startX, this.startY, x, y);

151. Selecting Colors: JColorChooser
public class LineDrawer { private double startX, startY; private Color currentColor = Color.black; public LineDrawer (){ UI.setMouseListener(this::doMouse); UI.addButton("Color", this::doChooseColour); } public void doMouse(String action, double x, double y) { if (action.equals("pressed") ) { this.startX = x; this.startY = y; } else if (action.equals("released") ) { UI.drawLine(this.startX, this.startY, x, y); } public void doChooseColour(){ this.currentColor = JColorChooser.showDialog(null, "Choose Color", this.currentColor); UI.setColor(this.currentColor);

152. Numbers program Program for constructing files of numbers:
Allow user to select a new file
Allow user to enter a set of numbers with the mouse (height of mouse click is the number)
Display numbers as bar chart and list in text pane
Save numbers to the file as they are entered
User Interface:
Button to clear screen and select new file.
Graphics pane to select (with mouse) and display the numbers
Text pane to display list of numbers
Numbers
130 72
281 98
264 97
New

153. Numbers: Design Design: When does something happen? button presses
mouse clicks
Fields
to store the file (PrintStream) that the numbers are being saved to
to remember the horizontal position of the next bar.
Constructor
set up the interface
Methods to respond to mouse
record a new number
Method to respond to button
clear and start a new file
Numbers
New

154. Numbers: Design public class Numbers { private PrintStream outputFile;
private double barX = 0;
private static final double BASE= 450;
public Numbers(){
UI.setMouseListener(this::doMouse);
UI.addButton("New", this::doNew);
UI.drawLine(0, BASE, 600, BASE); }
public void doNew() {…
public void doMouse( …
public static void main(String[ ] args){
new Numbers();
Numbers
New

155. Respond to Mouse: When user clicks/releases:
work out the number they meant
draw a bar on the graphics pane
display it in the text pane
print it to the file
public void doMouse(String action, double x, double y) {
if (action.equals("released")) {
double number = BASE - y;
this.barX = this.barX + 10;
UI.fillRect(this.barX, y, 5, number);
UI.println(number);
this.outputFile.println(number); }
Numbers
130
What's the problem?
New
if (this.outputFile != null) {
this.outputFile.println(number); }

156. Respond to "New" button public void doNew(){ UI.clearPanes();
UI.drawLine(0, BASE, 600, BASE);
this.barX = 0;
this.outputFile.close();
try{
this.outputFile = new PrintStream(new File(UIFileChooser.save()));
} catch(IOException e) { UI.println("File error: "+e); } }
// Alternative for the long one line:
String fname = UIFileChooser.save();
File file = new File(fname);
this.outputFile = new PrintStream(file);
if (this.outputFile != null) {
this.outputFile.close(); }
Still a problem!

157. PuppetMaster: Problem 1
Suppose we have two characters!
Problem:
Which character should smile/turn/walk/speak?
Event-driven input can be tricky!
Smile
Frown
Left
Right
Walk
Speak
Distance

158. GUI design: choosing object to act on
One typical simple GUI interaction mechanism
Select object you want to act on
Choose action.
Must remember the currently selected object:
in a field, because the action will be performed in a later method
this.selectedCC = cc1;
Typically, the “selected object” doesn’t change until user selects another object.

159. PuppetMaster Problem: two characters
Smile
Frown
Speak
Distance
Walk
PuppetMaster-3
fields:
walkDistance:
cc1: CartoonCharacter-11
cc2: CartoonCharacter-12
selectedCC: CartoonCharacter-11
CartoonCharacter-11
emotion: "smile"
figX: 110
figY: 200
direction: "right"
imgBaseName: "blue"
CartoonCharacter-12
emotion: "frown"
figX: 350
figY: 200
direction: "left"
imgBaseName: "green"

160. PuppetMaster: selecting a character.
public class PuppetMaster{
private CartoonCharacter cc1= new CartoonCharacter(“blue", 100, 100);
private CartoonCharacter cc2= new CartoonCharacter(“green", 500, 100);
private CartoonCharacter selectedCC = cc1; // the selected one
private double walkDistance = 20;
public PuppetMaster(){
UI.addButton( "Smile", this::doSmile); ⋮ }
public void doSmile(){
this.selectedCC.smile();
public void doFrown(){
this.selectedCC.frown();
How do we change
the selected character?

161. PuppetMaster: buttons for selecting
public PuppetMaster() {
UI.addButton( "Jim", this::doJim);
UI.addButton( "Jan", this::doJan);
UI.addButton( "Smile", this::doSmile); ⋮ }
public void doJim() {
this.selectedCC = this.cc1;
public void doJan() {
this.selectedCC = this.cc2;
public void doSmile(){
this.selectedCC.smile();
public void doWalk() {
this.selectedCC.walk(this.walkDistance );

162. PuppetMaster: TextFields & Sliders
Jim
Jan
Speak
Distance
Walk
Smile
Frown
Hello
Hello
PuppetMaster-3
walkDistance: 20
cc1: CartoonCharacter-11
cc2: CartoonCharacter-12
selectedCC: CartoonCharacter-11
CartoonCharacter-11
emotion: "smile"
figX: 110
figY: 200
direction: "right"
imgBaseName: "blue"
CartoonCharacter-12
emotion: "frown"
figX: 350
figY: 200
direction: "left"
imgBaseName: "green" 60

163. Shorthand: “Lambda expressions”
public class PuppetMaster{
private CartoonCharacter selectedCC = new CartoonCharacter(200, 100, "blue");
public PuppetMaster(){
UI.addButton("Smile", this::doSmile);
UI.addButton("Frown", this::doFrown);
UI.addTextField(“Say", this::doSpeak); : }
public void doSmile(){
this.selectedCC.smile();
public void doFrown(){
this.selectedCC.frown();
public void doSpeak(String words){
this.selectedCC.speak(words);
Lots of typing for just
one line

164. Shorthand: “Lambda expressions”
public class PuppetMaster{
private CartoonCharacter selectedCC = new CartoonCharacter(200, 100, "blue");
public PuppetMaster(){
UI.addButton("Smile", () -> { this.selectedCC.smile(); } );
UI.addButton("Frown", this::doFrown);
UI.addTextField(“Say", this::doSpeak); : }
public void doSmile(){
this.char.smile();
this.selectedCC.smile();
public void doSpeak(String words){
this.selectedCC.speak(words);
Lambda Expression:
Unnamed method!!
- has parameters
- has body
- has no name
It is a value!!

165. Shorthand: “Lambda expressions”
public class PuppetMaster{
private CartoonCharacter selectedCC = new CartoonCharacter(200, 100, "blue");
public PuppetMaster(){
UI.addButton("Smile", () -> { this.selectedCC.smile(); } );
UI.addButton("Frown", () -> { this.selectedCC.frown(); } );
UI.addButton( "Left", () -> { this.selectedCC.lookLeft(); } );
UI.addButton( "Right", () -> { this.selectedCC.lookRight(); } );
UI.addTextField(“Say", (String wds) -> { this.selectedCC.speak(wds); } );
UI.addButton(“Walk", () -> { this.selectedCC.walk(this.walkDist); } );
UI.addSlider( "Distance", 1, 100, 20,
(double val) -> { this.walkDist = val; } ); }
You do NOT HAVE TO USE THESE!!
It is always safe to have an explicit, named method.

166. Shorthand: “Lambda expressions”
public class PuppetMaster{ private CartoonCharacter cc1= new CartoonCharacter(“blue", 100, 100); private CartoonCharacter cc2= new CartoonCharacter(“green", 500, 100); private CartoonCharacter selectedCC= cc1; // the selected one private double walkDistance = 20; public PuppetMaster(){ UI.addButton(“Jim", () -> { this.selectedCC = cc1; } ); UI.addButton(“Jan", () -> { this.selectedCC = cc2; } ); UI.addButton("Smile", () -> { this.selectedCC.smile(); } ); UI.addButton("Frown", () -> { this.selectedCC.frown(); } ); UI.addButton( "Left", () -> { this.selectedCC.lookLeft(); } ); UI.addButton( "Right", () -> { this.selectedCC.lookRight(); } ); UI.addTextField(“Say", (String wds) -> { this.selectedCC.speak(wds); } ); UI.addButton(“Walk", () -> { this.selectedCC.walk(this.walkDist); } ); UI.addSlider( "Distance", 1, 100, 20, (double val) -> { this.walkDist = val; } ); }