1. Fields, Constructors COMP 102 #14 2016T2

2. Assignments and term test
A4 due after the break, Thursday 10am
No labs, lectures, tutorials during the break
Term test: 18 August during lecture time
50 minutes
Covers materials in lecture 1-11, A1-A4
Variables, arguments parameters
If, while,
Program with two classes: one call the other
Review lecture slides and lecture code
Complete the core part of the assignments
Do the previous years term test questions
2016T1, 2015T2

3. Menu A program with two classes The second class Data fields
Constructor
Methods

4. Define a method Input Output Ask user Parameters From data fields
Print/draw to screen
Return a value
Update data fields

5. The main class: create new objects
public class NewObjects{ public void createNewObj(){ BankAccount b1 = new BankAccount("Alice"); b1.deposit(100); BankAccount b2 = new BankAccount("Bob"); b2.deposit(200); b1.withdraw(50); b2.statement(); b1.statement(); }

6. The second class with fields, constructor
public class BankAccount { private String name; private double balance; public BankAccount(String n){ this.name = n; this.balance = 0.0; } public void deposit(double x){ this.balance = this.balance + x; } public void withdraw(double x){ this.balance = this.balance - x; } public void statement(){ UI.println(this.name + " $"+this.balance); }

7. A typical class Classes define objects:
Fields: places in an object that store the information associated with the object
Constructors: specify how to set up an object when it is first created.
Methods: can be called on any object of the class
methods can refer to fields of the object they were called on using: this.fieldname

8. Class in the library UI, Math, System, Trace, UIFileChooser
Constants
static methods
Scanner, PrintStream, File,
Data fields
Constructor
Non-static methods
Color, String, JColorChooser, Integer, Double,
non-static methods

9. Our programs Most of your assignments: one class 2 classes
Constants
Methods
2 classes
CartoonStrip, with methods only
CartoonFigure
Data fields
Constructor
Non-static methods
Typical classes with fields, constructor
Animal, Frog, Snake (in A5)
Car, Flower, Balloon, StudentMarkRecord (in A5 exercises)
BankAccount, (in lecture)
Butterfly (in lecture, only give documentation version)
In future: Customer, Product, Employee, Student, Tutor, Course, …

10. Program has two classes: the main class
Simple class:
- no fields
- no constructor
- just methods
public class CartoonStrip{
public void playStory(){
CartoonFigure cf1 = new CartoonFigure(“green”, 150, 100);
cf1.turnLeft();
cf1.turnRight();
cf1.frown()
cf1.talk("Is anyone here?");
CartoonFigure cf2 = new CartoonFigure(“blue”, 300, 100);
cf2.talk("Hello");
cf2.turnLeft() ;
cf1.smile();
cf1.talk("Hi there, I'm Jim");
cf2.talk("I'm Jan"); }

11. CartoonFigure class: wishful methods
public class CartoonFigure {
public void talk(String msg) { public void move(double dist) {
// draw msg in bubble this.erase();
// wait // change position
// erase msg this.draw();
} }
public void turnLeft() { public void turnRight() {
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 erase() { public void draw() {
??? ???

12. CartoonFigure: draw But where are those variables defined?
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 = baseImgNm+“-"+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?

13. Remembering state Each CartoonFigure must remember:
its state:
position
emotion
direction
the set of image files that it is using.
its size
Can’t be stored in local variables in a method
local variables are “lost” when the method finishes.
Have to be stored in the Object itself
⇒ fields

14. Syntax of Field declarations:
public class CartoonFigure {
// 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 baseImgNm; // 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

15. Setting up an object How do we 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!)
otherwise, 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).

16. 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 CartoonFigure(“green” 150, 100)
Like a method but some differences
Has parameters that can be used to set up the new object.

17. CartoonFigure class public class CartoonFigure { // 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 baseImgNm; // folder where images stored
private double wd = 40, ht=80; // dimensions
// constructor
public CartoonFigure(String base, double x, double y){
this.baseImgNm = base;
this.figX = x;
this.figY = y;
this.draw(); }
// methods …….
public void turnLeft() {
this.erase(); …..
Name of the class, instead of
return type and method name

18. Syntax of Constructors
public
class name (
type
parameter name ) { ,
statement }
public CartoonFigure(String base, double x, double y){
this.baseImgNm = base;
this.figX = x;
this.figY = y;
this.draw(); }

19. 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 set up the user interface, if there is one.
may call other methods on the object,
can do anything an ordinary method can do.

20. What happens with new ? When an object is created “ smile ” “ right ”
CartoonFigure-24
figX:
figY:
emotion:
direction:
baseImgNm :
wd:
ht:
When an object is created
eg new CartoonFigure("yellow", 100, 200);
New chunk of memory is allocated (new filing card).
Reference (ID) to object is constructed CartoonFigure-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.
150.
100.
“ smile ”
“ right ”
“ yellow ”
null
40.
80.

21. CartoonFigure: fields & constructor
public class CartoonFigure {
// 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 baseImgNm; // base name of image set
private double wd = 40; // dimensions
private double ht=80;
// constructor
public CartoonFigure(String base, double x, double y){
this.baseImgNm = base;
this.figX = x;
this.figY = y;
this.draw(); }

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

23. CartoonFigure: methods
public void turnLeft() { public void turnRight() {
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 move(double dist) {
this.erase();
if ( this.direction.equals(“right”) {
this.figX = this.figX + dist ; }
else {
this.figX = this.figX – dist ;

24. CartoonFigure Objects
Objects need Fields to store values – places inside the object
Objects are like filing cards
CartoonFigure-24
figX:
figY:
emotion:
direction:
baseImgNm:
wd:
ht:
CartoonFigure-27
figX:
figY:
emotion:
direction:
baseImgNm:
wd:
ht: . . . .
“ ”
“ ”
“ ”
“ ”
“ ”
“ ” . . . .

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

26. Using fields: CartoonFigure-24 figX: wd: figY: ht: emotion: baseImgNm:
direction: :
cf1. turnLeft();
cf1. move(20);
public void turnLeft() {
this.erase() ;
this.direction = “left”;
this.draw() ; }
Object
150 40
300 80
“smile”
“green”
“right”
“left”
cf1: CartoonFigure-24
ID of Object
Method
this: CartoonFigure-

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

28. Using fields: CartoonFigure-24 Object figX: wd: figY: ht: 150 40
emotion: baseImgNm:
direction: :
cfg1. turnLeft();
cfg1. move(20);
public void turnLeft() {
this.erase() ;
this.direction = “left”;
this.draw() ; }
Object
150 40
300 80
“smile”
“green”
“left”
cfg1: CartoonFigure-24
ID of Object
this: CartoonFigure-

29. Using fields: CartoonFigure-24 figX: wd: figY: ht: 150 40
emotion: baseImgNm:
direction:
cfg1.turnLeft();
cfg1.move(20); :
public void move (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: CartoonFigure-24
this: CartoonFigure-