1. Method - CIS 1068 Program Design and Abstraction
Zhen Jiang
CIS Dept.
Temple University
SERC 347, Main Campus
6/14/2018

2. Table of Contents Introduction (use of method) Declaring a method
Calling a method
Methods calling methods
Control flow of method call
Generalizing method
Parameterization
Multiple parameters
Parameter mystery
Return values (vs. void)
Summary
6/14/2018

3. Procedural decomposition using methods
6/14/2018

4. Redundancy in programs
public class FraggleRock {
public static void main(String[] args) {
System.out.println("Dance your cares away,");
System.out.println("Worry's for another day.");
System.out.println("Let the music play,");
System.out.println("Down at Fraggle Rock.");
System.out.println(); }
6/14/2018

5. Compared with the timing problem in loop: 1st, 2nd, 3rd … iterations
This is a space & timing problem: to locate the redundancy of any part of program running at any time.
Compared with the timing problem in loop: 1st, 2nd, 3rd … iterations
Compared with the space problem in arrays: 1st, 2nd, 3rd … elements
1st, 2nd, 3rd … appearances of the same/similar procedure in your program!
6/14/2018

6. public class FraggleRock { public static void singChorus() {
finclaration,
definition,
body
public class FraggleRock {
public static void singChorus() {
System.out.println("Dance your cares away,");
System.out.println("Worry's for another day.");
System.out.println("Let the music play,");
System.out.println("Down at Fraggle Rock."); }
public static void main(String[] args) {
singChorus();
System.out.println();
First time, four print
lines repeated
The second time,
four prints repeated
6/14/2018

7. static method: A group of statements given a name.
To use a static method:
1. define it (write down the recipe)‏
Write a group of statements and give it a name.
2. call it (cook using the recipe)‏
Tell our program to execute the method.
6/14/2018

8. public class FraggleRock { public static void singChorus() {
Method signature
public class FraggleRock {
public static void singChorus() {
System.out.println("Dance your cares away,");
System.out.println("Worry's for another day.");
System.out.println("Let the music play,");
System.out.println("Down at Fraggle Rock."); }
public static void main(String[] args) {
singChorus();
System.out.println();
Method body
Method call, 1st time
Method call, 2nd time
6/14/2018

9. Declaring a static method
The syntax for declaring a static method:
public class <class name> {
...
public static void <method name>() {
<statement>; … }
6/14/2018

10. Calling a static method
The syntax for calling a static method (cooking using the recipe):
<method name>();
6/14/2018

11. Declaring a static method
public static void printAffirmation() {
System.out.println("I am good enough!");
System.out.println("I am smart enough!");
System.out.println("People like me!"); }
Calling a static method (possibly multiple times)‏
printAffirmation();
Output
I am good enough!
I am smart enough!
People like me!
6/14/2018

12. Methods calling methods
Output:
This is message1.
This is message2.
Done with message2.
Done with main.
One static method can call another (p310):
public class MethodsExample {
public static void main(String[] args) {
message1();
message2();
System.out.println("Done with main."); }
public static void message1() {
System.out.println("This is message1.");
public static void message2() {
System.out.println("This is message2.");
System.out.println("Done with message2.");
6/14/2018

13. Control flow of methods
When a method is called, the execution
"jumps" into that method
executes all of the method’s statements
"jumps" back to the statement after the method call
6/14/2018

14. Output: public class MethodsExample {
This is message1.
This is message2.
Done with message2.
Done with main.
public class MethodsExample {
public static void main(String[] args) {
message1();
message2();
... }
public static void message1() {
System.out.println("This is message1."); }
public static void message2() {
System.out.println("This is message2");
message1();
System.out.println("Done with message2."); }
public static void message1() {
System.out.println("This is message1."); }
6/14/2018

15. Example: Figure drawing
______
/ \
/ \
\ /
\______/
| STOP |
Example: Figure drawing
Write a program to print the figures. Use static methods to capture structure and and eliminate redundancy.
6/14/2018

16. Version 1: Unstructured
______
/ \
/ \
\ /
\______/
| STOP |
Version 1: Unstructured
Create an empty program with a skeletal header and main method.
Copy the expected output into it, surrounding each line with System.out.println syntax.
Run and verify that it produces the correct output.
6/14/2018

17. 6/14/2018 // Suzy Student, CSE 142, Autumn 2047
// This program prints several assorted figures. //
public class Figures1 {
public static void main(String[] args) {
System.out.println(" ______");
System.out.println(" / \\");
System.out.println("/ \\");
System.out.println("\\ /");
System.out.println(" \\______/");
System.out.println();
System.out.println(" ");
System.out.println("| STOP |"); }
6/14/2018

18. Version 2: Structured with redundancy
______
/ \
/ \
\ /
\______/
| STOP |
Version 2: Structured with redundancy
Identify the overall structure of the output, and divide the main method into several static methods based on this structure.
6/14/2018

19. The structure of the output: initial "egg" figure
______
/ \
/ \
\ /
\______/
| STOP |
Identify the overall structure of the output, and divide the main method into several static methods based on this structure.
The structure of the output:
initial "egg" figure
second "teacup" figure
third "stop sign" figure
fourth "hat" figure
This structure can be represented by methods:
drawEgg
drawTeaCup
drawStopSign
drawHat
6/14/2018

20. 6/14/2018 // Suzy Student, CSE 142, Autumn 2047
// Prints several assorted figures, with methods for structure. //
public class Figures2 {
public static void main(String[] args) {
drawEgg();
drawTeaCup();
drawStopSign();
drawHat(); }
// Draws a figure that vaguely resembles an egg.
public static void drawEgg() {
System.out.println(" ______");
System.out.println(" / \\");
System.out.println("/ \\");
System.out.println("\\ /");
System.out.println(" \\______/");
System.out.println();
// Draws a figure that vaguely resembles a teacup.
public static void drawTeaCup() {
System.out.println(" ");
6/14/2018

21. 6/14/2018 // Draws a figure that vaguely resembles a stop sign.
public static void drawStopSign() {
System.out.println(" ______");
System.out.println(" / \\");
System.out.println("/ \\");
System.out.println("| STOP |");
System.out.println("\\ /");
System.out.println(" \\______/");
System.out.println(); }
// Draws a figure that vaguely resembles a hat.
public static void drawHat() {
System.out.println(" ");
6/14/2018

22. Version 3: Structured without redundancy
______
/ \
/ \
\ /
\______/
| STOP |
Version 3: Structured without redundancy
Further divide the program to eliminate all redundancy.
6/14/2018

23. Further divide the program to eliminate all redundancy.
______
/ \
/ \
\ /
\______/
| STOP |
Further divide the program to eliminate all redundancy.
The redundancy:
top half of egg (purple)‏
bottom half of egg (green)‏
divider line (yellow)‏
This redundancy can be fixed by methods:
drawEggTop
drawEggBottom
drawLine
6/14/2018

24. 6/14/2018 // Suzy Student, CSE 142, Autumn 2047
// Prints several figures, with methods for structure and redundancy.
public class Figures3 {
public static void main(String[] args) {
drawEgg();
drawTeaCup();
drawStopSign();
drawHat(); }
// draws redundant part that looks like the top of an egg
public static void drawEggTop() {
System.out.println(" ______");
System.out.println(" / \\");
System.out.println("/ \\");
// draws redundant part that looks like the bottom of an egg
public static void drawEggBottom() {
System.out.println("\\ /");
System.out.println(" \\______/");
6/14/2018

25. 6/14/2018 // Draws a figure that vaguely resembles an egg.
public static void drawEgg() {
drawEggTop();
drawEggBottom();
System.out.println(); }
// Draws a figure that vaguely resembles a teacup.
public static void drawTeaCup() {
System.out.println(" ");
// Draws a figure that vaguely resembles a stop sign.
public static void drawStopSign() {
System.out.println("| STOP |");
// Draws a figure that vaguely resembles a hat.
public static void drawHat() {
6/14/2018

26. BBBBB
B B
AAAA
A A
AAAAAA
N N
NNN N
N NNN
Exercise
Write a program to print the block letters spelling "banana". Use static methods to capture structure and and eliminate redundancy.
6/14/2018

27. Generalizing Method
Consider the task of drawing the following figures:
*************
*******
***********************************
**********
* *
*****
* *
The lines and figures are similar, but not exactly the same.
6/14/2018

28. Observation: Methods are redundant.
public class Stars {
public static void main(String[] args) {
drawLineOf13Stars();
drawLineOf7Stars();
drawLineOf35Stars();
draw10x3Box();
draw5x4Box(); }
public static void drawLineOf13Stars() {
for (int i = 1; i <= 13; i++) {
System.out.print("*");
System.out.println();
public static void drawLineOf7Stars() {
for (int i = 1; i <= 7; i++) {
public static void drawLineOf35Stars() {
for (int i = 1; i <= 35; i++) {
...
Observation: Methods are redundant.
Would constants help us solve this problem?
Other ideas?
6/14/2018

29. What if we had the following?
drawLine - A method to draw a line of any number of stars.
drawBox - A method to draw a box of any size. 7
main
drawLine
******* 13
drawLine
*************
Character of each
repetition
General format of
repetition
6/14/2018

30. Parameterization parameterized method:
A method that is given extra information (e.g. number of stars to draw) when it is called.
The common part of all calls in a generalization: 1st, 2nd, 3rd, …
parameter: A value passed to a method by its caller, mapping the difference of each call.
Examples:
System.out.println(“*”) or System.out.println(“**”)
random.nextInt(10)
6/14/2018

31. Parameterized method declaration syntax:
public static void <name> (<type> <name>) {
<statement(s)>; }
The scope of the parameter is the entire method. P284
Example:
public static void printSpaces(int count) {
for (int i = 1; i <= count; i++) {
System.out.print(" ");
Whenever printSpaces is called, the caller must specify how many spaces to print.
count’s scope
6/14/2018

32. Parameterized method call syntax:
passing an argument for parameter: Calling a parameterized method and specifying a value for its parameter(s). P291
Parameterized method call syntax:
<name>(<expression>);
Example:
System.out.print("*");
printSpaces(7);
System.out.print("**");
int x = 3 * 5;
printSpaces(x + 2);
System.out.println("***");
Output:
* ** ***
6/14/2018

33. 7 13 When the parameterized method call executes:
the value passed to the method is copied into the parameter variable
the method's code executes using that value
public static void main(String[] args) {
printSpaces(7);
printSpaces(13); }
public static void printSpaces(int count) {
for (int i = 1; i <= count; i++) {
System.out.print(" "); 7 13
count:
6/14/2018

34. value semantics: When primitive variables (such as int or double) are passed as parameters, their values are copied into the method's parameter variable. P
Modifying the method’s parameter variable will NOT affect the the variable which was passed to the method.
...
int x = 23;
strange(x);
System.out.println("2. x = " + x); // this x unchanged }
public static void strange(int x) {
x = x + 1; // modifies my x
System.out.println("1. x = " + x);
Output:
1. x = 24
2. x = 23
6/14/2018

35. printSpaces(); // ERROR: parameter value required
ERROR: Not passing a parameter to a method that accepts parameters.
printSpaces(); // ERROR: parameter value required
ERROR: Passing a parameter of the wrong type.
printSpaces(3.7); // ERROR: must be of type int
The parameter must satisfy the domain of the method.
6/14/2018

36. Change the Stars program to use parameterized methods.
public class Stars {
public static void main(String[] args) {
drawLineOf13Stars();
drawLineOf7Stars();
drawLineOf35Stars();
draw10x3Box();
draw5x4Box(); }
public static void drawLineOf13Stars() {
for (int i = 1; i <= 13; i++) {
System.out.print("*");
System.out.println();
public static void drawLineOf7Stars() {
for (int i = 1; i <= 7; i++) {
...
6/14/2018

37. // Prints several lines of stars.
// Uses a parameterized method to remove redundancy.
public class Stars2 {
public static void main(String[] args) {
drawLine(13);
drawLine(7);
drawLine(35); }
// Prints the given number of stars plus a line break.
public static void drawLine(int count) {
for (int i = 1; i <= count; i++) {
System.out.print("*");
System.out.println();
6/14/2018

38. Multiple parameters Methods can accept as many parameters as you like.
When the method is called, it must be passed values for each of its parameters.
Multiple parameters declaration syntax:
public static void <name> (<type> <name>,
<type> <name>, ..., <type> <name>) {
<statement(s)>; }
Multiple parameters call syntax:
<name>(<expression>, <expression>, ..., <expression>);
6/14/2018

39. // Prints several lines and boxes made of stars.
// Third version with multiple parameterized methods.
public class Stars3 {
public static void main(String[] args) {
drawLine(13);
drawLine(7);
drawLine(35);
System.out.println();
drawBox(10, 3);
drawBox(5, 4);
drawBox(20, 7); }
// Prints the given number of stars plus a line break.
public static void drawLine(int count) {
for (int i = 1; i <= count; i++) {
System.out.print("*");
6/14/2018

40. // Prints a box of stars of the given size.
public static void drawBox(int width, int height) {
drawLine(width);
for (int i = 1; i <= height - 2; i++) {
System.out.print("*");
printSpaces(width - 2);
System.out.println("*"); }
// Prints the given number of spaces.
public static void printSpaces(int count) {
for (int i = 1; i <= count; i++) {
System.out.print(" ");
6/14/2018

41. Exercise Output: public static void main(String[] args) {
printNumber(4, 9);
printNumber(17, 6);
printNumber(8, 0);
printNumber(0, 8); }
Output:
6/14/2018

42. Output: public static void main(String[] args) { printNumber(4, 9);}
Output:
public static void printNumber(int number, int count) {
for(int i=0; i<count; i++) {
System.out.print(number);
System.out.println();
6/14/2018

43. Write a method named printDiamond that accepts a height as a parameter and prints a diamond figure.*
***
*****
Write a method named multiplicationTable that accepts a maximum integer as a parameter and prints a table of multiplication from 1 x 1 up to that integer times itself.
6/14/2018

44. Rewrite the following program to use parameterized methods:
// Draws triangular figures of stars.
public class Loops {
public static void main(String[] args) {
for (int i = 1; i <= 5; i++) {
for (int j = 1; j <= i - 1; j++) {
System.out.print(" "); }
for (int j = 1; j <= * i + 1; j++) {
System.out.print("*");
System.out.println();
for (int i = 1; i <= 12; i++) {
for (int j = 1; j <= * i; j++) {
6/14/2018

45. 6/14/2018 // Draws triangular figures using parameterized methods.
public class Loops {
public static void main(String[] args) {
triangle(5);
triangle(12); }
// Draws a triangle figure of the given size.
public static void triangle(int height) {
for (int i = 1; i <= height; i++) {
printSpaces(i - 1);
drawLine(2 * height * i);
6/14/2018

46. Parameter mystery (value vs. object)
What does this code do?
public static void main(String[] args) {
int a = 7;
int b = 35;
System.out.println(a + " " + b);
int temp = a;
a = b;
b = temp; }
6/14/2018

47. Does this work? Why or why not?
public static void main(String[] args) {
int a = 7;
int b = 35;
System.out.println(a + " " + b);
// swap a with b
swap(a, b); }
public static void swap(int a, int b) {
int temp = a;
a = b;
b = temp;
Does this work? Why or why not?
6/14/2018

48. Class type vs. primitive type, P345!
When a primitive type of argument is passed for parameter, the value is copied. A method cannot change the value of an argument.
When an object is passed as a parameter, the object is not copied. The same object is referred to by both the original argument and the method's parameter.
If a method is called on the parameter, it will affect the original object that was passed to the method.
6/14/2018

49. Object are reference value like this:
Dog balto = new Dog();
balto.name = “Balto”;
name:
breed:
age:
balto:
“Balto” ? ?
Variable, with slot in memory
Reference, a pointer to the object’s data
6/14/2018

50. p1 x: -1 2 y: 3 -2 p Example: public static void main(String[] args) {
Point p1 = new Point(2, 3);
move(p1); }
public static void move(Point p) {
p.setLocation(-1, -2); p1 x: -1 2 y: 3 -2 p
6/14/2018

51. What is the output of the following program?
public class Mystery {
public static void main(String[] args) {
int x = 5, y = 9, z = 2;
mystery(z, y, x);
System.out.println(x + " " + y + " " + z);
mystery(y, x, z); }
public static void mystery(int x, int z, int y) {
x++;
y = x - z * 2;
x = z + 1;
x: y: z:
x: z: y:
6/14/2018

52. ParametersDemo.java and DemoSpecies.java
6/14/2018

53. Return Values (vs. Void)
return: To send a value out as the result of a method, which can be used in an expression.
A return value is like the opposite of a parameter.
Parameters pass information in from the caller to the method.
Return values pass information out from a method to its caller.
How would this be useful?
6/14/2018

54. Java has a class called Math that has several useful static methods to perform mathematical calculations.
logarithm base e
log(value)‏
nearest whole number
round(value)‏
square root
sqrt(value)‏
random double between 0 and 1
random()‏
base to the exponent power
pow(base, exponent)‏
smaller of two values
min(value1, value2)‏
larger of two values
max(value1, value2)‏
logarithm base 10
log10(value)‏
cosine, in radians
cos(value)‏
absolute value
abs(value)‏
Description
Method name
6/14/2018

55. Math method call syntax:
Math.<method name>(<parameter(s)>)‏
Examples:
double squareRoot = Math.sqrt(121.0);
System.out.println(squareRoot); // 11.0
int absoluteValue = Math.abs(-50);
System.out.println(absoluteValue); // 50
System.out.println(Math.min(3, 7) + 2); // 5
Notice that the preceding calls are used in expressions; they can be printed, stored into a variable, etc…
6/14/2018

56. The Math methods do not print results to the console.
Instead, each method evaluates to produce (or return) a numeric result, which can be used in an expression.
main
Math.abs
-42
Math.round
2.71 42 3
6/14/2018

57. Evaluate the following expressions:
Math.abs(-1.23)‏
Math.pow(3, 2)‏
Math.pow(10, -2)‏
Math.sqrt(121.0) - Math.sqrt(256.0)‏
Math.ceil(6.022) + Math.floor( )‏
Math.abs(Math.min(-3, -5))‏
Math.max and Math.min can be used to bound numbers.
Consider an int variable named age.
What statement would replace negative ages with 0?
What statement would cap the maximum age to 40?
6/14/2018

58. Declaring a method that returns a value (p277):
public static <type> <name>(<parameters>) {
<statement(s)>; }
Returning a value from a method:
return <expression>;
Example:
// Returns the given number cubed (to the third power).
public static int cube(int number) {
return number * number * number;
6/14/2018

59. ERROR: Writing statements after a return statement.
public static int increment(int x) {
return (x + 1);
x = x + 1; // ERROR: statement unreachable! }
ERROR: Confusing the return variable with a variable in the calling method, AKA ignoring the return value.
public class ReturnExample {
public static void main(String[] args) {
int x = 1;
addOne(x);
System.out.println("x = " + x);
public static int addOne(int x) {
x = x + 1;
return x;
6/14/2018

60. Write a method called isFactor that accepts two integers as parameters, and returns true if the first is a factor of the second.
6/14/2018

61. Methods can return different values under different conditions:
public static int min(int a, int b) {
if (a > b) {
return b;
} else {
return a; }
public static String message(int place) {
if (place == 1) {
return "You won!";
return "If you're not first, you're last!";
6/14/2018

62. The compiler will complain about a "missing return statement". Why?
public static int min(int a, int b) {
if (a > b) {
return b; }
The compiler will complain about a "missing return statement". Why?
ERROR: Not returning a value in every path. In the above example, what if a <= b?
6/14/2018

63. It still produces the "missing return statement" error. Why?
public static int min(int a, int b) {
if (a > b) {
return b;
} else if (a <= b) {
return a; }
It still produces the "missing return statement" error. Why?
To our eyes, it is clear that all paths (greater, equal, less) do return a value.
But the compiler thinks that if/else if code might choose not to execute any branch, so it refuses to accept this code.
How can we fix it?
6/14/2018

64. Write a program that prompts the user for a maximum integer and prints out a list of all prime numbers up to that maximum. Here is an example log of execution:
Maximum number? 50
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47
15 total primes
6/14/2018

65. 6/14/2018 import java.util.*; public class PrintPrimes {
public static void main(String[] args) {
Scanner console = new Scanner(System.in);
printPrimes(getNumber(console)); }
public static int countFactors(int num) {
int count = 0;
for (int i = 1; i <= num; i++) {
if (num % i == 0) {
count++;
return count;
...
6/14/2018

66. 6/14/2018 public static int getNumber(Scanner console) {
System.out.print("Maximum number? ");
return console.nextInt(); }
public static void printPrimes(int max) {
int numPrimes = 0;
if (max >= 2) {
System.out.print(2);
numPrimes++;
for (int i = 3; i <= max; i++) {
if (countFactors(i) == 2) {
System.out.print(", " + i);
System.out.println();
System.out.println(numPrimes + " total primes");
6/14/2018

67. Summary Procedure decomposition Method declaration, p277
Method call & control flow
Exercises on slide 26
Scope of local variable, p , p290
Figure drawing, sample code, slides 18-25
Parameter and argument, p291
Primitive type (p291) vs. class type (p )
DemoSpecies on slide 52
Return (vs. void) , p277
6/14/2018