Methods Functions

(1)

(2)

Methods / Functions

•

A method/function is a set of statements

grouped together to perform an operation

•

Useful to break code into methods/functions to

keep code simpler

(3)

Method Components

•

Name

•

Modifiers

•

Return type

(4)

Method Example

public class HelloWorld

{

public static void main(String args[])

{

//code

}

(5)

Method Example

{

//code

}

(6)

Method Example

{

//code

}

(7)

Method Example

{

//code

}

(8)

Method Example

{

public static void main(String args[])

{

//code

}

(9)

Method Example

{

//code

}

(10)

Method Example

{

//code

}

String args[] is the

type and name of

(11)

Other Examples

•

private int returnSum(int[] list)

•

void setValue()

(12)

Modifiers

•

Access control / Scope

•

Static

•

Final

•

Abstract

(13)

Access Control / Scope

•

Will discuss in more detail in future classes

Modifier

Class

Scope

Package

Scope

Subclass

Scope

Package Scope

Outside

public

TRUE

(14)

Access Control / Scope

•

public

–

public void go(){

//code here

}

•

protected

–

protected void go(){

//code here

}

•

default

–

void go(){

//code here

}

•

private

(15)

Static

•

Whether or not the method is called by the

class or the object

•

Static methods do not need an instance of the

class to be created to be called

–

ClassName.staticMethod();

(16)

Final

•

Different than a final variable

(17)

Abstract

•

(18)

Synchronized

•

For multithreading, ensures that the method

(19)

Return Types

•

The type of value the function returns

•

Can be any object or primitive, or nothing

–

void – no return value

(20)

Return Types

public void returnNothing() {

}

public int returnInt() {

return 1; }

public int returnInt2() {

(21)

Parameters

•

Input values the method takes

•

Can be any object or primitive, or none

•

Comma separated list

•

Limit is 255

–

Generally only 3-4 is acceptable

(22)

Parameters

//no parameters

void doSomething(){}

//one integer parameter

void doSomething(int x){}

(23)

Method Signature

•

All methods within the same class must have a

unique method signature

•

Method signature

(24)

Example - SignatureExamples.java

•

SignatureExamples.java

(25)

Calling Methods

•

Must call methods with the expected

parameters

•

Methods can be called three ways

(26)

Calling Methods

•

Class name

–

Static methods are called with the class name

//calls the floor method of the Math class

Math

.floor(5.2);

(27)

Calling Methods

•

Object name

–

Regular methods are called with the object name

(28)

Calling Methods

•

No name

–

Methods in the same class can be called without

any name

(29)

Example – Method Calling

•

MethodCallExample.java

•

MethodCallExample2.java

(30)

Parameter Passing

•

Pass by Value

–

For primitive types

–

Makes a copy of the data and uses the copy

•

Pass by Reference

–

For object types and arrays

(31)

Pass by Value

public static int increment(int a) {

a++; return a; }

public static void main(String args[]) {

int a = 5;

(32)

Pass by Value

a++; return a; }

int a = 5;

int b = increment(a); System.out.println(a); System.out.println(b);

(33)

Pass by Value

a++; return a; }

int a = 5;

int b = increment(a); System.out.println(a);

(34)

Pass by Value

a++; return a; }

int a = 5;

int b = increment(a); System.out.println(a); System.out.println(b);

Copy the value of

‘a’ into a new local variable

defined for the function which

(35)

Pass by Value

a++; return a; }

int a = 5;

(36)

Pass by Value

a++; return a; }

int a = 5;

(37)

Pass by Value

a++; return a; }

int a = 5;

return a

a

6

(38)

Pass by Value

a++; return a; }

int a = 5;

System.out.println(b);

5

a

Store return

value in b

6

(39)

Pass by Value

a++; return a; }

int a = 5;

a

b

(40)

Pass by Value

a++; return a; }

int a = 5;

(41)

Example – PassByValue.java

(42)

Pass by Reference

public static int[] increment(int[] a) {

a[0]++; return a; }

int a[] = {5, 10};

int[] b = increment(a);

System.out.println(a[0] + “ ” + a[1]); System.out.println(b[0] + “ ” + b[1]);

Set variable

a[0] = 5

a[1] = 10

(43)

Pass by Reference

a[0]++; return a; }

int a[] = {5, 10};

System.out.println(a[0] + “ ” + a[1]);

Call increment

with a

(44)

Pass by Reference

a[0]++; return a; }

int a[] = {5, 10};

a

A new pointer is made that

points to the SAME memory

(45)

Pass by Reference

a[0]++; return a; }

int a[] = {5, 10};

increment the first value

6

, 10

a

(46)

Pass by Reference

a[0]++; return a; }

int a[] = {5, 10};

return the array

6, 10

a

(47)

Pass by Reference

a[0]++; return a; }

int a[] = {5, 10};

Store the return value in b

6, 10

a

(48)

Pass by Reference

a[0]++; return a; }

int a[] = {5, 10};

Print out array a

6, 10

a

(49)

Pass by Reference

a[0]++; return a; }

int a[] = {5, 10};

Print out array b

6, 10

a

(50)

Example – PassByReference.java

(51)

Method Overloading

•

Method with the same name but different

parameters

–

Assumed to have the same purpose

–

Useful when the same task can be done with a

different set of initial parameters

(52)

Example - OverloadExample.java

(53)

Variable Scope

•

Local Variable = variable defined within the

method

•

Scope starts at declaration

(54)

Example - SelectionSort.java

•

Week3



SelectionSort.java

•

What is the scope of unsortedArray?

•

What is the scope of i?

•

What is the scope of j?

•

What is the scope of totalSelectionSort?