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Addison Wesley

is an imprint of

Starting Out with Java:

From Control Structures through Objects

Fourth Edition

by Tony Gaddis

Chapter 5: Methods

Reading Quiz

1. In Java a named block of code that accomplishes a specific task is called a: a) Function

b) Subroutine

c) Loop

d) Method

2. A _______ method is one that does not return a value when finished. a) boolean

b) String

c) void

d) class

5-3

Chapter Topics

Chapter 5 discusses the following main topics:

– Introduction to Methods

– Passing Arguments to a Method

– More About Local Variables

– Returning a Value from a Method

– Problem Solving with Methods

What is a Method?

A named group of Java commands.

Using Methods to Build A Shakespearean Insult Generator

Insult me!

How it works

PseudoCode for Insult Generator

1. Clear the screen

2. Display “Shakespearean Insult Generator”

3. Begin the insult with Thou

4. Add adjectives

5. Add noun

6. Add insult ending

clearScreen()

Method call

Method header

Method body

Method declaration

5-8

The Method Call

public static void main(String[] args)

{

clearScreen();

}

Call

5-9

Two Parts of Method Declaration

public static void clearScreen()

{

System.out.println(“\f");

}

Header

Body

public class Checkpoint

{

public static void main(String[] args)

{

method1();

method1();

method2();

}

public static void method1()

{

System.out.println("abra");

}

public static void method2()

{

System.out.print("cad");

method1();

}

}

What is the output of this program?

What is the output?

Circle method headers.

Box method calls

public class Checkpoint

{

public static void main(String[] args)

{

method1();

method1();

method2();

}

public static void method1()

{

System.out.println("abra");

}

public static void method2()

{

System.out.print("cad");

method1();

}

}

What is the output of this program?

circle method headers

box method calls

output:

abra

abra

cadabra

beginInsult()

adjective() - use if-else-if

loop in main() for many insults

2 adjectives

noun() - use switch

endInsult()

Let’s write some methods

5-13

Why Write Methods?

• Methods are commonly used to break a problem down into small manageable pieces. This is called divide and conquer.

• Methods simplify programs. If a specific task is performed in several places in the program, a method can be written once to perform that task, and then be executed anytime it is needed. This is known as code reuse.

generateInsults()

generateInsults(500) Arguments and parameters

Data types must match

Can pass many arguments like Math.pow()

Pass by reference; scope

Better and Deeper

Checkpoint

Checkpoint

• argument is given to method call

• parameter receives argument in method body

99 1.5

99 1.5

0 0

99 1.5

Ask user for # of insults.

getNumInsults()

Returns an int

User Input and Passing Data

Methods we know already. Do they receive and/or return data?

int i = keyboard.nextInt();

System.out.println(“Hello World”);

String upper = name.toUpperCase();

clearScreen();

System.out.println( Math.sqrt(49) );

5-21

Defining a Value-Returning Method

public static int sum(int num1, int num2)

{

int result;

result = num1 + num2;

return result;

}

Return type

This expression must be of the

same data type as the return type

The return statement

causes the method to end

execution and it returns a

value back to the

statement that called the

method.

5-22

Calling a Value-Returning Method

total = sum(value1, value2);

public static int sum(int num1, int num2)

{

int result;

result = num1 + num2;

return result;

}

20 40

60

is it void or value-returning?

Checkpoint

5.3b Is the following line of code a method header or method call? void or

value-returning?

is it void or value-returning?

Checkpoint

a void method has no return value

when invoked it appears on a line by itself

a value returning method comes back with a value when complete

must appear in an assignment statement or output line.

5.3b Is the following line of code a method header or method call? void or

value-returning?

• Any: int, double, boolean, String, …

• Any primitive or object

• Parameter type must match Argument type.

• Return type must match the use in the call.

What data types can be passed or returned?

5-26

Returning a Reference to a String Object

customerName = fullName("John", "Martin");

public static String fullName(String first, String last)

{

String name;

name = first + " " + last;

return name;

}

See example:

ReturnString.java

address

“John Martin”

Local variable name holds

the reference to the object.

The return statement sends

a copy of the reference

back to the call statement

and it is stored in customerName.

public class ReturnString

{

public static void main(String[] args)

{

String customerName;

customerName = fullName("John", "Martin");

System.out.println(customerName);

}

public static String fullName(String first, String last)

{

String name;

name = first + " " + last;

return name;

}

}

String Returning Method Example

Checkpoint

Checkpoint

public static int days( int years, int months, int

weeks)

public static double distance(double rate, double time)