Chapter 5 Methods

Chapter 5 Chapter 5 Methods 1 2 Contents Introduction to Methods Passing Arguments to a Method More about Local Variables Returning a Value from a Method Problem Solving with Methods 2 3 1 Introductio.

Chapter 5

Methods

Contents

1 Introduction to Methods

performs a specific task

such as

1 Introduction to Methods

into small manageable pieces

all of the statements, several small methods that each solve a specific part of the problem can be written

because a large problem is divided into several

problems

1 Introduction to Methods

simplify programs

a program, a method can be written once and

then be executed anytime it is needed

reuse.

void Methods and

Value-Returning Methods

task, but also sends a value back to the code that called it

Defining a void Method

which consists of two general parts:

when the method is executed

public static void main(String[] args)

{

System.out.println(“Hello World!”);

} Header

Body

Defining a void Method

public static void displayMessage ()

{

System.out.println(“Hello from the displayMessage method.”);

}

Method Modifiers:

- public: The method is publicly available to code outside the class

- static: The method belongs to the class, not a specific object.

Return Type:

- void: The method is a void method, and does not return a value.

Method Name: A descriptive name that can describes the function of the method.

Parentheses:

- The method name is always followed by a set of parentheses.

- List of arguments will appear inside the parentheses.

Method Modifiers

Return Type

Method Name

Parentheses The method header is never terminated with a semicolon.

Calling a Method

program starts

statements

that method and executes the statements in its

body

displayMessage();

Method call statement:

- The statement is simply the name of the method followed by a set of parentheses.

- The method modifiers and the void return type are not written.

Calling a Method

Branching in the SimpleMethod.java Program

public static void main(String[] args)

Calling a Method

Calling a Method

his annual salary and credit rating The program determines whether the user qualifies for a credit card One of two void methods, qualify or

noQualify, is called to display a message

Credit rating:

The user qualifies for a credit card if his salary is not less than $20,000 and his credit rating is not less than 7

Calling a Method

Calling a Method

Hierarchical Method Calls

Hierarchical Method Calls

Using Documentation Comments

with Methods

comments that appear just before the method's

definition

of the method's purpose

with /** and end with */ These types of

comments can be read and processed by a

program named javadoc which produces

attractive HTML documentation

2 Passing Arguments to a

Method

to as a parameter, is a special variable that holds

a value being passed into a method

public static void displayValue(int num)

{

System.out.println(“The value is “ + num);

}

integer value as an argument

various arguments passed:

displayValue(x);

displayValue(x * 4);

The argument 5 is copied into the parameter variable num.

2 Passing Arguments to a

Method

Argument and Parameter Data

Type Compatibility

sure that the argument's data type is compatible with the parameter variable's data type

conversation if the argument's data type is ranked lower than the parameter variable's data type

short s = 1;

displayValue(s); // converts short to int

Argument and Parameter Data

Type Compatibility

byte b = 2;

displayValue(b); // converts byte to int

to a lower-ranking data type

double d = 1.0;

displayValue(d); // Error! Can't convert

// double to int.

Parameter Variable Scope

which the parameter is declared

parameter variable by its name

public static void displayValue(int num)

Passing Multiple Arguments

variables

public static void showSum(double num1, double num2) {

sum = num1 + num2;

System.out.println(“The sum is ” + sum);

sum = num1 + num2;

System.out.println(“The sum is ” + sum);

Passing Multiple Arguments

have data type listed before its name

public static void showSum(double num1, num2) // Error!

public static void showSum(double num1, double num2)

Arguments Are Passed by Value

are passed by value

parameter variable

distinct from the arguments

method, it has no affect on the original argument

Arguments Are Passed by Value

Arguments Are Passed by Value

Arguments Are Passed by Value

changed in the changeMe method, the argument number is not mofified

the number variable

Passing String Object References to a Method

String objects as arguments

public static void showLength(String str) {

into the str parameter variable.

both name and str reference the same object

Passing String Object References to a Method

The name variable holds

the address of a String object.

The str parameter variable holds

the address of the same

Passing String Object References to a Method

method can change the contents of any String

object that has been passed to it as an argument

which means that they can not be changed

Passing String Object

References to a Method

Passing String Object

References to a Method

The str parameter variable holds

the address of the same

The str parameter variable holds

the address of a different

“Dickens”

A String object

Using the @param Tag in Documentation Comments

method, you can provide a description of each

parameter by using a @param tag

sum = num1 + num2;

System.out.println(“The sum is ” + sum);

}

3 More about Local Variables

in which they are declared

method's local variables

method, other method may have own local

variables with the same name

More about Local Variables

More about Local Variables

More about Local Variables

the program can only see one of them at a time

because they are in different methods

variable declared inside texas is visible

birds variable declared inside california is

visible

Local Variable Lifetime

method is executing This is known as the lifetime

of a local variable

its parameter variables are created in memory,

and when the method ends, the local variables

and parameter variables are destroyed

Initializing Local Variables with

Parameter Values

initialize a local variable

public static void showSum(double num1, double num2) {

double sum; // To hold the sum

sum = num1 + num2;

System.out.println(“The sum is ” + sum);

}

We can combine these Statements into one.

Initializing Local Variables with

Parameter Values

entire method in which it is declared, we can use parameter variables to initialize local variables

public static void showSum(double num1, double num2) {

double sum = num1 + num2;

System.out.println(“The sum is ” + sum);

}

Warning !

with a default value They must be given value

before they can be used

been given a value, a compiler error will result

variable x has not been given a value.

to the statement that called it.

known as value-returning methods.

int num;

num = Integer.parseInt(“700”);

Defining a Value-Returning

Method

we must decide what type of value the method will return

in the method header

a value, uses the key word void as its return type

in the method header

Defining a Value-Returning

Method

double, boolean, … or other valid data type in its header

public static int sum(int num1, int num2)

return Statement

value-returning method

returns value to the statement that called the

return Statement

variable, literal, or mathematical expression

public static int sum(int num1, int num2) {

return num1 + num2;

}

the same data type as the return type specified in the method header, or compatible with it

return expression, if necessary

Calling a Value-Returning Method

Calling a Value-Returning Method

Calling a Value-Returning Method

total = sum(value1, value2);

public static int sum(int num1, int num2) {

Using the @return Tag in Documentation Comments

the return value when writing the documentation comments

@return Description

description of the method

the end of the documentation comment (*/

symbol), or at the beginning of another tag

Returning a boolean Value

public static boolean isValid(int number) {

System.out.println(“The value is out

of range.”);

Retuning a Reference to a String

Object

reference to a non-primitive type, such as a

String object

Retuning a Reference to a String

Object

Retuning a Reference to a String

Object

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

public static String fullName(String first, String last) {

A String object

Problem Solving with Method

at a time by methods

smaller pieces is called functional decomposition.

one long method, small methods are written

These small methods can then be executed in the desired order to solve the problem

Problem Solving with Methods

sales amounts from a file, and then display the the total sales and average daily sales

Problem Solving with Methods

Problem Solving with Methods

Problem Solving with Methods

Problem Solving with Methods

Exercises

Exercises