Chapter 2 Method in Java OOP

1Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671
Chapter 2 Methods

Motivations
A method is a construct for grouping statements together
to perform a function. Using a method, you can write the
code once for performing the function in a program and
reuse it by many other programs. For example, often you
need to find the maximum between two numbers.
Whenever you need this function, you would have to write
the following code:
int result;
if (num1 > num2)
result = num1;
else
result = num2;
If you define this function for finding a
maximum number between any two
numbers in a method, you don’t have to
repeatedly write the same code. You
need to define it just once and reuse it
by any other programs.

Objectives
To define methods, invoke methods, and pass arguments to a
method.
To develop reusable code that is modular, easy-to-read, easy-to-
debug, and easy-to-maintain.
To determine the scope of variables.

Defining Methods
A method is a collection of statements that are
grouped together to perform an operation.
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
Define a method Invoke a method
int z = max(x, y);
actual parameters
(arguments)
method
signature

Method Signature
Method signature is the combination of the method name and the
parameter list.
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
int z = max(x, y);
actual parameters
(arguments)
method
signature

Formal Parameters
The variables defined in the method header are known as
formal parameters.
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
int z = max(x, y);
actual parameters
(arguments)
method
signature

Actual Parameters
When a method is invoked, you pass a value to the parameter. This
value is referred to as actual parameter or argument.
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
int z = max(x, y);
actual parameters
(arguments)
method
signature

Return Value Type
A method may return a value. The returnValueType is the data
type of the value the method returns. If the method does not return
a value, the returnValueType is the keyword void. For example, the
returnValueType in the main method is void.
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
int z = max(x, y);
actual parameters
(arguments)
method
signature

Calling Methods
Listing 2.1 Testing the max method
This program demonstrates calling a method max
to return the largest of the int values
TestMaxTestMax

Calling Methods, cont.
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
pass the value of i
pass the value of j
animation

Trace Method Invocation
int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
i is now 5
animation

int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
j is now 2
animation

int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
invoke max(i, j)
animation

int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
invoke max(i, j)
Pass the value of i to num1
Pass the value of j to num2
animation

int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
declare variable result
animation

int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
(num1 > num2) is true since
num1 is 5 and num2 is 2
animation

int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
result is now 5
animation

int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
return result, which is 5
animation

int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
return max(i, j) and assign the
return value to k
animation

int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Execute the print statement
animation

CAUTION
A return statement is required for a value-returning method. The
method shown below in (a) is logically correct, but it has a
compilation error because the Java compiler thinks it possible that
this method does not return any value.
To fix this problem, delete if (n < 0) in (a), so that the compiler
will see a return statement to be reached regardless of how the if
statement is evaluated.
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else if (n < 0)
return –1;
}
(a)
Should be
(b)
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else
return –1;
}

Reuse Methods from Other Classes
NOTE: One of the benefits of methods is for reuse. The max
method can be invoked from any class besides TestMax. If
you create a new class Test, you can invoke the max method
using ClassName.methodName (e.g., TestMax.max).

void Method Example
This type of method does not return a value. The method
performs some actions.
TestVoidMethodTestVoidMethod

Scope of Local Variables
A local variable: a variable defined inside a
method.
Scope: the part of the program where the
variable can be referenced.
The scope of a local variable starts from its
declaration and continues to the end of the
block that contains the variable. A local
variable must be declared before it can be
used.

Scope of Local Variables, cont.
You can declare a local variable with the
same name multiple times in different non-
nesting blocks in a method, but you cannot
declare a local variable twice in nested
blocks.

A variable declared in the initial action part of a for loop
header has its scope in the entire loop. But a variable
declared inside a for loop body has its scope limited in the
loop body from its declaration and to the end of the block
that contains the variable.
public static void method1() {
.
.
for (int i = 1; i < 10; i++) {
.
.
int j;
.
.
.
}
}
The scope of j
The scope of i

int x = 1;
int y = 1;
for (int i = 1; i < 10; i++) {
x += i;
}
for (int i = 1; i < 10; i++) {
y += i;
}
}
It is fine to declare i in two
non-nesting blocks
int i = 1;
int sum = 0;
for (int i = 1; i < 10; i++) {
sum += i;
}
}
It is wrong to declare i in
two nesting blocks

// Fine with no errors
public static void correctMethod() {
int x = 1;
int y = 1;
// i is declared
for (int i = 1; i < 10; i++) {
x += i;
}
// i is declared again
for (int i = 1; i < 10; i++) {
y += i;
}
}

// With no errors
public static void incorrectMethod() {
int x = 1;
int y = 1;
for (int i = 1; i < 10; i++) {
int x = 0;
x += i;
}
}

Chapter 2 Method in Java OOP

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