Transcript Chapter 4 Methods - Computer Science

Chapter 5 Methods
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
1
Objectives








To define methods, invoke methods, and pass arguments to a
method (§5.2-5.5).
To develop reusable code that is modular, easy-to-read, easy-todebug, and easy-to-maintain. (§5.6).
To use method overloading and understand ambiguous
overloading (§5.7).
To design and implement overloaded methods (§5.8).
To determine the scope of variables (§5.9).
To know how to use the methods in the Math class (§§5.105.11).
To learn the concept of method abstraction (§5.12).
To design and implement methods using stepwise refinement
(§5.12).
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
2
Calling a Methods
 In
order to use the method, you need to call or invoke
the methods;
 Two ways to call a methods depend upon if method:
– return value
int larger = max (3, 4); // calling max (3, 4) ad assigns the result to larger
System.out.println(max(3, 4);
– not returns a value

the method returns void, a call to method must be a
statement.
System.out.println(“Welcome to Java!”);
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
3
Method
A Java method is a collection of statements that are grouped
together to perform an operation. When you call the
System.out.println method, for example, the system actually
executes several statements in order to display a message on the
console.
Now you will learn how to create your own methods with or
without return values, invoke a method with or without parameters,
overload methods using the same names, and apply method
abstraction in the program design.
modifier returnValueType methodName(list of parameters)
{
// Method body;
}
A method definition consists of a method header and a method
body. The Java Tutorials – Defining Methods
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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4
// this program contains the main method and max method.
// The main method is just like any other method except that it is invoked by JVM.
public class TestMax {
/** Main method */
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j); // is calling method max
System.out.println("The maximum between " + i +
" and " + j + " is " + k);
}
/** when the max method is calling, variable i’s value 5 is passing to num1, and variable j’s
value 2 is passing to num in the max method
*/
/** Return the max between two numbers */
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
}
Listing 5.1 –TestMax
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
5
Methods headers
Here are all the parts of a method:
Modifiers: The modifier, which is optional, tells
the compiler how to call the method. This
defines the access type of the method.
Return Type: A method may return a value. The
returnValueType is the data type of the value the
method returns.
Method Name: This actual name of method,
named max, has two int parameters, num1 &
num2, the larger of which it returned by the
methods.. The method name and the parameter
list together constitute the method signature.
Parameters: A parameter is like a placeholder.
When a method is invoked, you pass a value to
the parameter.
Method Body: The method body contains a
collection of statements that define what the
method does.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
6
Object-Oriented Programming Concepts
Defining Methods
 http://docs.oracle.com/javase/tutorial/java/c
oncepts/index.html

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
7
Problem
Find the sum of integers from 1 to 10, from 20 to 30,
and from 35 to 45, respectively.
int sum = 0;
for (int i = 1; i <= 10; i++)
sum += i;
System.out.println("Sum from 1 to 10 is " + sum);
sum = 0;
for (int i = 20; i <= 30; i++)
sum += i;
System.out.println("Sum from 20 to 30 is " + sum);
sum = 0;
for (int i = 35; i <= 45; i++)
sum += i;
System.out.println("Sum from 35 to 45 is " + sum);
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
8
Problem
int sum = 0;
for (int i = 1; i <= 10; i++)
sum += i;
System.out.println("Sum from 1 to 10 is " + sum);
sum = 0;
for (int i = 20; i <= 30; i++)
sum += i;
System.out.println("Sum from 20 to 30 is " + sum);
sum = 0;
for (int i = 35; i <= 45; i++)
sum += i;
System.out.println("Sum from 35 to 45 is " + sum);
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
9
Solution
public class SumRang {
public static int sum(int i1, int i2) {
int sum = 0;
for (int i = i1; i <= i2; i++)
sum += i;
return sum;
}
public static void main(String[] args) {
System.out.println("Sum from 1 to 10 is " + sum(1, 10));
System.out.println("Sum from 20 to 30 is " + sum(20, 30));
System.out.println("Sum from 35 to 45 is " + sum(35, 45));
} // end of main
} //end of class SumRange
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
10
Defining Methods
A method is a collection of statements that are
grouped together to perform an operation.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
11
Method Signature
Method signature is the combination of the method name and the
parameter list.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
12
Formal Parameters
The variables defined in the method header are known as
formal parameters. Simply parameters are place holders.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
13
Actual Parameters
When a method is invoked, you pass a value to the parameter. This
value is referred to as actual parameter or argument.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
14
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.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
15
Calling a Method
To use a Method, you have to call or invoke it.
There are two ways to call a Methods, whether the method returns
a value or not.
 If the method return a value, a call to the method is usually
treated as value
int larger = max (3, 4);
Call max (3, 4) and assigns the result to methods to the variable
larger.
System.out.println(max(3, 4);
 If the method returns void, then a call of the method must be
statement
System.out.println(“Welcome to Java!”);
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
16
Calling Methods
Testing the max method. This program demonstrates calling a
method max to return the largest of the int valuesListing 5.1 Video Link (This program contain main method and the max method.)
public class TestMax {
/** Main method is just like any other method except that it is invoked by the JVM*/
public static void main(String[] args) { // main method is always the same
int i = 5;
int j = 2;
int k = max(i, j); //the main method invokes max (i, j), then variable I’s value 5 is passed to num1, & j’s value 2
System.out.println("The maximum between " + i +
" and " + j + " is " + k);
}
/** Return the max between two numbers */
public static int max(int num1, int num2) { // max method
int result;
TestMax
if (num1 > num2)
result = num1;
else
result = num2;
return result;
Run
// the max method is executed when the return statement in the max method is executed, the max method returns //the
control to its caller Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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rights reserved. 0132130807
}
animation
Calling Methods, cont.
When a program calls a method, program control is transferred to the called
method.
The statement in main may invoke other methods that are defined in the class.
In this example, the main method invokes max(i, j), which is define in the
same class with the main method.
pass the value of i
pass the value of j
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
18
animation
Trace Method Invocation
i is now 5
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
19
animation
Trace Method Invocation
j is now 2
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
20
animation
Trace Method Invocation
invoke max(i, j)
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
21
animation
Trace Method Invocation
invoke max(i, j)
Pass the value of i to num1
Pass the value of j to num2
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
22
animation
Trace Method Invocation
declare variable result
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
23
animation
Trace Method Invocation
(num1 > num2) is true since num1
is 5 and num2 is 2
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
24
animation
Trace Method Invocation
result is now 5
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
25
animation
Trace Method Invocation
return result, which is 5
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
26
animation
Trace Method Invocation
return max(i, j) and assign the
return value to k
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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animation
Trace Method Invocation
Execute the print statement
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
28
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.
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
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else
return –1;
}
(b)
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. //return -1; (does not keep all the variable)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
29
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).
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
30
Call Stacks
Each time a method is invoked (called), the system stores parameters
and variables in the memory known as a stack, which stores
elements in LIFO (Last-in-first-out).
Space required for
the max method
num2: 2
num1: 5
Space required for
the max method
result: 5
num2: 2
num1: 5
Space required for
the main method
k:
j:
2
i:
5
Space required for
the main method
k:
j:
2
i:
5
Space required for
the main method
k:
j:
2
i:
5
Space required for
the main method
k:
5
j:
2
i:
5
(a) The main
method is invoked.
(b) The max
method is invoked.
(c) The max method
is being executed.
(d) The max method is
finished and the return
value is sent to k.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Stack is empty
(e) The main
method is finished.
31
animation
Trace Call Stack
i is declared and initialized
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);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
i: 5
The main method
is invoked.
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
32
animation
Trace Call Stack
j is declared and initialized
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);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
j: 2
i: 5
The main method
is invoked.
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
33
animation
Trace Call Stack
Declare k
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);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
Space required for the
main method
k:
j: 2
i: 5
The main method
is invoked.
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
34
animation
Trace Call Stack
Invoke max(i, j)
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);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
Space required for the
main method
k:
j: 2
i: 5
The main method
is invoked.
return result;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
35
animation
Trace Call Stack
pass the values of i and j to num1
and num2
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);
num2: 2
num1: 5
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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animation
Trace Call Stack
pass the values of i and j to num1
and num2
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);
result:
num2: 2
num1: 5
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
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rights reserved. 0132130807
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animation
Trace Call Stack
(num1 > num2) is true
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);
result:
num2: 2
num1: 5
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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animation
Trace Call Stack
Assign num1 to result
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);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Space required for the
max method
result: 5
num2: 2
num1: 5
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
39
animation
Trace Call Stack
Return result and assign it to k
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);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Space required for the
max method
result: 5
num2: 2
num1: 5
Space required for the
main method
k:5
j: 2
i: 5
The max method is
invoked.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
40
animation
Trace Call Stack
Execute print statement
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);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
Space required for the
main method
k:5
j: 2
i: 5
The main method
is invoked.
return result;
}
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void Method & Example
void is a Java keyword.
Used at method declaration and definition to specify that
the method does not return any type, the method returns
void. It is not a type and there is no void references
The example listing 5.2 program is a type of method
does not return a value. The method performs some
actions.
Video Link on void method
TestVoidMethod
Run
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42
Passing Parameters
• The power of a method is its ability to work with parameters.
• You can use println to print any string;
• You can use max to find the maximum between any two int values.
• When calling a method, you need to provide arguments, this know
as parameter order association.
The following method prints a message n times.
public static void nPrintln(String message, int
n) {
for (int i = 0; i < n; i++)
System.out.println(message);
}
Suppose you invoke the method using
nPrintln(“Computer Science”, 15);
What is the output?
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Pass by Value
This program demonstrates passing values to the methods.
public class TestPassByValue { // the program creates a method for swapping two variables.
/** Main method */
public static void main(String[] args) {
// Declare and initialize variables
int num1 = 1;
int num2 = 2;
System.out.println("Before invoking the swap method, num1 is " +
num1 + " and num2 is " + num2);
// Invoke the swap method to attempt to swap two variables
swap(num1, num2);
System.out.println("After invoking the swap method, num1 is " +
num1 + " and num2 is " + num2);
}
/** Swap two variables- The swap method is invoked by passing two arguments. */
public static void swap(int n1, int n2) {
System.out.println("\tInside the swap method");
System.out.println("\t\tBefore swapping n1 is " + n1
Listing 5.5
+ " n2 is " + n2);
// Swap n1 with n2
int temp = n1;
n1 = n2;
n2 = temp;
System.out.println("\t\tAfter swapping n1 is " + n1
+ " n2 is " + n2);
}
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}
44
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Increment
Run
Pass by Value, cont.
Swap method is invoked, num1 is 1 and num2 is 2. After swap
method is invoked, num1 is still 1 and num2 is still 2. Their
values have not been swapped, but the value of the arguments
num1 and num2 are passed to n1 and n2 (use temp location) with
their own memory locations independet
The values of num1 and num2 are
passed to n1 and n2. Executing swap
does not affect num1 and num2.
Space required for the
swap method
temp:
n2: 2
n1: 1
Space required for the
main method
num2: 2
num1: 1
The main method
is invoked
Space required for the
main method
num2: 2
num1: 1
The swap method
is invoked
Space required for the
main method
num2: 2
num1: 1
The swap method
is finished
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Stack is empty
The main method
is finished
45
Modularizing Code
Methods can be used to reduce redundant coding
and enable code reuse. Methods can also be used to
modularize code and improve the quality of the
program. Listing 5.6 program.
Video Note Modularize
GreatestCommonDivisorMethod
Run
PrimeNumberMethod
Run
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46
Overloading Methods
Overloading the max Method
public static double max(double num1, double
num2) {
if (num1 > num2)
return num1;
else
return num2;
}
TestMethodOverloading
Run
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47
Ambiguous Invocation
Sometimes there may be two or more
possible matches for an invocation of a
method, but the compiler cannot determine
the most specific match. This is referred to
as ambiguous invocation. Ambiguous
invocation is a compilation error.
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Ambiguous Invocation
public class AmbiguousOverloading {
public static void main(String[] args) {
System.out.println(max(1, 2));
}
public static double max(int num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
public static double max(double num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
}
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49
Problem: Converting Decimals to
Hexadecimals
Write a method that converts a decimal integer to
a hexadecimal.
Decimal2HexConversion
Run
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50
Scope of Local Variables
Scope: the part of the program where the
variable can be referenced.
A local variable: a variable defined inside a
method.
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.
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51
Scope of Local Variables, cont.
You can declare a local variable with the
same name multiple times in different nonnesting blocks in a method, but you cannot
declare a local variable twice in nested
blocks.
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52
Scope of Local Variables, cont.
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.
The scope of i
The scope of j
public static void method1() {
.
.
for (int i = 1; i < 10; i++) {
.
.
int j;
.
.
.
}
}
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53
Scope of Local Variables, cont.
A local variable must be declared and assigned a value before
it can be used.
It is fine to declare i in two
non-nesting blocks
public static void method1() {
int x = 1;
int y = 1;
It is wrong to declare i in
two nesting blocks
public static void method2() {
int i = 1;
int sum = 0;
for (int i = 1; i < 10; i++) {
x += i;
}
for (int i = 1; i < 10; i++) {
y += i;
}
for (int i = 1; i < 10; i++) {
sum += i;
}
}
}
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Scope of Local Variables, cont.
// 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;
}
}
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55
Scope of Local Variables, cont.
// 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;
}
}
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56
Method Abstraction
You can think of the method body as a black box
that contains the detailed implementation for the
method.
Optional arguments
for Input
Optional return
value
Method Header
Black Box
Method body
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57
Benefits of Methods
• Write a method once and reuse it anywhere.
• Information hiding. Hide the implementation
from the user.
• Reduce complexity.
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58
The Math Class
 Class
constants:
– PI
–E
 Class
methods:
– Trigonometric Methods
– Exponent Methods
– Rounding Methods
– min, max, abs, and random Methods
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59
Trigonometric Methods

sin(double a)

cos(double a)

tan(double a)

acos(double a)

asin(double a)

atan(double a)
Radians
Examples:
Math.sin(0) returns 0.0
Math.sin(Math.PI / 6)
returns 0.5
Math.sin(Math.PI / 2)
returns 1.0
Math.cos(0) returns 1.0
Math.cos(Math.PI / 6)
returns 0.866
Math.cos(Math.PI / 2)
returns 0
toRadians(90)
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Exponent Methods

exp(double a)
Returns e raised to the power of a.
Examples:

log(double a)
Returns the natural logarithm of a.

log10(double a)
Math.exp(1) returns 2.71
Math.log(2.71) returns 1.0
Math.pow(2, 3) returns 8.0
Math.pow(3, 2) returns 9.0
Math.pow(3.5, 2.5) returns
22.91765
Math.sqrt(4) returns 2.0
Math.sqrt(10.5) returns 3.24
Returns the 10-based logarithm of
a.

pow(double a, double b)
Returns a raised to the power of b.

sqrt(double a)
Returns the square root of a.
Web Link: Class Math
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Rounding Methods

double ceil(double x)
x rounded up to its nearest integer. This integer is returned as a double
value.

double floor(double x)
x is rounded down to its nearest integer. This integer is returned as a
double value.

double rint(double x)
x is rounded to its nearest integer. If x is equally close to two integers,
the even one is returned as a double.

int round(float x)
Return (int)Math.floor(x+0.5).

long round(double x)
Return (long)Math.floor(x+0.5).
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Rounding Methods Examples
Math.ceil(2.1) returns 3.0
Math.ceil(2.0) returns 2.0
Math.ceil(-2.0) returns –2.0
Math.ceil(-2.1) returns -2.0
Math.floor(2.1) returns 2.0
Math.floor(2.0) returns 2.0
Math.floor(-2.0) returns –2.0
Math.floor(-2.1) returns -3.0
Math.rint(2.1) returns 2.0
Math.rint(2.0) returns 2.0
Math.rint(-2.0) returns –2.0
Math.rint(-2.1) returns -2.0
Math.rint(2.5) returns 2.0
Math.rint(-2.5) returns -2.0
Math.round(2.6f) returns 3
Math.round(2.0) returns 2
Math.round(-2.0f) returns -2
Math.round(-2.6)
returns -3
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min, max, and abs

max(a, b)and min(a, b)
Returns the maximum or
minimum of two parameters.

abs(a)
Returns the absolute value of the
parameter.

random()
Returns a random double value
in the range [0.0 , 1.0).
Examples:
Math.max(2, 3) returns 3
Math.max(2.5, 3) returns
3.0
Math.min(2.5, 3.6)
returns 2.5
Math.abs(-2) returns 2
Math.abs(-2.1) returns
2.1
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64
Math Class
Math Class: Method Summary
Java Math Engine: Build-In Functions
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The random Method
Generates a random double value greater than or equal to 0.0 and less
than 1.0 (0 <= Math.random() < 1.0).
Examples:
(int)(Math.random() * 10)
Returns a random integer
between 0 and 9.
50 + (int)(Math.random() * 50)
Returns a random integer
between 50 and 99.
In general,
a + Math.random() * b
Returns a random number between
a and a + b, excluding a + b.
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66
Case Study: Generating Random
Characters
Computer programs process numerical data and characters.
You have seen many examples that involve numerical
data. It is also important to understand characters and how
to process them.
As introduced in Section 2.9, each character has a unique
Unicode between 0 and FFFF in hexadecimal (65535 in
decimal). To generate a random character is to generate a
random integer between 0 and 65535 using the following
expression: (note that since 0 <= Math.random() < 1.0, you
have to add 1 to 65535.)
(int)(Math.random() * (65535 + 1))
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Case Study: Generating Random
Characters, cont.
Now let us consider how to generate a random
lowercase letter. The Unicode for lowercase letters
are consecutive integers starting from the Unicode
for 'a', then for 'b', 'c', ..., and 'z'. The Unicode for 'a'
is
(int)'a'
So, a random integer between (int)'a' and (int)'z' is
(int)((int)'a' + Math.random() * ((int)'z' - (int)'a' + 1)
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Case Study: Generating Random
Characters, cont.
Now let us consider how to generate a random
lowercase letter. The Unicode for lowercase letters
are consecutive integers starting from the Unicode
for 'a', then for 'b', 'c', ..., and 'z'. The Unicode for 'a'
is
(int)'a'
So, a random integer between (int)'a' and (int)'z' is
(int)((int)'a' + Math.random() * ((int)'z' - (int)'a' + 1)
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Case Study: Generating Random
Characters, cont.
As discussed in Chapter 2., all numeric operators
can be applied to the char operands. The char
operand is cast into a number if the other operand
is a number or a character. So, the preceding
expression can be simplified as follows:
'a' + Math.random() * ('z' - 'a' + 1)
So a random lowercase letter is
(char)('a' + Math.random() * ('z' - 'a' + 1))
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Case Study: Generating Random
Characters, cont.
To generalize the foregoing discussion, a random character
between any two characters ch1 and ch2 with ch1 < ch2
can be generated as follows:
(char)(ch1 + Math.random() * (ch2 – ch1 + 1))
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The RandomCharacter Class
// RandomCharacter.java: Generate random characters
public class RandomCharacter {
/** Generate a random character between ch1 and ch2 */
public static char getRandomCharacter(char ch1, char ch2) {
return (char)(ch1 + Math.random() * (ch2 - ch1 + 1));
}
/** Generate a random lowercase letter */
public static char getRandomLowerCaseLetter() {
return getRandomCharacter('a', 'z');
}
/** Generate a random uppercase letter */
public static char getRandomUpperCaseLetter() {
return getRandomCharacter('A', 'Z');
}
/** Generate a random digit character */
public static char getRandomDigitCharacter() {
return getRandomCharacter('0', '9');
}
RandomCharacter
TestRandomCharacter
/** Generate a random character */
public static char getRandomCharacter() {
return getRandomCharacter('\u0000', '\uFFFF');
}
Run
}
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72
Stepwise Refinement (Optional)
The concept of method abstraction can be applied
to the process of developing programs. When
writing a large program, you can use the “divide
and conquer” strategy, also known as stepwise
refinement, to decompose it into subproblems. The
subproblems can be further decomposed into
smaller, more manageable problems.
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73
PrintCalender Case Study
Let us use the PrintCalendar example to demonstrate the
stepwise refinement approach.
PrintCalendar
Run
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74
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
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Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
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76
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
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77
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
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78
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
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79
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
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80
Implementation: Top-Down
Top-down approach is to implement one method in the
structure chart at a time from the top to the bottom. Stubs
can be used for the methods waiting to be implemented. A
stub is a simple but incomplete version of a method. The
use of stubs enables you to test invoking the method from
a caller. Implement the main method first and then use a
stub for the printMonth method. For example, let
printMonth display the year and the month in the stub.
Thus, your program may begin like this:
A Skeleton for printCalendar
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Implementation: Bottom-Up
Bottom-up approach is to implement one method in the
structure chart at a time from the bottom to the top. For
each method implemented, write a test program to test it.
Both top-down and bottom-up methods are fine. Both
approaches implement the methods incrementally and
help to isolate programming errors and makes debugging
easy. Sometimes, they can be used together.
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