Download Chapter 4 Methods

Chapter 6
Methods
1
Objectives






To declare methods, invoke methods, and pass arguments to
a method.
To use method overloading and know ambiguous
overloading.
To determine the scope of local variables.
To learn the concept of method abstraction.
To know how to use the methods in the Math class.
To design and implement methods using stepwise
refinement .
2
Introducing Methods
A method is a collection of statements that are grouped together to
perform an operation.
Example: when you call the System.out.println method, the system
actually executes several statements in order to display a message
on the console.
3
Introducing Methods
In general, a method has the following syntax:
Modifier returnValueType methodName(list of parameters) {
// Method body;
}
Example: a method to find which of two integers is bigger:
Define a method
modifier
method
header
return value type
Invoke a method
method name
formal parameters
public static int max(int num1, int num2) {
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
parameter list
int z = max(x, y);
actual parameters
(arguments)
return value
return result;
}
4
Introducing Methods, cont.
• Method signature is the combination of the method
name and the parameter list.
• The variables defined in the method header are known
as formal parameters.
• When a method is invoked, you pass a value to the
parameter. This value is referred to as actual
parameter or argument.
• The modifier which is optional, tells the compiler how
to call the method. The static modifier is used for all
the methods in this chapter.
5
Introducing Methods, cont.
• 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, as well as in System.exit,
System.out.println, and
JOptionPane.showMessageDialog. The method
that returns a value is called a nonvoid method,
and the method that does not return a value is
called a void method.
6
Calling Methods
Testing the max method
This program demonstrates calling a method max to return the largest of the int values:
public class TestMax {
/** Main method */
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);
} // end main 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;
} //end method max
} // end class TestMax
7
animation
Calling Methods, cont.
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;
}
When the return statement in the max method is executed, the max
method returns the control to its caller (in this case the caller is the
main method).
8
animation
Trace Method Invocation
i is now 5
9
animation
Trace Method Invocation
j is now 2
10
animation
Trace Method Invocation
invoke max(i, j)
11
animation
Trace Method Invocation
invoke max(i, j)
Pass the value of i to num1
Pass the value of j to num2
12
animation
Trace Method Invocation
declare variable result
13
animation
Trace Method Invocation
(num1 > num2) is true since num1
is 5 and num2 is 2
14
animation
Trace Method Invocation
result is now 5
15
animation
Trace Method Invocation
return result, which is 5
16
animation
Trace Method Invocation
return max(i, j) and assign the
return value to k
17
animation
Trace Method Invocation
Execute the print statement
18
CAUTION
A return statement is required for a nonvoid method. The following method 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;
}
To fix this problem, delete if (n<0) in the code, so that the compiler will see a
return statement to be reached regardless of how the if statement is evaluated as
follows:
public static int sign(int n) {
if (n > 0) return 1;
else if (n == 0) return 0;
else return –1;
}
19
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).
public class Test {
/** Main method */
public static void main (String[] args) {
int i = 5;
int j = 2;
System.out.println("The maximum between " + i +
" and " + j + " is " + TestMax.max(i, j));
}
}
20
Call Stacks
Each time a method is invoked, the system stores parameters and variables in an area of
memory, known as a stack, which stores elements in last-in first-out fashion. When a
method calls another method the caller’s stack space is kept intact, and new space is
created to handle the new method call. When a method finishes its work, and returns to
Its caller, its associated space is released.
Understanding call stack helps you to understand how methods are invoked:
Space required for the
max method
result: 5
num2: 2
num1: 5
Space required for the
main method
k:
j: 2
i: 5
The main method
is invoked.
Space required for the
main method
k:
j: 2
i: 5
Space required for the
main method
k: 5
j: 2
i: 5
The max method is
invoked.
The max method is
finished and the return
value is sent to k.
Stack is empty
The main method
is finished.
21
animation
Trace Call Stack
i is declared and initialized
i: 5
The main method
is invoked.
22
animation
Trace Call Stack
j is declared and initialized
j: 2
i: 5
The main method
is invoked.
23
animation
Trace Call Stack
Declare k
Space required for the
main method
k:
j: 2
i: 5
The main method
is invoked.
24
animation
Trace Call Stack
Invoke max(i, j)
Space required for the
main method
k:
j: 2
i: 5
The main method
is invoked.
25
animation
Trace Call Stack
pass the values of i and j to num1
and num2
num2: 2
num1: 5
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
26
animation
Trace Call Stack
pass the values of i and j to num1
and num2
result:
num2: 2
num1: 5
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
27
animation
Trace Call Stack
(num1 > num2) is true
result:
num2: 2
num1: 5
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
28
animation
Trace Call Stack
Assign num1 to 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.
29
animation
Trace Call Stack
Return result and assign it to k
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.
30
animation
Trace Call Stack
Execute print statement
Space required for the
main method
k:5
j: 2
i: 5
The main method
is invoked.
31
JBuilder
Optional
Call Stack in JBuilder Debugger
You can see the chain of
method invocations in the
stack view of the JBuilder
debugger.
Stack view
Invoke max
Invoke main
32
void Method Example
public class TestVoidMethod {
public static void main(String[] args) {
printGrade(78.5);
}
public static void printGrade(double score) {
if (score < 0 || score > 100) {
System.out.println("Invalid score");
//return statement to terminate the function
//when the score is invalid
return;
}
Program Output:
if (score >= 90.0) {
System.out.println('A');
}
else if (score >= 80.0) {
System.out.println('B');
}
else if (score >= 70.0) {
System.out.println('C');
}
else if (score >= 60.0) {
System.out.println('D');
}
else {
System.out.println('F');
}
}
}
33
void Method Example using GUI
import javax.swing.JOptionPane;
public class TestVoidMethodGui {
public static void main(String[] args) {
String s = JOptionPane.showInputDialog("Enter score: ");
double d = Double.parseDouble(s);
Program Output:
printGrade(d);
}
public static void printGrade(double score) {
if (score < 0 || score > 100) {
String output = "Student score is invalid";
JOptionPane.showMessageDialog(null, output);
return;
}
if (score >= 90.0) {
String output = "Student score is " +score +" and student grade is A";
JOptionPane.showMessageDialog(null, output);
}
else if (score >= 80.0) {
String output = "Student score is " +score +" and student grade is B";
JOptionPane.showMessageDialog(null, output);
}
else if (score >= 70.0) {
String output = "Student score is " +score +" and student grade is C";
JOptionPane.showMessageDialog(null, output);
}
else if (score >= 60.0) {
String output = "Student score is " +score +" and student grade is D";
JOptionPane.showMessageDialog(null, output);
}
else {
String output = "Student score is " +score +" and student grade is F";
JOptionPane.showMessageDialog(null, output);
}
}
}
34
Passing Parameters
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(“Welcome to Java”, 5);
What is the output?
Suppose you invoke the method using
nPrintln(“Computer Science”, 15);
What is the output?
35
Passing Parameters
Suppose you invoke the method using
nPrintln(“Welcome to Java”, 5);
What is the output?
public class Invoke {
public static void main (String args[]) {
nPrintln("Welcome to Java", 5);
} //end main
public static void nPrintln(String message, int n) {
for (int i = 0; i < n; i++)
System.out.println(message);
} // end nPrintln
} // end Invoke
36
Passing Parameters
Suppose you invoke the method using
nPrintln(“Computer Science”, 15);
What is the output?
public class Invoke {
public static void main (String args[]) {
nPrintln("Computer Science", 15);
} //end main
public static void nPrintln(String message, int n) {
for (int i = 0; i < n; i++)
System.out.println(message);
} // end nPrintln
} // end Invoke
37
Pass by Value
Testing Pass by value. This program demonstrates passing values to the
methods:
public class TestPassByValue {
/** 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);
} //end main
Program Output:
/** Swap two variables */
public static void swap(int n1, int n2) {
System.out.println("Inside the swap method");
System.out.println("Before swapping n1 is " + n1
+ " n2 is " + n2);
// Swap n1 with n2
int temp = n1;
n1 = n2;
n2 = temp;
System.out.println("After swapping n1 is " + n1
+ " n2 is " + n2);
} //end swap
} //end TestPassByValue
38
Pass by Value, cont.
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
Stack is empty
The main method
is finished
39
Exercise
a method named footToMeter() that
converts feet to meters. The method receives a
double variable with actual parameter 68.1 and
formal parameter named foot and a return value in
meter. (meter = 0.305 x foot)
 Write a method named divisibleBy3() that
return true or false. The method receives an
integer named number and evaluate whether
number is divisible by 3 or not.
 Write
40
Solution

Write a method named footToMeter() that converts feet to meters. The method
receives a double variable with actual parameter 68.1 and formal parameter named
foot and a return value in meter. (meter = 0.305 x foot)
public class ConvertFootToMeter {
public static void main (String args[]) {
System.out.println(footToMeter(68.1));
} //end main
public static double footToMeter(double foot) {
double meter;
Program Output:
meter = 0.305*foot;
return meter;
} // end footConvert
} // end ConvertFootToMeter
41
Solution
Write a method named divisibleBy3() that return true or false. The method receives an integer named number and evaluate whether number
is divisible by 3 or not.
import java.util.Scanner;
public class DivisibleThree {
public static void main (String args[]) {
/* Create scanner object */
Scanner scanner = new Scanner(System.in);
/* Read a number */
System.out.print("Enter an integer number: ");
int num = scanner.nextInt();
System.out.println();
/* Print the result */
System.out.println(num + " can be divided by 3? " +divisibleBy3(num));
System.out.println();
Program Output:
} //end main method
/* divisibleBy3 method */
public static boolean divisibleBy3(int number) {
if (number%3==0)
return true;
else
return false;
} // end divisibleBy3 method
} // end DivisibleThree class
42
Overloading Methods
The max method that was used earlier works only with the int data type. But what if
you need to find which of two floating-point numbers has the maximum value? The
solution is to create another method with the same name but different parameters as
follows:
public static double max(double num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
If you call max with int parameters, the max method that expects int parameters will be
invoked; if you call max with double parameters, the max method that expects
double parameters will be invoked. This is referred to as method overloading; that is
two methods have the same name but different parameter lists within one class. The
Java compiler determines which method is used based on the method signature.
43
Overloading Methods
This program creates three methods. The first finds the maximum integer, the second finds the maximum double, and the third finds the maximum among three double
values. All three methods are named max.
public class TestMethodOverloading {
/** Main method */
public static void main(String[] args) {
// Invoke the max method with int parameters
System.out.println("The maximum between 3 and 4 is "
+ max(3, 4));
// Invoke the max method with the double parameters
System.out.println("The maximum between 3.0 and 5.4 is "
+ max(3.0, 5.4));
// Invoke the max method with three double parameters
System.out.println("The maximum between 3.0, 5.4, and 10.14 is "
+ max(3.0, 5.4, 10.14));
} // end main
/** Return the max between two int values */
public static int max(int num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
} // end max with 2 int parameters
/** Find the max between two double values */
public static double max(double num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
} // end max with 2 double parameters
/** Return the max among three double values */
public static double max(double num1, double num2, double num3) {
return max(max(num1, num2), num3);
} // end max with 3 double parameters
} // end class TestMethodOverloading
44
Overloading Methods
If caller method is max(2,2.5), which method will be invoked?
The max method for finding the maximum of two double values will
be invoked. The argument 2 is automatically converted into a double
value and passed to this method.
BUT, Why max(double,double) is not invoked for the call max(3,4)?
Both max(double,double) and max(int,int) are possible matches for
max(3,4). The Java compiler finds the most specific method for a
method invocation. Since max(int,int) is more specific than
max(double,double), max(int,int) is used to invoke max(3,4).
45
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.
46
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;
}
}
Both max(int,double) and max(double,int) are possible candidates to match max(1,2). Since neither of them
is more specific than the other, the invocation is ambiguous, resulting in a compilation error.
47
Case Study: Computing Taxes
with Methods
“Computing Taxes,” uses if statements to check the filing
status and computes the tax based on the filing status.
Simplify the problem using methods. Each filing status has
six brackets.
The code for computing taxes is nearly same for each filing
status except that each filing status has different bracket
ranges. For example, the single filer status has six brackets
[0, 6000], (6000, 27950], (27950, 67700], (67700,
141250], (141250, 307050], (307050, ), and the married
file jointly status has six brackets [0, 12000], (12000,
46700], (46700, 112850], (112850, 171950], (171950,
307050], (307050, ).
48
Case Study cont.
The first bracket of each filing status is taxed at 10%, the second
15%, the third 27%, the fourth 30%, the fifth 35%, and the sixth
38.6%. So you can write a method with the brackets as arguments to
compute the tax for the filing status. The signature of the method is:
public static double computeTax(double income,
int r1, int r2, int r3, int r4, int r5)
400000
[0, 6000], (6000, 27950], (27950, 67700], (67700, 141250], (141250, 307050], (307050, )
For example, you can invoke computeTax(400000, 6000, 27950,
67700, 141250, 307050) to compute the tax for single filers with
$400,000 of taxable income:
ComputeTaxWithMethod
Run
49
Case Study cont.
else if (income <= r5)
tax = r1 * 0.10 + (r2 - r1) * 0.15 + (r3 - r2) * 0.27 +
(r4 - r3) * 0.30 + (income - r4) * 0.35;
else
tax = r1 * 0.10 + (r2 - r1) * 0.15 + (r3 - r2) * 0.27 +
(r4 - r3) * 0.30 + (r5 - r4) * 0.35 + (income - r5) * 0.386;
return tax;
import javax.swing.JOptionPane;
public class ComputeTaxWithMethod {
public static void main(String[] args) {
// Prompt the user to enter filing status
String statusString = JOptionPane.showInputDialog(
"Enter the filing status:");
int status = Integer.parseInt(statusString);
}
public static double computeTax(int status, double income) {
switch (status) {
case 0: return
computeTax(income, 6000, 27950, 67700, 141250, 307050);
case 1: return
computeTax(income, 12000, 46700, 112850, 171950, 307050);
case 2: return
computeTax(income, 6000, 23350, 56425, 85975, 153525);
case 3: return
computeTax(income, 10000, 37450, 96700, 156600, 307050);
default: return 0;
}
}
// Prompt the user to enter taxable income
String incomeString = JOptionPane.showInputDialog(
"Enter the taxable income:");
double income = Double.parseDouble(incomeString);
// Display the result
JOptionPane.showMessageDialog(null, "Tax is " +
(int)(computeTax(status, income) * 100) / 100.0);
}
public static double computeTax(double income,
int r1, int r2, int r3, int r4, int r5) {
double tax = 0;
}
if (income <= r1)
tax = income * 0.10;
else if (income <= r2)
tax = r1 * 0.10 + (income - r1) * 0.15;
else if (income <= r3)
tax = r1 * 0.10 + (r2 - r1) * 0.15 + (income - r2) * 0.27;
else if (income <= r4)
tax = r1 * 0.10 + (r2 - r1) * 0.15 +
(r3 - r2) * 0.27 + (income - r3) * 0.30;
50
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.
51
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 as shown below:
The scope of i
The scope of j
public static void method1() {
.
.
for (int i = 1; i < 10; i++) {
.
.
int j;
.
.
.
}
}
52
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.
53
Scope of Local Variables, cont.
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;
}
}
}
54
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;
}
}
55
Scope of Local Variables, cont.
// With errors
public static void incorrectMethod() {
int x = 1;
int y = 1;
for (int i = 1; i < 10; i++) {
int x = 0;
x += i;
}
}
56
Scope of Local Variables, cont.
CAUTION! DO NOT declare a variable inside a block and then attempt
to use it outside the block, as follows:
// With errors
for (int i = 0; i < 10; i++) {
}
System.out.println(i)
}
The last statement will cause a syntax error because variable i is not
defined outside of the for loop
57
The Math Class
The Math class contains the methods needed to
perform basic mathematical functions.
 Class
constants:
– PI
–E
 Class
methods:
– Trigonometric Methods
– Exponent Methods
– Rounding Methods
– min, max, abs, and random Methods
58
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)
59
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.
60
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).
61
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
0.0 <= Math.random() < 1.0
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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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JBuilder
Optional
View java.lang.Math Documentation
You can view the complete
documentation for the Math
class online from
http://java.sun.com/j2se/1.5.
2/docs/api/index.htm.
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JBuilder
Optional
View java.lang.Math from JBuilder
You can view the complete
documentation for the Math
class in JBuilder by
choosing Search, Find
Classes to display the Find
Classes dialog box. Type
java.lang.Math in the Class
name field to display the
documentation of the Math
class in the content pane.
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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.
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.
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');
}
/** Generate a random character */
public static char getRandomCharacter() {
return getRandomCharacter('\u0000', '\uFFFF');
}
}
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Test Program for the RandomCharacter Class
public class TestRandomCharacter {
/** Main method */
public static void main(String args[]) {
final int NUMBER_OF_CHARS = 175;
final int CHARS_PER_LINE = 25;
// Print random characters between '!' and '~', 25 chars per line
for (int i = 0; i < NUMBER_OF_CHARS; i++) {
char ch = RandomCharacter.getRandomLowerCaseLetter();
if ((i + 1) % CHARS_PER_LINE == 0)
System.out.println(ch);
else
System.out.print(ch);
}
}
}
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Test Program for The RandomCharacter Class:
Output
74
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 Signature
Black Box
Method body
75
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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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.
77
PrintCalender Case Study
Let us use the PrintCalendar example to demonstrate the
stepwise refinement approach.
78
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
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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.
Refer to page chapter 5, page 176.
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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.
Refer to PrintCalendar class on page 177-179.
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