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CS1101: Programming Methodology
http://www.comp.nus.edu.sg/~cs1101x/
Aaron Tan
This is Week 5
 Last week:
 Repetition constructs
 ‘while’, ‘do … while’, ‘for’
 Testing and Debugging
 This week:
 Last week’s Exercise 5 (Prime number)
 A mini programming test! (argh!)
 Chapter 5: Using Pre-Built Methods
 Other classes: Random, DecimalFormat
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Last week’s exercise 5

Primality test is a classic programming problem.

Given a positive integer, determine whether it is a
prime number or not.

A prime number has two distinct factors (divisors): 1
and itself. Examples: 2, 3, 5, 7, 11, …

Write a program PrimeTest.java. Some sample runs
shown below:

Enter integer: 131
131 is a prime.

Enter integer: 713
713 is not a prime.
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Mini-Programming Test

Are you ready? 
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API Specification
 Important: Bookmark this website!
 http://java.sun.com/j2se/1.5.0/docs/api/
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Overloaded Methods (1/2)
 Refer to the abs() method in Math class.

public static int abs(int num)


Returns the absolute value of num.
public static double abs(double num)

Returns the absolute value of num.
 Note that there are two ‘versions’ of abs().
 This is called overloading.
 Just as we have overloaded operators (eg: ‘+’ that
serves as addition or concatenation), we also have
overloaded methods.
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Overloaded Methods (2/2)

Methods can share the same name as long as
 they
have a different number of parameters (Rule 1)
or
 their parameters are of different data types when
the number of parameters is the same (Rule 2)
public void myMethod(int x, int y) { ... }
public void myMethod(int x) { ... }
Rule 1
public void myMethod(double x) { ... }
public void myMethod(int x) { ... }
Rule 2
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Instance Methods and Class Methods (1/3)
 There are two types of method.
 Instance method
 Operates on a variable (instance) of a class
 Example: The length() method in String class
String str = "Hello";
int len = str.length();
 Class method
 Do not need to call it on a variable.
 Examples:
double ans = Math.abs(-4.5);
char ch = Character.toUpperCase('e');
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Instance Methods and Class Methods (2/3)
 How do we know a method is an instance

method or a class method?
Look at the API page
 If you see the ‘static’ modifier, it is a class method.
 If not, then it is an instance method.
abs() is a class method.
length() is an instance
method.
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Instance Methods and Class Methods (3/3)
 Some classes provide only class methods
 Example: Math
 Some classes provide only instance methods
 Example: Scanner
 Some classes provide a mix
 Example: String
 We will learn more about classes and
instances (objects) after the recess.
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Some Other Classes
 I would like to introduce 2 more classes:
 Random class
 DecimalFormat class
 Compare them with
 random() method in Math class
 printf() method
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Random Numbers (1/3)



We learnt about the random() method in the Math
class.
We will learn another way here, using the Random
class.
Notes:




Need to import java.util.*;
Random(): constructs a new random number generator
Random whose seed is based on the current time.
int nextInt(int n): returns the next pseudo-random, from the
interval [0, 1, … n-1], uniformly distributed value of a Random
object.
Refer to the API specification for the complete description of
class Random.
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Random Numbers (2/3)

See API of Random class.
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Random Numbers (3/3)

Example: To generate 5 random integers in the range [0, 100).
import java.util.*;
public class TestRandom {
public static void main(String[] args) {
Random num = new Random();
for (int i=0; i<5; i++)
System.out.println("Next random number is "
+ num.nextInt(100));
}
}
Next
Next
Next
Next
Next
random
random
random
random
random
number
number
number
number
number
is
is
is
is
is
48
14
89
7
44
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DecimalFormat (1/2)
 To print a decimal value in certain format.
 Notes
 Need to import java.text.*;
 Refer to the API specification for the complete
description of class DecimalFormat.
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DecimalFormat (2/2)
 Example: To print a value in 2 decimal places.
import java.util.*;
import java.text.*;
Enter a value: 90.7281
value = 90.7281
Formatted = 90.73
public class TestDecimalFormat {
public static void main(String[] args) {
Scanner stdIn = new Scanner(System.in);
DecimalFormat df = new DecimalFormat("0.00");
System.out.print("Enter a value: ");
double value = stdIn.nextDouble();
System.out.println("value = " + value);
System.out.println("formatted = " + df.format(value));
}
}
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Announcement/Reminder
 Lab #1
 Deadline: 10 September (Wednesday), 2359hr.
 CourseMarker
 How to check CM feedback after you submit your
program? I will give a demonstration.
 Programming Style
 I see programs with poor style! (Bad indentation,

poor use of white spaces, etc.)
Please refer to course website, “Resources”,
“Online”, “Java Style Guides”:
http://www.comp.nus.edu.sg/~cs1101x/2_resources/online.html
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This is Week 5
 Next week?
 Chapter 10: Arrays
 Only sections 10.1 to 10.6
 We will cover the other sections some other time.
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End of file
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