CS1101: Programming Methodology Aaron Tan

2 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

3 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: is a prime.  Enter integer: is not a prime.

4 Mini-Programming Test Are you ready?

5 API Specification  Important: Bookmark this website! 

6 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.

7 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

8 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');

9 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.

10 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.

11 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

12 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.

13 Random Numbers (2/3) See API of Random class.

14 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 random number is 48 Next random number is 14 Next random number is 89 Next random number is 7 Next random number is 44

15 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.

16 DecimalFormat (2/2)  Example: To print a value in 2 decimal places. import java.util.*; import java.text.*; 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)); } Enter a value: value = Formatted = 90.73

17 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”:

18 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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