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11.2. Objects and Classes

In object-oriented programming the programmer writes a class which defines what all objects of the class know and can do. You can think of the class as like a cookie cutter or factory that produces objects. All objects created by the same class have the same fields and methods. A field is something the object knows about itself and a method is a thing the object can do.

A class also has constructors which initialize the fields when the object is created. A class can also have a main method which can be used to test the class.

11.2.1. Person Class

What should we want to know about a person? What we want to know depends on what problem we are trying to solve. In one situation we might want to know the person’s name and phone number and email. We need ways to get and set the data (fields) so we create getters and setters.

public class Person { // fields private String name; private String email; private String phoneNumber; // constructor public Person(String theName) { this.name = theName; } // methods - getters public String getName() { return this.name;} public String getEmail() { return this.email;} public String getPhoneNumber() { return this.phoneNumber;} // methods - setters public void setName(String theName) { this.name = theName;} public void setEmail(String theEmail) {this.email = theEmail;} public void setPhoneNumber(String thePhoneNumber) { this.phoneNumber = thePhoneNumber;} public String toString() { return this.name + " " + this.email + " " + this.phoneNumber; } // main method for testing public static void main(String[] args) { Person p1 = new Person("Sana"); System.out.println(p1); Person p2 = new Person("Jean"); p2.setEmail("jean@gmail.com"); p2.setPhoneNumber("404 899-9955"); System.out.println(p2); } }

Modify the code above to add more constructors. Also modify the main method to test the new constructors.

11.2.2. Die Class

What if you wanted to represent a die which has 6 sides and you can roll it? You might also want to keep track of the last value rolled. Does the following class have everything it needs?

Figure 1: One six sided die from a pair of dice

public class Die { private int lastValue; public int roll() { lastValue = (int) (Math.random() * 6) + 1; return lastValue; } public static void main(String[] args) { Die d = new Die(); for (int i = 0; i < 10; i++) { System.out.println(d.roll()); } } }

Can you modify the Die class to keep a record of all the values this dice has rolled? How would you do that?

11.2.3. Coin Class

What if you just wanted to simulate flipping a coin? What would you need the objects of the class to know and do? You would want to flip the coin (set it randomly to heads or tails) and ask if the value is heads or tails. See the class below for one way to do this. Notice the use of a constant for HEADS. Any field that is declared to be static can’t be changed and so is a constant.

public class Coin { // constant to represent heads private static int HEADS = 1; // current value of the coin private int value = 0; // method to randomly set the value of the coin to heads or tails public void flip() { if (Math.random() < 0.5) { value = 0; } else { value = 1; } } // return true if the value is heads or false otherwise public boolean isHeads() { return value == HEADS; } // convert the value to a string public String toString() { if (value == HEADS) return "Heads"; else return "Tails"; } // test the class public static void main(String[] args) { Coin myCoin = new Coin(); for (int i = 0; i < 10; i++) { myCoin.flip(); System.out.println(myCoin); System.out.println(myCoin.isHeads()); } } }

Modify the class to add a isTails method that returns true when the value is not heads.

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