CS 1063  Week 4:  An Introduction to Objects and Classes 

Modifying a Class

Objectives

• Construct and use Objects
• State the public interface of a class given the class
• Use the Dice class
• Use and modify the Greeter class

Assignments

• Reading Week 4
• Lab 2 due (Monday)
  
• Begin Lab 3
• Take Quiz 3 (Thursday, Friday and Saturday)

The Dice Class

public class Dice { // Private data private int mySides; private int myLastRoll; // construct and initialize the data public Dice (int sides) { mySides = sides; myLastRoll = 1; } // accessor methods - These methods will not change the values of the private data // return the number of sides of the Dice public int getSides() { return mySides; } // return the value of the last roll public int getLastRoll() { return myLastRoll; } public String toString() { return "sides: " + mySides + " Last Roll: " + myLastRoll; } // modifier method - This method will change the value(s) of the private data // Rolls the Dice and returns this value public int roll() { myLastRoll = (int)(Math.random() * mySides) + 1; return myLastRoll; } }

Activity 1  Write the public interface for the Dice class

Activity 2  Testing the Dice Class

• Folder Name:  Dice
• Create a new project named Dice (select the New option on the Project menu).
• Save the project in the Dice folder.
• Create a new class with name:  DiceMain

public class DiceMain {
  public static void main(String[] args) {

  }
}

• Save the class.
• Download the following file Dice.java and save in in your Dice folder.
• Open Dice.java in DrJava.
• Save the project.

Part 1.  Construct and initialize (instantiate) two Dice objects named d1 and d2 with 6 sides.

Part 2.  Print d1 and d2 using the toString method.

Part 3.  Roll d1 and d2 placing each value in int variables roll1 and roll2.  Print both values with appropriate labels.

int roll1 = d1.roll();
...

Part 4.  Find the sum of the two rolls and place this value in an int variable sum.  Print the sum with an appropriate label.

Part 5.  Roll each of the two Dice 3 more times and print the final value of each Dice  using the getLastRoll method.

Part 6.  Test the getsides method for each Dice object using the System.out.println method

Modifying the Greeter Class

public class Greeter {                         

   // Default Constructor   
   public Greeter() {
   }

   // Public accessor methods
   public String sayHello() {
      String message = "Hello World!";
      return message;
   }

   public String sayGoodbye() {
      String message = "Goodby World!";   
      return message;
   }

}

Activity 3 List the public interface for the Greeter class

Activity 4 Testing the Greeter Class

• Folder Name:  Greeter
• Create a new project named Greeter (select the New option on the Project menu).
• Save the project in the Greeter folder.
• Create a new class with name:  GreeterTest

public class GreeterTest {
  public static void main(String[] args) {

  }
}

• Save the class.
• Download the following file Greeter.java and save in in your Greeter folder.
• Open Greeter.java in DrJava.
• Save the project.

Part 1.  Construct and initialize (instantiate) a Greeter object named g1.

Part 2.  Ask g1 to sayHello and store the message in a String variable named message.

Part 3.  Print message.

Part 4.  Ask g1 to sayGoodbye and store the message in the variable message.  (What will happen to the first hello message?)

Part 5.  Print message.

Activity 5:  Modifying the Greeter Class

The Greeter class can only say Hello World or Goodbye World.  Suppose we would like our class to be able to say Hello Cathy or Hello Jeff.  In order to do this the Greeter class must store a name to place after Hello.  This means that we would have to add a private data field and modify the constructor so that it can initialize the data field.  The name should come from the client that is using our Greeter class.  Below are some very important rules that we will follow when adding data fields and methods.

1. Instance fields are generally declared private


2. An instance field (data) declaration consists of the following parts:
   • An access specifier (private)
   • The type of the variable such as String
   • The name of the variable such as myName


3. Constructors contain instructions to initialize objects.  The constructor name is always the same as the class name.


4. The new operator invokes the constructor.


5. A method definition contains the following parts:
• An access specifier such as public
• The return type of the method (such as String or void)
• The name of the method (such as sayHello)
• A list of the parameters of the method, enclosed in parentheses(the sayHello method has no parameters)
• The body of the method: a sequence of statements enclosed in braces

Part 1:  Modify the Greeter class  to say Hello <some name>

Add the following instance field to our Greeter class:

private String myName;

The constructor method for the Greeter Class will be:

public Greeter(String name) {
  myName = name;
}

How will the sayHello and sayGoodbye methods need to be modified?  Modify the assignment statement:

String message = ...

for the sayHello and the sayGoodbye methods.

It should return a string such as "I am sorry, Dave.  I am afraid I can't do that."

Activity 6:  Vocabulary