Week 3

Introduction to Computer Science and Object-Oriented Programming

COMP 111

George Basham

Week 3 Topics

3.1.1 Constructing Objects

3.1.2 Accessor and Mutator Methods

3.1.3 The API Documentation

3.1.4 Implementing a Test Program

3.1.5 Object References

3.1.1 Constructing Objects

• We will learn how to construct objects that allow us to go beyond our earlier String object and System.out object creation examples

• Let’s look at the Java class library Rectangle class

• A Rectangle object isn’t a rectangular shape, it is an object that contains a set of numbers that describes a rectangle

3.1.1 Constructing Objects Cont.

• Each rectangle is described by the x and y coordinates of its top-left corner and its width and height

• Rectangle box = new Rectangle(5,10,20,30)

x = 5 y = 10 width = 20height = 30

3.1.1 Constructing Objects Cont.

• The new operator makes a Rectangle object

• It uses parameters in the call to one of the constructors to initialize the data of the object

• It returns the object’s address in memory

• In our example, the address (object reference) is stored in the object variable identified by box, of type Rectangle

3.1.2 Accessor and Mutator Methods

• A method of a class that accesses an object and returns some information about it without changing the object is called an accessor method

• A method whose purpose is to modify the state of an object (its data) is called a mutator method

3.1.2 Accessor and Mutator Methods Cont.

• The length method of the String class is an accessor method. It does not modify the state of the string object, it just counts the number of stored characters

• The Rectangle class has a number of accessor methods, getX, getY, getWidth and getHeight

• double width = box.getWidth();

3.1.2 Accessor and Mutator Methods Cont.

• The translate method of the Rectangle class is a mutator method

• Translate moves a rectangle by a certain distance in the x and y directions

• box.translate(15, 25);• The above method call moves the

rectangle by 15 units in the x direction and 25 units in the y direction

3.1.2 Accessor and Mutator Methods Cont.

Illustration:

Effect of translate

method

Using the translate method to move a rectangle

3.1.3 The API Documentation

• The classes and methods of the Java library are listed in the API documentation (application programming interface)

• The API documentation can be found on the web at http://java.sun.com

• The API for each class starts out with a purpose section, then summary tables for the constructors and methods

• Click on a method link to get detailed info

3.1.4 Implementing a Test Program

1. Provide a new class

2. Supply a main method

3. Inside the main method, construct one or more objects

4. Apply methods to the objects

5. Display the results of the method calls

import java.awt.Rectangle;

public class MoveTester{ public static void main(String[] args) { Rectangle box = new Rectangle(5, 10, 20, 30); // Move the rectangle box.translate(15, 25); // Print information about the moved rectangle System.out.println("After moving, the top-left corner is:"); System.out.println(box.getX()); System.out.println(box.getY()); }}

Output: text message then 20 followed by 35

/** * Test the translate method. */ protected void testTranslate() { Rectangle box = new Rectangle(5, 10, 20, 30); box.translate(15, 25); assertEquals(20.0, box.getX(), .0001); assertEquals(35.0, box.getY(), .0001); }

Example: JUnit test method.

Upon doing Run Tests a green checkmark indicates successful test, a red X indicates that test failed.

3.1.5 Object References

• In Java a variable whose type is a class does not hold an object, it holds the memory location of an object.

• Object reference is the technical term to denote the memory location of an object.

• Rectangle box = new Rectangle(5,10,20,30)

• The variable box refers to the object that the new operator constructed.

3.1.5 Object References Cont.

• The new operator returned a reference to the new object, and that reference is stored in the box variable.

• Again, the box variable does not contain the object, it refers to the object

• Remember that number variables actually store numbers, not a reference to the number

3.1.5 Object References Cont.

• A number variable and an object variable behave differently when you make a copy of the variable:

// number variable 1:

int luckyNbr = 13;

// number variable 2 has value 13:

int luckyNbr2 = luckyNbr;

// number variable 2 now has value 12, but

// number variable 1 still has value 13:

luckyNbr2 = 12;

3.1.5 Object References Cont.

• Note object variable behavior:

// object variable 1:

Rectangle box = new Rectangle(5, 10, 20, 30);

// object variable 2 will have same value

// as object variable 1, a MEMORY ADDRESS:

Rectangle box2 = box;

// calling a mutator method on object variable

// 2 will change the object referenced by both

// variables:

box2.translate(15, 25);

3.1.5 Object References Cont.

• Thus, both these statements will produce identical output since the object variables reference the same object:

System.out.println(box);System.out.println(box2);

Output:

java.awt.Rectangle[x=20,y=35,width=20,height=30]

Reference: Big Java 4th Edition by Cay Horstmann

3.1.1 Constructing Objects (section 2.6 in Big Java)3.1.2 Accessor and Mutator Methods (section 2.7 in Big Java)3.1.3 The API Documentation (section 2.8 in Big Java)3.1.4 Implementing a Test Program (section 2.9 in Big Java)3.1.5 Object References (section 2.10 in Big Java)