Class Object and Method

Defining Classes

To define a class, we use the class keyword and the name of the class:

class MyClassName
{
...
}

By default, classes inherit from the Object class.

If this class is a subclass of another specific class (that is, inherits from another class), use extends to indicate the superclass of this class:

class myClassName extends mySuperClassName
{
...
}

Following is the code for a class called SimplePoint that represents a point in 2D space:

public class SimplePoint
{
public int x = 0;
public int y = 0;
}

This segment of code declares a class-- a new data type really-- called SimplePoint. The SimplePoint class contains two integer member variables, x and y. The public keyword preceding the declaration for x and y means that any other class can freely access these two members.

We create an object from a class such as SimplePoint by instantiating the class. When we create a new SimplePoint object space is allocated for the object and its members x and y. In addition, the x and y members inside the object are initialized to 0 because of the assignment statements in the declarations of these two members.

Ex:

public class SimpleRectangle
{
public int width = 0;
public int height = 0;
public SimplePoint origin = new SimplePoint();
}

Here the segment of code declares a class SimpleRectangle-- that contains two integer members, width and height.

SimpleRectangle also contains a third member, origin, whose data type is SimplePoint.

Here the class name SimplePoint is used in a variable declaration as the variable's type. We can use the name of a class anywhere we can use the name of a primitive type.

As with SimplePoint, when we create a new SimpleRectangle object, space is allocated for the object and its members, and the members are initialized according to their declarations.

The initialization for the origin member creates a SimplePoint object with this code: new SimplePoint() as illustrated here:

This diagram shows the difference between primitive types and reference types