Creating Subclasses

You declare that a class is the subclass of another class within The Class Declaration. For example, suppose that you wanted to create a subclass named SubClass of another class named SuperClass. You would write:
class SubClass extends SuperClass {
    . . .
}
This declares that SubClass is the subclass of the Superclass class. It also implicitly declares that SuperClass is the superclass of SubClass. A subclass also inherits variables and methods from its superclass's superclass, and so on up the inheritance tree. For purposes of making our discussion easier, when this tutorial refers to a class's superclass it means the class's direct ancestor as well as all of its ascendant classes.
A Java class can have only one direct superclass. Java does not support multiple inheritance.
Creating a subclass can be as simple as including the extends clause in your class declaration (such as in the declaration in ImaginaryNumber above). However, you usually have to make other provisions in your code when subclassing a class, such as overriding methods.
What Member Variables Does a Subclass Inherit?

Rule:A subclass inherits all of the member variables within its superclass that are accessible to that subclass (unless the member variable is hidden by the subclass).
That is, subclasses

inherit those member variables declared as public or protected
inherit those member variables declared with no access specifier as long as the subclass is in the same package as the superclass. These variables are sometimes known as "friendly".
don't inherit a superclass's member variable if the subclass declares a member variable using the same name. The subclass's member variable is said to hide the member variable in the superclass.
don't inherit private member variables

Hiding Member Variables
As mentioned in the previous section, member variables defined in the subclass hide member variables of the same name in the superclass.
While this feature of the Java language is powerful and convenient, it can be a fruitful source of errors: hiding a member variable can be done deliberately or by accident. So, when naming your member variables be careful to only hide those member variables that you actually wish to hide.
One interesting feature of Java member variables is that a class can access a hidden member variable through its superclass. Consider this superclass and subclass pair:
class Super {
    Number aNumber;
}
class Sub extends Super {
    Float aNumber;
}
The aNumber variable in Sub hides aNumber in Super. But you can access aNumber from the superclass with:
super.aNumber
super is a Java language keyword that allows a method to refer to hidden variables and overriden methods of the superclass.
What Methods Does a Subclass Inherit?

The rule that specifies which methods get inherited by a subclass is similar to that for member variables.
Rule:A subclass inherits all of the methods within its superclass that are accessible to that subclass (unless the method is overriden by the subclass).
That is, subclasses

inherit those methods declared as public or protected
inherit those methods declared with no access specifier as long as the subclass is in the same package as the superclass
don't inherit a superclass's method if the subclass declares a method using the same name. The method in the subclass is said to override the one in the superclass.
don't inherit private methods

Overriding Methods

The ability of a subclass to override a method in its superclass allows a class to inherit from a superclass whose behavior is "close enough" and then supplement or modify the behavior of that superclass.

Objects, Classes, and Interfaces