Java Language Inheritance


Example

With the use of the extends keyword among classes, all the properties of the superclass (also known as the Parent Class or Base Class) are present in the subclass (also known as the Child Class or Derived Class)

public class BaseClass {

    public void baseMethod(){
        System.out.println("Doing base class stuff");
    }
}

public class SubClass extends BaseClass {

}

Instances of SubClass have inherited the method baseMethod():

SubClass s = new SubClass();
s.baseMethod();  //Valid, prints "Doing base class stuff"

Additional content can be added to a subclass. Doing so allows for additional functionality in the subclass without any change to the base class or any other subclasses from that same base class:

public class Subclass2 extends BaseClass {

    public void anotherMethod() {
        System.out.println("Doing subclass2 stuff");
    }
}

Subclass2 s2 = new Subclass2();
s2.baseMethod(); //Still valid , prints "Doing base class stuff"
s2.anotherMethod(); //Also valid, prints "Doing subclass2 stuff" 

Fields are also inherited:

public class BaseClassWithField {

    public int x;

}

public class SubClassWithField extends BaseClassWithField {

    public SubClassWithField(int x) {
        this.x = x; //Can access fields
    }
}

private fields and methods still exist within the subclass, but are not accessible:

public class BaseClassWithPrivateField {

    private int x = 5;

    public int getX() {
        return x;
    }
}

public class SubClassInheritsPrivateField extends BaseClassWithPrivateField {

    public void printX() {
        System.out.println(x); //Illegal, can't access private field x
        System.out.println(getX()); //Legal, prints 5
    }
}

SubClassInheritsPrivateField s = new SubClassInheritsPrivateField();
int x = s.getX(); //x will have a value of 5.

In Java, each class may extend at most one other class.

public class A{}
public class B{}
public class ExtendsTwoClasses extends A, B {} //Illegal

This is known as multiple inheritance, and while it is legal in some languages, Java does not permit it with classes.

As a result of this, every class has an unbranching ancestral chain of classes leading to Object, from which all classes descend.