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javainheritancesubclasssuperclass

Is it possible to create a subclass which extends from another subclass?

I am still learning how to code java and had a question about how inheritance works. Class A is the parent class and Class B is the subclass which inherits all the methods from Class A. Suppose I create a third class, Class C. If I do Class C extends Class B, is this different than doing Class C extends Class A? If so, how? Thank You. (Sorry if the format sucked)

Solution

  • The simplest way to visualize this is to consider that inheritance is like a parent/child relationship. You can have Parent -> Child -> Grand Child, etc.

    When you have:

    class A {}
class B extends A{}
class C extends B{}

    C is like a grand child of A. And that means C inherits all the methods from B, including those methods that B itself inherited from A. In OOP words,C**is**A`.

    However, when you have

    class A {}
class B extends A{}
class C extends A{}

    C and B are sibling classes, meaning they both inherit A's methods, but they are incompatible with one another.

    In the first case, these are valid:

    C c = new C();
c.methodFromA(); //C inherits methods from A by being its grand-child
c.methodFromB(); //C inherits methods from B by being its child
c.methodFromC();

    In the second case, however, when both B and C extends A directly:

    C c = new C();
B b = new B();
c.methodFromA(); //C inherits methods from A by being its child
b.methodFromA(); //B inherits methods from A by being its child

c.methodFromB(); //not allowed
b.methodFromC(); //not allowed

    However, there's no direct relationship between B and C. These are invalid:

    B b = new B();
C c = new C();

b = (B) c; //invalid, won't compile

A b = b;
c = (C) b; //will compile, but cause a ClassCastException at runtime