是否可以创建从另一个子类扩展的子类？

Question

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)

Answer 1

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.

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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();

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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