java类如何扩展另一个类并同时实现具有相同方法名称的接口

Question

I have an interface(A) the contain method myMethod() and a class(B) also contain the same method myMethod() and another class(c) extends and implement A and B.

No i created the object of class A a = new C() and call the a.myMethod(); .

How it is executing the myMethod of class B.

interface A {
    void myMethod();
}

class B {
    public void myMethod() {
        System.out.println("My Method");
    }
}

class C extends B implements A {

}

class MainClass {
    public static void main(String[] args) {
        A a = new C();
        a.myMethod();
    }
}

Output of the program -

 My Method

Answer 1

The methods have the same signature. This means, that the methods look the same, after all types have been reduced to their most compact form, and all (seemingly) redundant information has been removed during compilation. It is what is ultimately 'precompiled' to the .class files.

What the compiler remembers about the interface method: name: myMethod, returns: void, parameters: {}

What the compiler remembers about the interface method: name: myMethod, returns: void, parameters: {}

(For both methods, the compiler also remembers, that they are instance-methods, as opposed to static methods, and therefore there actually is a parameter [the 'this' object]. But this is only interesting much later in your developer life :))

So in essence, they are the same method, to the compiler. In other words, the class you extend covers that method for you, and you do not have to implement it explicitly. You can however (if you wish to) override the behavior of your extended class B and do something else, if required.

Answer 2

First it will look for implementation of myMethod() in class C. since it is not there it will check in your extended class B. Then it will execute that class B myMethod(). But if you have implemented myMethod() in class C your program will execute class C myMethod().

Answer 3

The executed method depends on the type of the object and not the variable (polymorphism). In your case the object is of type C which inherits the implementation of myMethod from type B .