如果我调用一个接口方法，它会从实现类中获取方法体并执行吗？

Question

I have an interface Interface1 . I have its implementation Imple implements Interface1 (all methods have been implemented :) ).

Now, consider a third class CheckCall , can I do a call in the class CheckCall like I mention below :

Interface1 interface1;
interface1.method();

All necessary imports have been done. Please tell me is it possible or not , if not then ok and if yes, then tell me what will happen if I've more than one implementing classes for the same interface and I'm doing the same call.

Answer 1

Well, you cannot call the method directly on the interface, but you can do what you wrote, more or less.

You wrote:

Interface1 interface1;
interface1.method();

This will work if you do this:

Interface1 interface1 = new CheckCall();
interface1.method();

then tell me what will happen if i have more than one impl classes for the same interface and i am doing the same call

Well, that's the nice thing about Java: the problem you're referring to is called the "diamond problem":

http://en.wikipedia.org/wiki/Diamond_problem

And it doesn't really exist in Java because Java fully supports multiple inheritance, but only through "multiple (Java) interface inheritance" (* see comment).

So in your case when you call interface1.method() you're either calling Impl 's method or CheckCall 's method and there's no confusion possible.

Answer 2

Sure, your code works fine! You just have to initialize your variable interface1 with an actual implementation (ie new Imple()).

Check out this example, I used your class-names:

public class CheckCall {

    interface Interface1 {
        void method();
    }

    static class Imple implements Interface1 {
        @Override
        public void method() {
            System.out.println("Imple.method1()");
        }
    }

    public static void main(String[] args) {
        Interface1 interface1;
        interface1 = new Imple();
        interface1.method();
    }

}

Answer 3

Yes, but not as you have it written. You'd have to say:

Interface1 interface1 = new Imple();

You can create a variable of type Interface, and then instantiate it to the class that implements the interface. You see this quite often with the Java collections, for example:

List<String> a = new ArrayList<String>();

Answer 4

Actually, you should refer to methods by the interfaces that define them, but you need an actual implementing class. So I would usually do it like this:

// the variable is of type Interface1
Interface1 interface1 = new Imple();
// but the method will be executed on the Imple object
interface1.method();

Answer 5

No, you need to make the call on the object that implements the interface, not on the interface itself.

Edit: A variable with that type could be used but it would still need to be an class that implements that type. You can't create an instance of an interface.

Answer 6

No.

As you can't instanciate an interface, you'll have to create a Imple object, to use it as an Interface1 one like this :

Interface1 interface1 = new Imple();

interface1.method();

in fact, an interface main interest come from the ability to have a method return any object implementing it, without having to worry about given implementation. in your case, it could shows up as

public class CheckCall {
    public Interface1 factoryMethod() {
        return new Imple();
    }

    public void test() {
        Interface1 used = factoryMethod();

        used.method();
    }
}