Java – generics – what am I missing or what’s the point?

I have the following code:

public class AClass<X extends AnInterface> implements AnotherInterface {

    public AClass(X x){
    }

}

Why do I use X as the constructor parameter type when I can replace it with aninterface? Of course, they mean the same. Is everything a subtype of aninterface?

I tried to keep all parameter types generic and only mentioned the interface name in the generic parameter declaration, such as "x extends aninterface", but I encountered a problem because it said that the value I passed in (type aninterface) was not equal to the input X

Edit:

public void<X extends AnInterface> myMethod(){


    AnInterface b = new ADiffClass();
    AnotherInterface a = new AClass(b);
}

public class ADiffClass<X extends AnInterface> implements AnInterface {

    public ADiffClass(){
    }

}

This is where I encountered the problem. I got a compilation error, saying that the type of B is aninterface, and the required type in the constructor of aclass is X

Solution

If you declare such a variable:

private AClass<Foo> a;

The following are valid:

a = new AClass<Foo>(new Foo());

But the following are not:

a = new AClass<Foo>(new Bar());

(suppose Foo and bar are two implementations of aninterface)

In this case, this is generics: limiting types to specific types that implement (or extend) aninterface