Question

Stimulated by the question Interface vs Abstract Classes and the accepted answer, I would like to have a more detailed and clarified answer. In particular I cannot understand the statement “a field in an interface is implicitly static and final”. Does it mean that a class A implementing an interface containing method foo() can invoke the method as A.foo()?

What concerning final: As long as interfaces contain only methods, given an abstract class A which implements an interface with a method foo() and an ordinary class B which extends the abstract class A, cannot the class B override the foo method? As far as I am concerned, final methods are impossible to be overridden. What is true finally?

Answer 1

“a field in an interface is implicitly static and final”.

In an interface writing

int N = 1;
public int N = 1;
static int N = 1;
public static int N = 1;
// also
final int N = 1;
public final int N = 1;
static final int N = 1;
public static final int N = 1;

are all the same.

Does it mean that a class A implementing an interface containing method foo() can invoke the method as A.foo()

Fields and method are both members, but methods and fields are not the same thing.

Methods in an interface cannot be static or final, but are implicitly public and abstract

int foo();
public int foo();
abstract int foo();
public abstract int foo();

are all the same for an interface.

As far as I am concerned, final methods are impossible to be overridden

final instances methods cannot be overridden and final static methods cannot be hidden.

Similarly nested interfaces, classes and annotations are public and static. Nested interfaces and annotations are also implicitly abstract.

interface A {
    public static class C { }
    public static /* final */ enum E {; }
    public static abstract interface I { }
    public static abstract @interface A { }
}