Question

Extending an interface simply adds additional operations to be defined in any implementors and cannot break any existing implementations (unlike extending a class). But it can change (EDIT 3 WHICH CONSTANTS) and hence the PERCIEVED value of constants (EDIT 2 AS SEEN BY THE IMPLEMENTATION CLASSES).

For instance, the following:

interface A {
  int TEST = 6;
}

interface B extends A {
  int TEST = 7;
}

public class InterfacesTest implements B {
  public static void main(final String[] args) {
    System.out.println(TEST);
  }
}

yields 7, when perhaps the intent of interface A was that any implementation of A contain a test field valued at 6.

If A were to be declared final we could be assured all implementations of A see the same value of test.

So does anyone understand why this isn’t possible?

• P.S.: This is NOT a duplicate of
  this question, I know they
  can’t be final, I’m interested in the thinking behind the design
  decision that led to this outcome.

• P.P.S.: I understand constants in
  interfaces is usually a bad idea,
  that isn’t the issue here.

EDIT: Please check the revision history, the title of this question was edited in a way which did not reflect the question’s intent. Sorry to everyone who answered the question I wasn’t asking. Yes, interface fields are implicitly public static final, unfortunately that’s not what I’m interested in at all.

EDIT 2 To be absolutely clear: this question is about why can’t an interface block other interfaces from extending it (by being final or some equivalent).

Answer 1

Primary purpose of interface is not being a container of constants, it’s to define some APIs which to be implemented by concrete classes.

And, from a lang spec (9.4. Abstract method declarations):

Note that a method declared in an interface must not be declared final or a compile-time error occurs. However, a method declared in an interface may be implemented by a method that is declared final in a class that implements the interface.