Java - Why all fields in an interface are implicitly static and final?

Question

I am just trying to understand why all fields defined in an Interface are implicitly static and final. The idea of keeping fields static makes sense to me as you can't have objects of an interface but why they are final (implicitly)?

Any one knows why Java designers went with making the fields in an interface static and final?

Answer 1

The fields must be static because they can't be abstract (like methods can). Because they can't be abstract, the implementers will not be able to logically provide the different implementation of the fields.

The fields must be final, I think, because the fields may be accessed by many different implementers allows they to be changeable might be problematic (as synchronization). Also to avoid it to be re-implemented (hidden).

Just my thought.

Answer 2

An interface can't have behavior or state because it is intended to specify only an interaction contract, no implementation details. No behavior is enforced by not allowing method/constructor bodies or static/instance initializing blocks. No state is enforced by only allowing constants. A constant in Java is defined by a static final field (and by convention the name uses UPPER_CASE_AND_UNDERSCORES).

Answer 3

There are a couple of points glossed over here:

Just because fields in an interface are implicitly static final does not mean they must be compile-time constants, or even immutable. You can define e.g.

interface I {
  String TOKEN = SomeOtherClass.heavyComputation();
  JButton BAD_IDEA = new JButton("hello");
}

(Beware that doing this inside an annotation definition can confuse javac, relating to the fact that the above actually compiles to a static initializer.)

Also, the reason for this restriction is more stylistic than technical, and a lot of people would like to see it be relaxed.

Answer 4

prove that the feilds in an interface are implicitly state and final?

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 then  i had answered that to question as that by keeping all methods  in interface and implementing the methods in another class whether my answer is corrcet or wrong please tell me its very urgent