Question

This question may be silly but i accidentally checked in java source code that run method in runnable interface is defined with abstract keyword.
But according to interface definition all methods in an interface are by default abstract.
Then i am confused why specially Runnable interface has abstract keyword for run method.
I check with other interfaces like map , list etc but no one has abstract keyword.

Please give me an idea why it is written like this in java source code.

public abstract void run();

Thanks

Answer 1

From the java language specification:

Every method declaration in the body
of an interface is implicitly
abstract, so its body is always
represented by a semicolon, not a
block.

Every method declaration in the body
of an interface is implicitly public.

For compatibility with older versions
of the Java platform, it is permitted
but discouraged, as a matter of style,
to redundantly specify the abstract
modifier for methods declared in
interfaces.

It is permitted, but strongly
discouraged as a matter of style, to
redundantly specify the public
modifier for interface methods.

Hey, just learned that my interface declarations are bad style, because I always use the public modifier.

---

apache Harmony just adds the public modifier (uuuh – bad style!). SUNOracle has both modifiers? I guess that’s because in the ‘older versions’ it was mandatory to add the abstract modifier – just because the JLS mentions ‘compatibilty with older versions’.

And then: Never change a Runnable system 🙂