Question

Does any Java Interface implicitly implements java.lang.Object?

This question arose when I did something like this:

public static String[] sizeSort(String[] sa) {

Comparator<String> c = new Comparator<String>() {
            public int compare(String a, String b) {
                if (a.length() > b.length()) return 1; 
                else if (a.length() < b.length())
                    return -1;
                else 
                    return 0;
                 }
        };

// more code

}

It worked fine even though I did not implement equals method of this interface.
Your answers clears this up. But does any one know if above is anonymous local inner class or named local inner class?

Answer 1

Sort of. Citing the Java Language Specification:

If an interface has no direct
superinterfaces, then the interface
implicitly declares a public abstract
member method m with signature s,
return type r, and throws clause t
corresponding to each public instance
method m with signature s, return type
r, and throws clause t declared in
Object, unless a method with the same
signature, same return type, and a
compatible throws clause is explicitly
declared by the interface. It is a compile-time error if the interface explicitly
declares such a method m in the case
where m is declared to be final in
Object.

Note that Object has a number of final and protected methods that all interfaces “inherit” this way, and you couldn’t have those modifiers in an interface (which would be implied by your “implements java.lang.Object”).