Question

Just by experiment I discovered that Java non static methods overrides all same named methods in scope even at static context. Even without allowing parameter overloading. Like

import java.util.Arrays;    
import static java.util.Arrays.toString;

public class A {
    public static void bar(Object... args) {
        Arrays.toString(args);
        toString(args);     //toString() in java.lang.Object cannot be applied to (java.lang.Object[])
    }
}

I can’t find anything about this in spec. Is this a bug? If it isn’t, are there any reasons to implement language like that?

UPD: Java 6 do not compile this example. The question is – why?

Answer 1

The explanation is simple although it doesn’t change the fact that the behavior is highly unintuitive:

When resolving the method to be invoked the first thing the compiler does is find the smallest enclosing scope that has a method of the right name. Only then come other things like overload resolution and co in game.

Now what is happening here is that the smallest enclosing scope that contains a toString() method is class A which inherits it from Object. Hence we stop there and don’t search farther. Sadly next the compiler tries to find the best fit of the methods in the given scope and notices that it can’t call any of those and gives an error.

Which means never statically import methods with a name that is identical to a method in Object, because methods that are naturally in scope take precedence over static imports (the JLS describes method shadowing in detail, but for this problem I think it’s much simpler to just remember that).

Edit: @alf kindly submitted the right part of the JLS that describes the method invocation for those who want the whole picture. It’s rather complex, but then the problem isn’t simple either so that’s to be expected.