Why are static imports of static methods with same names legal?

Question

Lets say we have these packages and classes:

package p1;

public class A1 {
    public static void a() {}
}

package p2;

public class A1 {
    public static void a() {}
}

package p3;

import static p1.A1.a;
import static p2.A1.a;

public class A1 {
    public static void test() {

    }
}

I am wondering, why the static import of methods is legal (won't result in compile time error) in package p3? We won't be able to use them further in the test() method as such usage will result in the compile time error.

Why it is not the same as with a normal import of classes. Lets say we would like to import classes A1 from packages p1 and p2 into p3:

package p3;
import p1.A1;
import p2.A1;

such import is illegal and will result in the compile time error.

Because the package and class combination uniquely distinguish one method from the other. — duffymo, Oct 22 '12 at 10:34
@duffymo - He's specifically asking why there is no name clash when the methods are imported statically, which you don't address. — Dave Richardson, Oct 22 '12 at 10:37
I think I do. I have no problem distinguishing betweek a.b.c and a.d.c; neither does the compiler. — duffymo, Oct 22 '12 at 11:24

Joachim Sauer · Accepted Answer · 2012-10-22T10:43:04.513

44

The ambiguity of the static imports of methods could be resolved at the point of the method invocation.

For example if you had a static import for two methods that look like this:

void frobnicate(int i);
// and
void frobnicate(boolean b);

Then you could import and use both, because the compiler could tell which one to use, based on the arguments you pass in (frobnicate(1) calls the first one, frobnicate(true) calls the second one).

With classes, that's not possible: Foobar a; alone is not sufficient to tell you which of the two Foobar classes you want.

Also note that a single static import can import multiple names. According to the relevant section of the JLS (emphasis mine):

A single-static-import declaration imports all accessible static members with a given simple name from a type.

For example if the two frobnicate methods above where located in the same class, a single static import could import them both.

edited Oct 22 '12 at 10:43

answered Oct 22 '12 at 10:37

Joachim Sauer

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Fun fact: Groovy allows this too, however at runtime it'll throw `groovy.lang.MissingMethodException: No signature of method` at you if you're not careful enough like me. It'll also list `Possible solutions` in the error message and that'll contain the method name that hides the one that you have tried to call in the first place. Took me an hour to figure this out. – Kohányi Róbert Apr 21 '16 at 10:12
The question remains as to why the compiler allows this, when it "knows" that two of the static methods with the same name have the same signature. – Dávid Horváth Jan 22 '17 at 01:45
@DávidHorváth if you can reproduce this ambiguity, then I suggest filing a bug. It definitely sounds like something the compiler should prevent. – Gili Feb 01 '17 at 04:29

score 2 · Answer 2 · answered Oct 22 '12 at 10:36

It is because you have named all of these classes the same thing. Every time you call this static method it is looking in the most local class which in this case is the A1 in p3 which does not contain the static method a(). Always remember a class name should be unique and never the same as another.

score 0 · Answer 3 · answered Oct 22 '12 at 15:26

I'm guessing, not backed by language spec, that methods can be overloaded, but fields/types cannot; therefore java allows importing methods of same name from different classes.

However two methods of the same signature cannot exist in same class; they ought not to be allowed either by imports.

Even if the two methods have different signatures, it is still a bad idea. The meaning of a name is very important, it should have clarity. We can handle overloaded methods from 1 class, but "overloading" across class is a stretch.

Why are static imports of static methods with same names legal?

3 Answers3