Is an enum constant-specific class body static or non-static?

Question

I have a enum type class:

public enum Operation {     PLUS() {         @Override         double apply(double x, double y) {                    // ERROR: Cannot make a static reference             // to the non-static method printMe()...             printMe(x);             return x + y;         }     };      private void printMe(double val) {         System.out.println("val = " + val);     }      abstract double apply(double x, double y); } 

As you see above, I have defined one enum type which has value PLUS. It contains a constant-specific body. In its body, I tried to call printMe(val);, but I got the compilation error:

Cannot make a static reference to the non-static method printMe().

Why do I get this error? I mean I am overriding an abstract method in PLUS body. Why is it in static scope? How to get rid of it?

I know adding a static keyword on printMe(){...} solves the problem, but I am interested to know if there is another way if I want to keep printMe() non-static?

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Another issue, quite similar as above one, but this time the error message sounds the other way around, i.e. PLUS(){...} has non-static context:

public enum Operation {     PLUS() {         // ERROR: the field "name" can not be declared static         // in a non-static inner type.         protected static String name = "someone";          @Override         double apply(double x, double y) {             return x + y;         }     };      abstract double apply(double x, double y); } 

I try to declare a PLUS-specific static variable, but I end up with error:

the field "name" can not be declared static in a non-static inner type.

Why can I not define static constant inside PLUS if PLUS is an anonymous class? The two error messages sound contradictory to each other, as the 1st error message says PLUS(){...} has static context while the 2nd error message says PLUS(){...} has non-static context. I am even more confused now.

Answer 1

Well this is a strange case.

It appears that the problem is:

• In this case, the private member should be accessible (6.6.1.):

  Otherwise, the member or constructor is declared private, and access is permitted if and only if it occurs within the body of the top level class that encloses the declaration of the member or constructor.

• However, private members are not inherited (8.2):

  Members of a class that are declared private are not inherited by subclasses of that class.

• Therefore, printMe is not a member of the anonymous subclass and the compiler searches for it within the superclass* Operation (15.12.1):

  If there is an enclosing type declaration of which that method is a member, let T be the innermost such type declaration. The class or interface to search is T.

  This search policy is called the "comb rule". It effectively looks for methods in a nested class's superclass hierarchy before looking for methods in an enclosing class and its superclass hierarchy.

• And here is where it gets strange. Because printMe is found in a class that also encloses PLUS, the object that the method is called on is instead determined to be an enclosing instance of Operation, which doesn't exist (15.12.4.1):

  Otherwise, let T be the enclosing type declaration of which the method is a member, and let n be an integer such that T is the n'th lexically enclosing type declaration of the class whose declaration immediately contains the method invocation. The target reference is the n'th lexically enclosing instance of this.

  It is a compile-time error if the n'th lexically enclosing instance of this does not exist.

So in short, because printMe is only a member of Operation (and not inherited), the compiler is compelled to invoke printMe on a non-existent outer instance.

However, the method is still accessible and we can find it by qualifying the invocation:

@Override double apply(double x, double y) { //  now the superclass is searched //  but the target reference is definitely 'this' //  vvvvvv     super.printMe(x);     return x + y; } 

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The two error messages sound contradictory to each other [...].

Yes, this is a confusing aspect of the language. On the one hand, an anonymous class is never static (15.9.5), on the other hand, an anonymous class expression can appear in a static context and therefore has no enclosing instance (8.1.3).

An anonymous class is always an inner class; it is never static.

An instance of an inner class I whose declaration occurs in a static context has no lexically enclosing instances.

To help understand how this works, here is a formatted example:

 class Example {     public static void main(String... args) {         new Object() {             int i;             void m() {}         };     } } 

Everything in italics is the static context. The anonymous class derived from the expression in bold is considered inner and non-static (but has no enclosing instance of Example).

Since the anonymous class is non-static it cannot declare static non-constant members, despite that it is itself declared within a static context.

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* Besides obscuring the matter a little, the fact that Operation is an enum is completely irrelevant (8.9.1):

The optional class body of an enum constant implicitly defines an anonymous class declaration that extends the immediately enclosing enum type. The class body is governed by the usual rules of anonymous classes [...].

Answer 2

I don't think I have the answer about the nature of the error, but perhaps I can contribute a bit to the discussion.

When the Java compiler compiles your enum code, it produces a synthetic class that is somewhat as follows:

class Operation {      protected abstract void foo();     private void bar(){ }      public static final Operation ONE;      static {         ONE = new Operation() {             @Override             protected void foo(){                 bar();              }         };     } } 

You can verify the enum code looks somewhat like this by running javap in one of your enum classes.

This code above gives me the exact same error you're getting on your enum: "error: non-static method bar() cannot be referenced from a static context".

So here the compiler thinks you cannot invoke the bar() method, which is an instance method, from the static context in which the anonymous class is being defined.

It makes no sense to me, it should either be accesible or be denied access, but the error does not seem accurate. I am still puzzled, but this seems to be what is actually happening.

It would make more sense if the compilers said that that the anonymous class did not have access to foo since it is private on its parent class, but the compiler is firing this other error instead.