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javainner-classesstatic-members

Why does Java prohibit static fields in inner classes?

class OuterClass {
 class InnerClass {
  static int i = 100; // compile error
  static void f() { } // compile error
 }
}

Although it's not possible to access the static field with OuterClass.InnerClass.i, if I want to record something that should be static, e.g. the number of InnerClass objects created, it would be helpful to make that field static. So why does Java prohibit static fields/methods in inner classes?

EDIT: I know how to make the compiler happy with static nested class (or static inner class), but what I want to know is why java forbids static fields/methods inside inner classes (or ordinary inner class) from both the language design and implementation aspects, if someone knows more about it.

Solution

  • The idea behind inner classes is to operate in the context of the enclosing instance. Somehow, allowing static variables and methods contradicts this motivation?

    8.1.2 Inner Classes and Enclosing Instances

    An inner class is a nested class that is not explicitly or implicitly declared static. Inner classes may not declare static initializers (§8.7) or member interfaces. Inner classes may not declare static members, unless they are compile-time constant fields (§15.28).