Why should my local variables be final to be accessible from anonymous class? [duplicate]

Answer 1

When you access a final variable from an anonymous class, the compiler secretly copies their value into a member variable of the anonymous class. eg:

Runnable foo() {
  final int x = 42;
  return new Runnable() {
    void run() {
      System.out.writeln(x);
    }
  };
}

becomes:

// the actual name is generally illegal in normal java syntax
class internal_Runnable implements Runnable {
  final int x;
  internal_Runnable(int _x) { x = _x; }
  void run() {
    System.out.writeln(x);
  }
}

void foo() {
  final x = 42;
  return new internal_Runnable(x);
}

If the variable were not final and were allowed to change, the value cached in the anonymous class instance could go out of sync. This could have been avoided by use of a closure - that is, an object holding the values of all local variables, that both the original function and the new anonymous class instance access. .NET uses closures, for example. However, this can incur a performance hit, and perhaps for that reason, the Java language designers decided not to support full closures.

Answer 2

An anonymous class is a separate class. It has no access to the control flow inside your method. If you would reassign the variable in the anonymous class, you would actually only reassign the anonymous class' copy of the variable. That would be very error-prone, and hence the design choice was made to make it an error.

If you want to work around that, use an AtomicReference.

Answer 3

It's because the anonymous inner object might out-persist its context, which, if it were referring to non-final variables, would then have it talking to things that don't exist any more.

Answer 4

...when an object of the anonymous class is instantiated, copies of the final local variables and method parameters referred to by the object's methods are stored as instance variables in the object. The methods in the object of the anonymous class really access those hidden instance variables.

Thus, the local variables and method parameters accessed by the methods of the local class must be declared final to prevent their values from changing after the object is instantiated.

From here.