How to initialize a static field as an object of the same class in Java?

Issue

I have a simple nested class A. (In C I would use a struct.) I want to provide a default object A.DEFAULT of class A without having to call the constructor each time. So my idea was this:

class MyApplication {
   class A {
        int x;
        double y;
        public static final A DEFAULT = new A(0, 0.0);

        public A(int x, double y) {
            this.x = x;
            this.y = y;
        }
    }
    …

Obviously, this does not work, because the constructor cannot be called before the class has been loaded.

How to handle this? How does e. g. java.awt.Color.RED handle this?

Solution

Apart from the obvious problem that you missed a type (A) on the definition of DEFAULT...

The problem isn't that "the class has [not] been loaded". The problem is that, in Java, every instance of an inner class implicitly has a reference to the instance of the outer class that created it. Since there is no instance of the outer class when initializing the static field, you get an error:

MyApplication.java:5: error: non-static variable this cannot be referenced from a static context
        public static final A DEFAULT = new A(0, 0.0);
                                        ^

And in all but the most recent Java versions, even declaring a static field inside a non-static inner class is illegal, no matter what value you give it:

MyApplication.java:5: error: Illegal static declaration in inner class MyApplication.A
        public static final A DEFAULT = new A(0, 0.0);
                              ^
  modifier 'static' is only allowed in constant variable declarations

The solution is to make the inner class not have a reference to the outer class, which you do by declaring A as static:

class MyApplication {
    static class A {
    ^^^^^^

Answered By - Thomas