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

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?

CodePudding user response：

This has nothing to do with constructor calls, you've just made a couple of simple mistakes.

You haven't specified a type for DEFAULT:

public static final A DEFAULT

You cannot instantiate an A without an instance of MyApplication, as A is an inner class:

new MyApplication().new A(0, 0.0);

However you probably didn't intend to make A an inner class, so I would mark it as static. Without any extra context, it seems as if A has no relation to MyApplication so shouldn't depend on it.

Note: In Java 15 and below, static declarations weren't even allowed in inner classes (as pointed out by @Johannes Kuhn).

CodePudding user response：

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 {
    ^^^^^^