如何将 static 字段初始化为 ZD52387880E1EA21817A792D371 中相同 class 的 object？

Question

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 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. Color.RED handle this?

Answer 1

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 .

Answer 2

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 {
    ^^^^^^