Question

When a constructor uses a reference to a constant that is being lazily instantiated, java throws an ExceptionInInitializerError (specifically on the line “this(ClassA.INSTANCE1)”).

public class ClassA {
    public static final ClassA INSTANCE1 = get("INSTANCE1");
    public static final ClassA INSTANCE2 = get("INSTANCE2");

    private static final Map<String, ClassA> MULTITON_MAP = new HashMap<String, ClassA>();

    private String name;

    private ClassA(String name) {
        this.name = name;
    }

    public static ClassA get(String name) {
        ClassA toReturn = MULTITON_MAP.get(name);
        if (toReturn == null) {
            toReturn = new ClassA(name);
            MULTITON_MAP.put(name, toReturn);
        }
        return toReturn;
    }
}

public class ClassB {
    private ClassA type;

    public ClassB() {
        this(ClassA.INSTANCE1);
    }

    public ClassB(ClassA type) {
        this.type = type;
    }

    public static void main(String[] args) {
        new ClassB();
    }
 }

I’ve solved the problem by removing the lazy instantiation and moving instantiations into a static block.

public class ClassA {
    public static final ClassA INSTANCE1;
    public static final ClassB INSTANCE2;

    ...

    static {
        INSTANCE1 = new ClassA("INSTANCE1");
        INSTANCE2 = new ClassA("INSTANCE2");

        MULTITON_MAP.put("INSTANCE1", INSTANCE1);
        MULTITON_MAP.put("INSTANCE2", INSTANCE2);
    }

    ...
}

So, my question is, why is Java unable to handle what I did previously. What causes the error and why?

Thanks a bunch!

Answer 1

I think your problem is that you are trying to initialize INSTANCE1 before MULTITON_MAP, but the instantiation of INSTANCE1 depends on MULTITON_MAP.

E.g. when you call MULTITON_MAP.get(name); in get, MULTITON_MAP is still null.