Enums with static fields

suggest change

If your enum class is required to have static fields, keep in mind they are created after the enum values themselves. That means, the following code will result in a NullPointerException:

enum Example {
    ONE(1), TWO(2);

    static Map<String, Integer> integers = new HashMap<>();

    private Example(int value) {
        integers.put(this.name(), value);
    }
}

A possible way to fix this:

enum Example {
    ONE(1), TWO(2);

    static Map<String, Integer> integers;

    private Example(int value) {
        putValue(this.name(), value);
    }

    private static void putValue(String name, int value) {
        if (integers == null)
            integers = new HashMap<>();
        integers.put(name, value);
    }
}

Do not initialize the static field:

enum Example {
    ONE(1), TWO(2);

    // after initialisisation integers is null!!
    static Map<String, Integer> integers = null;

    private Example(int value) {
        putValue(this.name(), value);
    }

    private static void putValue(String name, int value) {
        if (integers == null)
            integers = new HashMap<>();
        integers.put(name, value);
    }
    // !!this may lead to null poiner exception!!
    public int getValue(){
        return (Example.integers.get(this.name()));
    }
}

initialisisation:

create the enum values
as side effect putValue() called that initializes integers
the static values are set
integers = null; // is executed after the enums so the content of integers is lost

Found a mistake? Have a question or improvement idea? Let me know.

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