Question

I’ve defined the following enum in Groovy, though for the purpose of this question it could be Java code:

enum FestivalType {

    BIG_MUSIC,
    SMALL_MUSIC,
    FILM,
    FOOD_AND_DRINK; 

    private static Set<String> allSearchTokens = new HashSet<String>();

    FestivalType() {
        String searchToken = this.name().tokenize('_').first().toLowerCase();

        if (searchToken in allSearchTokens) {
            throw new RuntimeException("Duplicate search token");

        } else {
            this.searchToken = searchToken;
            allSearchTokens.add(searchToken);
        }
    }

    final String searchToken;
}

What I’m trying to do in the constructor is establish whether the first token in the name of each enum constant is unique, where _ is used as the token separator.

However, this code doesn’t work because allSearchTokens is not initialized until after all the constants are instantiated, so I get a NullPointerException here

allSearchTokens.add(searchToken)

Answer 1

You can work around this as follows:

public enum FestivalType {

    BIG_MUSIC,
    SMALL_MUSIC,
    FILM,
    FOOD_AND_DRINK;

    private static class SetHolder {
        static Set<String> allSearchTokens = new HashSet<String>();
    }

    final String searchToken;

    FestivalType() {
        String searchToken = name().split("_")[0].toLowerCase();

        if (SetHolder.allSearchTokens.contains(searchToken))
            throw new RuntimeException("Duplicate search token");

        this.searchToken = searchToken;
        SetHolder.allSearchTokens.add(searchToken);
    }
}

This compiles because of the java specification that all static initializers must be completed before the class is used. By making the Set a static field of a sttic inner class, you guarantee that it will be initialized before the first enum is constructed.

Also, I took the liberty of changing/fixing a few things in your code:

• Use a Set rather than a List: Values are unique
• Use split(): There is not such method tokenize() for String in java
• Remove else: After a return or throws, else is always redundant because execution of the block is halted by these keywords (there is no “else” to handle)

As an aside, this technique is also great for lazy initialization of singletons:

public class MyLazySingleton() {
    private static class InstanceHolder {
        static MyLazySingleton INSTANCE = new MyLazySingleton();
    }

    public static MyLazySingleton getInstance() {
        return InstanceHolder.INSTANCE;
    }
}

The INSTANCE field is only constructed when the getInstance() method is first called!

Look mom! Lazy initialization without locks, without null checks, without synchronization of any kind and 100% bulletproof! (Object deserialization hacks notwithstanding)

It’s magic 🙂