Convert from String to a Java enum with large amount of values

Question

(This question already has answers here): question from:https://stackoverflow.com/questions/27703119/convert-from-string-to-a-java-enum-with-large-amount-of-values

Answer 1

You want a Map<String, Code>, but how to populate it neatly? Enums don't allow you to initialize a static fields before the enum instances are initialized, but there's a neat little trick, called the Initialization-on-demand holder idiom, that makes using a statically initialized map needed for this functionality easy to implement:

public enum Code {
    CODE_1("string1"),
    CODE_2("string2"),
    CODE_3("string3"),
    // etc
    ;

    private static class Holder {
        static Map<String, Code> CODE_MAP = new HashMap<>();
    }

    private final String code;

    private Code(String code) {
        this.code = code;
        Holder.CODE_MAP.put(code, this);
    }

    public String getCode() {
        return this.code;
    }

    public Code convertFromString(String code) {
        return Holder.CODE_MAP.get(code);
    }
}

This works because the class loader initializes inner static classes before initializing the enum class, so the map is assigned ready to load during enum instance initialization.

No loops. No special code to load the map (done in constructor). Minimal code.