从String转换为具有大量值的Java枚举

Question

假设我有一个包含100个值的枚举.为简单起见,请参考以下示例:

public enum code
{
    CODE_1("string1"),
    CODE_2("string2"),
    CODE_3("string3"),
    CODE_4("string4"),
    ...
}

我想创建一个公共方法与已知的格式字符串(如"字符串1","字符串2" ......)转换中选取适当的枚举值CODE_1,CODE_2 ......这通常是通过遍历所有的值来实现的,如果找到匹配项,返回该枚举值.(详情可在此问题中找到.)

但是,我关注的是对所有值进行reguraly循环.这可能是一个巨大的瓶颈吗？如果不是100个元素,那么有1000个？

作为我自己的练习,我尝试使用静态地图优化此查找,这可以确保给定任何字符串的O(1)查找时间.我喜欢这个额外的噱头,但我只想在我的代码中包含它,如果它确实是必要的.使用迭代方法和map方法有什么想法和发现？

public enum Code
{
    ...
    //enum values
    ...


    //The string-to-Code map
    private static final Map<String,Code> CODE_MAP = populateMap();

    private static Map<String,Code> populateMap()
    {
        Map<String,Code> map = new HashMap<String,Code>();

        for(Code c : Code.values())
        {
            map.put(c.getCode(), c);
        }

        return map;
    }


    private String code;

    private Code(String code)
    {
        this.code = code;
    }

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

    public Code convertFromString(String code)
    {
        //assume that the given string is actually a key value in the map

        return (Code) CODE_MAP.get(code);
    }
}

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 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.

没有循环.没有特殊的代码来加载地图(在构造函数中完成).最小的代码.