Java枚举自动生成getInstance方法？

Question

Lets say I have the following java enum

public enum Color {
  RED(10),
  GREEN(22),
  BLUE(33);

  private int value;

  Color(int value) {
    this.value = value;
  }

  public int intValue() {
    return value;
  }
}

In order to be able to obtain a color instance by a given integer value I need to add method like this:

public static Color getInstance(int value) {
  switch (value) {
    case 10: return RED;
    case 22: return GREEN;
    case 33: return BLUE;
    default: throw new IllegalArgumentException("Invalid color value");
  }
}

Is it possible this method to be autogenerated from the IDE? (Preferably IntelliJ)?

Answer 1

Instead of repeating the code in every enum , you may implement a central utility method for all kinds of enums with an int value. This is possible since enum s can implement interfaces, hence you can define a uniform way of accessing the int value:

public interface IntValued {
    int intValue();
}
/** get the particular {@link IntValued} {@code enum} constant. */
public static <T extends Enum<T>&IntValued> T get(Class<T> type, int value) {
    return type.cast(GET.get(type).apply(value));
}
private static ClassValue<IntFunction<Enum>> GET = new ClassValue<IntFunction<Enum>>() {
    protected IntFunction<Enum> computeValue(Class<?> type) {
        return prepare(type);
    }
};
// invoked only once per enum type
static IntFunction<Enum> prepare(Class<?> type) {
    Enum[] values=type.asSubclass(Enum.class).getEnumConstants();
    if(values.length==0) return i -> null;
    IntSummaryStatistics s=Arrays.stream(values)
        .mapToInt(o -> ((IntValued)o).intValue())
        .summaryStatistics();
    int min=s.getMin(), max=s.getMax();
    if((max-min+1)<s.getCount()*2) {
        Enum[] linear=new Enum[max-min+1];
        for(Enum e: values) linear[((IntValued)e).intValue()-min]=e;
        return i -> i<min||i>max? null: linear[i-min];
    }
    Map<Integer, Enum> map = Arrays.stream(values).collect(
        Collectors.toMap(o -> ((IntValued)o).intValue(), Function.identity()));
    return map::get;
}

This uses ClassValue , a JRE provided way of associating custom meta data to a class. ClassValue takes care of invoking the initialization code only once and still allows garbage collection/ class unloading. The code above dynamically determines which lookup structure to use. If the int values of a particular enum type are dense, it uses an array, otherwise it resorts to a Map .

You can use it as follows:

public enum Color implements IntValued {
    RED(10),
    GREEN(22),
    BLUE(33);

    private int value;

    Color(int value) {
      this.value = value;
    }

    public int intValue() {
      return value;
    }
}

Color color=get(Color.class, 22);
System.out.println(color);

Note that this solution, unlike generated switch statements, does not break if these int values are changed.

Answer 2

If you are using Java 8 you can make use of default methods in interface to add functionality to any class - even enum s.

Take a look here where I add a reverse-lookup to an enum just by making it implement ReverseLookup .