IndexValue.java

package com.company.util;

public interface IndexValue<Source, Value> {
    Value get(Source source);
}

ReverseEnumIndex.java

package com.company.util;


import com.sun.istack.internal.Nullable;

import java.util.HashMap;
import java.util.Map;

public final class ReverseEnumIndex<Key, Value extends Enum<Value>> {

    private final Map<Key, Value> map;

    @Nullable
    public Value get(Key key) {
        return map.get(key);
    }

    private ReverseEnumIndex(Builder<Key, Value> builder) {
        this.map = builder.map;
        for(Value value : builder.enumConstants) {
            map.put(builder.indexValue.get(value), value);
        }
    }

    public static <Value extends Enum<Value>> ReverseEnumIndex<Integer, Value> ordinalIndex(Class<Value> source) {
        return new Builder<Integer, Value>(source, new IndexValue<Value, Integer>() {
            @Override
            public Integer get(Value value) {
                return value.ordinal();
            }
        }).build();
    }

    public static <Value extends Enum<Value>> ReverseEnumIndex<String, Value> nameIndex(Class<Value> source) {
        return new Builder<String, Value>(source, new IndexValue<Value, String>() {
            @Override
            public String get(Value value) {
                return value.name();
            }
        }).build();
    }

    public static <Key, Value extends Enum<Value>> ReverseEnumIndex<Key, Value> fromSource(Class<Value> source, IndexValue<Value, Key> indexValue) {
        return new Builder<Key, Value>(source, indexValue).build();
    }

    public static final class Builder<Key, Value extends Enum<Value>> {

        private final Value[] enumConstants;
        private final IndexValue<Value, Key> indexValue;
        private Map<Key, Value> map;

        public Builder(Class<Value> source, IndexValue<Value, Key> indexValue) {
            if(source == null || indexValue == null) {
                throw new IllegalStateException("Enum class and IndexValue must be instantiated");
            }

            enumConstants = source.getEnumConstants();
            this.indexValue = indexValue;
        }

        public Builder withMap(Map<Key, Value> map) {
            this.map = map;
            return this;
        }

        public ReverseEnumIndex<Key, Value> build() {
            if(map == null) {
                map = new HashMap<>(enumConstants.length);
            }

            return new ReverseEnumIndex<>(this);
        }
    }
}