001/*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.base;
016
017import static com.google.common.base.Preconditions.checkNotNull;
018
019import com.google.common.annotations.GwtCompatible;
020import com.google.common.annotations.GwtIncompatible;
021import java.io.Serializable;
022import java.lang.ref.WeakReference;
023import java.lang.reflect.Field;
024import java.util.EnumSet;
025import java.util.HashMap;
026import java.util.Map;
027import java.util.WeakHashMap;
028import javax.annotation.Nullable;
029
030/**
031 * Utility methods for working with {@link Enum} instances.
032 *
033 * @author Steve McKay
034 *
035 * @since 9.0
036 */
037@GwtCompatible(emulated = true)
038public final class Enums {
039
040  private Enums() {}
041
042  /**
043   * Returns the {@link Field} in which {@code enumValue} is defined. For example, to get the
044   * {@code Description} annotation on the {@code GOLF} constant of enum {@code Sport}, use
045   * {@code Enums.getField(Sport.GOLF).getAnnotation(Description.class)}.
046   *
047   * @since 12.0
048   */
049  @GwtIncompatible // reflection
050  public static Field getField(Enum<?> enumValue) {
051    Class<?> clazz = enumValue.getDeclaringClass();
052    try {
053      return clazz.getDeclaredField(enumValue.name());
054    } catch (NoSuchFieldException impossible) {
055      throw new AssertionError(impossible);
056    }
057  }
058
059  /**
060   * Returns an optional enum constant for the given type, using {@link Enum#valueOf}. If the
061   * constant does not exist, {@link Optional#absent} is returned. A common use case is for parsing
062   * user input or falling back to a default enum constant. For example,
063   * {@code Enums.getIfPresent(Country.class, countryInput).or(Country.DEFAULT);}
064   *
065   * @since 12.0
066   */
067  public static <T extends Enum<T>> Optional<T> getIfPresent(Class<T> enumClass, String value) {
068    checkNotNull(enumClass);
069    checkNotNull(value);
070    return Platform.getEnumIfPresent(enumClass, value);
071  }
072
073  @GwtIncompatible // java.lang.ref.WeakReference
074  private static final Map<Class<? extends Enum<?>>, Map<String, WeakReference<? extends Enum<?>>>>
075      enumConstantCache =
076          new WeakHashMap<
077              Class<? extends Enum<?>>, Map<String, WeakReference<? extends Enum<?>>>>();
078
079  @GwtIncompatible // java.lang.ref.WeakReference
080  private static <T extends Enum<T>> Map<String, WeakReference<? extends Enum<?>>> populateCache(
081      Class<T> enumClass) {
082    Map<String, WeakReference<? extends Enum<?>>> result =
083        new HashMap<String, WeakReference<? extends Enum<?>>>();
084    for (T enumInstance : EnumSet.allOf(enumClass)) {
085      result.put(enumInstance.name(), new WeakReference<Enum<?>>(enumInstance));
086    }
087    enumConstantCache.put(enumClass, result);
088    return result;
089  }
090
091  @GwtIncompatible // java.lang.ref.WeakReference
092  static <T extends Enum<T>> Map<String, WeakReference<? extends Enum<?>>> getEnumConstants(
093      Class<T> enumClass) {
094    synchronized (enumConstantCache) {
095      Map<String, WeakReference<? extends Enum<?>>> constants = enumConstantCache.get(enumClass);
096      if (constants == null) {
097        constants = populateCache(enumClass);
098      }
099      return constants;
100    }
101  }
102
103  /**
104   * Returns a converter that converts between strings and {@code enum} values of type
105   * {@code enumClass} using {@link Enum#valueOf(Class, String)} and {@link Enum#name()}. The
106   * converter will throw an {@code IllegalArgumentException} if the argument is not the name of any
107   * enum constant in the specified enum.
108   *
109   * @since 16.0
110   */
111  public static <T extends Enum<T>> Converter<String, T> stringConverter(final Class<T> enumClass) {
112    return new StringConverter<T>(enumClass);
113  }
114
115  private static final class StringConverter<T extends Enum<T>> extends Converter<String, T>
116      implements Serializable {
117
118    private final Class<T> enumClass;
119
120    StringConverter(Class<T> enumClass) {
121      this.enumClass = checkNotNull(enumClass);
122    }
123
124    @Override
125    protected T doForward(String value) {
126      return Enum.valueOf(enumClass, value);
127    }
128
129    @Override
130    protected String doBackward(T enumValue) {
131      return enumValue.name();
132    }
133
134    @Override
135    public boolean equals(@Nullable Object object) {
136      if (object instanceof StringConverter) {
137        StringConverter<?> that = (StringConverter<?>) object;
138        return this.enumClass.equals(that.enumClass);
139      }
140      return false;
141    }
142
143    @Override
144    public int hashCode() {
145      return enumClass.hashCode();
146    }
147
148    @Override
149    public String toString() {
150      return "Enums.stringConverter(" + enumClass.getName() + ".class)";
151    }
152
153    private static final long serialVersionUID = 0L;
154  }
155}