001/*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.base;
018
019import static com.google.common.base.Preconditions.checkNotNull;
020
021import com.google.common.annotations.Beta;
022import com.google.common.annotations.GwtCompatible;
023import com.google.common.annotations.GwtIncompatible;
024
025import java.io.Serializable;
026import java.lang.reflect.Field;
027
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)
038@Beta
039public final class Enums {
040
041  private Enums() {}
042
043  /**
044   * Returns the {@link Field} in which {@code enumValue} is defined.
045   * For example, to get the {@code Description} annotation on the {@code GOLF}
046   * constant of enum {@code Sport}, use
047   * {@code Enums.getField(Sport.GOLF).getAnnotation(Description.class)}.
048   *
049   * @since 12.0
050   */
051  @GwtIncompatible("reflection")
052  public static Field getField(Enum<?> enumValue) {
053    Class<?> clazz = enumValue.getDeclaringClass();
054    try {
055      return clazz.getDeclaredField(enumValue.name());
056    } catch (NoSuchFieldException impossible) {
057      throw new AssertionError(impossible);
058    }
059  }
060
061  /**
062   * Returns a {@link Function} that maps an {@link Enum} name to the associated
063   * {@code Enum} constant. The {@code Function} will return {@code null} if the
064   * {@code Enum} constant does not exist.
065   *
066   * @param enumClass the {@link Class} of the {@code Enum} declaring the
067   *     constant values.
068   */
069  public static <T extends Enum<T>> Function<String, T> valueOfFunction(Class<T> enumClass) {
070    return new ValueOfFunction<T>(enumClass);
071  }
072
073  /**
074   * A {@link Function} that maps an {@link Enum} name to the associated
075   * constant, or {@code null} if the constant does not exist.
076   */
077  private static final class ValueOfFunction<T extends Enum<T>>
078      implements Function<String, T>, Serializable {
079
080    private final Class<T> enumClass;
081
082    private ValueOfFunction(Class<T> enumClass) {
083      this.enumClass = checkNotNull(enumClass);
084    }
085
086    @Override
087    public T apply(String value) {
088      try {
089        return Enum.valueOf(enumClass, value);
090      } catch (IllegalArgumentException e) {
091        return null;
092      }
093    }
094
095    @Override public boolean equals(@Nullable Object obj) {
096      return obj instanceof ValueOfFunction &&
097          enumClass.equals(((ValueOfFunction) obj).enumClass);
098    }
099
100    @Override public int hashCode() {
101      return enumClass.hashCode();
102    }
103
104    @Override public String toString() {
105      return "Enums.valueOf(" + enumClass + ")";
106    }
107
108    private static final long serialVersionUID = 0;
109  }
110
111  /**
112   * Returns an optional enum constant for the given type, using {@link Enum#valueOf}. If the
113   * constant does not exist, {@link Optional#absent} is returned. A common use case is for parsing
114   * user input or falling back to a default enum constant. For example,
115   * {@code Enums.getIfPresent(Country.class, countryInput).or(Country.DEFAULT);}
116   *
117   * @since 12.0
118   */
119  public static <T extends Enum<T>> Optional<T> getIfPresent(Class<T> enumClass, String value) {
120    checkNotNull(enumClass);
121    checkNotNull(value);
122    try {
123      return Optional.of(Enum.valueOf(enumClass, value));
124    } catch (IllegalArgumentException iae) {
125      return Optional.absent();
126    }
127  }
128}