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Java example source code file (Enums.java)

This example Java source code file (Enums.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

annotation, beta, class, enum, enums, gwtincompatible, map, nosuchfieldexception, object, optional, override, reflection, stringconverter, util, weakhashmap, weakreference

The Enums.java Java example source code

/*
 * Copyright (C) 2011 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.base;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;

import java.io.Serializable;
import java.lang.ref.WeakReference;
import java.lang.reflect.Field;
import java.util.EnumSet;
import java.util.HashMap;
import java.util.Map;
import java.util.WeakHashMap;

import javax.annotation.Nullable;

/**
 * Utility methods for working with {@link Enum} instances.
 *
 * @author Steve McKay
 *
 * @since 9.0
 */
@GwtCompatible(emulated = true)
@Beta
public final class Enums {

  private Enums() {}

  /**
   * Returns the {@link Field} in which {@code enumValue} is defined. For example, to get the
   * {@code Description} annotation on the {@code GOLF} constant of enum {@code Sport}, use
   * {@code Enums.getField(Sport.GOLF).getAnnotation(Description.class)}.
   *
   * @since 12.0
   */
  @GwtIncompatible // reflection
  public static Field getField(Enum<?> enumValue) {
    Class<?> clazz = enumValue.getDeclaringClass();
    try {
      return clazz.getDeclaredField(enumValue.name());
    } catch (NoSuchFieldException impossible) {
      throw new AssertionError(impossible);
    }
  }

  /**
   * Returns an optional enum constant for the given type, using {@link Enum#valueOf}. If the
   * constant does not exist, {@link Optional#absent} is returned. A common use case is for parsing
   * user input or falling back to a default enum constant. For example,
   * {@code Enums.getIfPresent(Country.class, countryInput).or(Country.DEFAULT);}
   *
   * @since 12.0
   */
  public static <T extends Enum Optional getIfPresent(Class enumClass, String value) {
    checkNotNull(enumClass);
    checkNotNull(value);
    return Platform.getEnumIfPresent(enumClass, value);
  }

  @GwtIncompatible // java.lang.ref.WeakReference
  private static final Map<Class, Map>>>
      enumConstantCache =
          new WeakHashMap<
              Class<? extends Enum, Map>>>();

  @GwtIncompatible // java.lang.ref.WeakReference
  private static <T extends Enum Map>> populateCache(
      Class<T> enumClass) {
    Map<String, WeakReference> result =
        new HashMap<String, WeakReference>();
    for (T enumInstance : EnumSet.allOf(enumClass)) {
      result.put(enumInstance.name(), new WeakReference<Enum(enumInstance));
    }
    enumConstantCache.put(enumClass, result);
    return result;
  }

  @GwtIncompatible // java.lang.ref.WeakReference
  static <T extends Enum Map>> getEnumConstants(
      Class<T> enumClass) {
    synchronized (enumConstantCache) {
      Map<String, WeakReference> constants = enumConstantCache.get(enumClass);
      if (constants == null) {
        constants = populateCache(enumClass);
      }
      return constants;
    }
  }

  /**
   * Returns a converter that converts between strings and {@code enum} values of type
   * {@code enumClass} using {@link Enum#valueOf(Class, String)} and {@link Enum#name()}. The
   * converter will throw an {@code IllegalArgumentException} if the argument is not the name of any
   * enum constant in the specified enum.
   *
   * @since 16.0
   */
  public static <T extends Enum Converter stringConverter(final Class enumClass) {
    return new StringConverter<T>(enumClass);
  }

  private static final class StringConverter<T extends Enum extends Converter
      implements Serializable {

    private final Class<T> enumClass;

    StringConverter(Class<T> enumClass) {
      this.enumClass = checkNotNull(enumClass);
    }

    @Override
    protected T doForward(String value) {
      return Enum.valueOf(enumClass, value);
    }

    @Override
    protected String doBackward(T enumValue) {
      return enumValue.name();
    }

    @Override
    public boolean equals(@Nullable Object object) {
      if (object instanceof StringConverter) {
        StringConverter<?> that = (StringConverter) object;
        return this.enumClass.equals(that.enumClass);
      }
      return false;
    }

    @Override
    public int hashCode() {
      return enumClass.hashCode();
    }

    @Override
    public String toString() {
      return "Enums.stringConverter(" + enumClass.getName() + ".class)";
    }

    private static final long serialVersionUID = 0L;
  }
}

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