概述
Enums提供了几个操作Enum的便利方法
常用方法
Field getField(Enum<?> enumValue): 返回变量名为enumValue变量值的Field
<T extends Enum<T>> Function<String, T> valueOfFunction(Class<T> enumClass): 返回一个可以将enum名字字符串转换成指定类型的enum的, 如果enum不存在时,Function将返回null
Optional<T> getIfPresent(Class<T> enumClass, String value): 使用Enum.valueOf()来返回指定名称和class的Enum的Optional,如果不存在则返回Absent, 常见用法Enums.getIfPresent(Country.class, countryInput).or(Country.DEFAULT)
示例代码
TestEnum a = TestEnum.A; System.out.println(Enums.getField(a)); Function<String, TestEnum> function = Enums.valueOfFunction(TestEnum.class); System.out.println(function.apply("B")); System.out.println(Enums.getIfPresent(TestEnum.class, "D").or(TestEnum.A)); // D不存在,则返回默认的A System.out.println(Enums.getIfPresent(TestEnum.class, "C").or(TestEnum.A)); // C存在,直接返回
public static final enumtest.TestEnum enumtest.TestEnum.A
B
A
C
源代码
/* * 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.reflect.Field; 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)}. * * 返回变量名为enumValue变量值得Field * * @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 a {@link Function} that maps an {@link Enum} name to the associated * {@code Enum} constant. The {@code Function} will return {@code null} if the * {@code Enum} constant does not exist. * * 返回一个可以将enum名字字符串转换成指定类型的enum的Function * 如果enum不存在时,Function将返回null * * @param enumClass the {@link Class} of the {@code Enum} declaring the * constant values. */ public static <T extends Enum<T>> Function<String, T> valueOfFunction(Class<T> enumClass) { return new ValueOfFunction<T>(enumClass); } /** * A {@link Function} that maps an {@link Enum} name to the associated * constant, or {@code null} if the constant does not exist. */ private static final class ValueOfFunction<T extends Enum<T>> implements Function<String, T>, Serializable { private final Class<T> enumClass; private ValueOfFunction(Class<T> enumClass) { this.enumClass = checkNotNull(enumClass); } @Override public T apply(String value) { try { // 通过valueOf方法实现将value转为enum return Enum.valueOf(enumClass, value); } catch (IllegalArgumentException e) { return null; } } @Override public boolean equals(@Nullable Object obj) { return obj instanceof ValueOfFunction && enumClass.equals(((ValueOfFunction) obj).enumClass); } @Override public int hashCode() { return enumClass.hashCode(); } @Override public String toString() { return "Enums.valueOf(" + enumClass + ")"; } private static final long serialVersionUID = 0; } /** * 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);} * * 使用Enum.valueOf()来返回指定名称和class的Enum的Optional,如果不存在则返回Absent * * @since 12.0 */ public static <T extends Enum<T>> Optional<T> getIfPresent(Class<T> enumClass, String value) { checkNotNull(enumClass); checkNotNull(value); try { return Optional.of(Enum.valueOf(enumClass, value)); } catch (IllegalArgumentException iae) { return Optional.absent(); } } }