Guava Enums

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();
        }
    }
}