不要让这个世界的复杂性阻碍你的前进。 要成为一个行动主义者，将解决人类的不平等视为己任。 它将成为你生命中最重要的经历之一。
——比尔·盖茨在哈佛大学的演讲
声明枚举
[public]enum 枚举类型名称[implements 接口名称列表]
{
枚举值；
变量成员声明及初始化；
方法声明及方法体；
}
简单的例子：
/**
* 简单的枚举类型举例
* @author wangbaofu
*
*/
public class ScoreTester {
public static void main(String[] args) {
giveScore(Score.EXCELLENT);
}
private static void giveScore(Score s) {
switch (s) {
case EXCELLENT:
System.out.println("Excellent");
break;
case QUALIFIED:
System.out.println("Qualified");
break;
case FAILED:
System.out.println("Failed");
break;
default:
break;
}
}
}
enum Score {
EXCELLENT, QUALIFIED, FAILED;
}
/*
* 输出 Excellent
* */
通过jad反编译一下：
// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3)
// Source File Name: ScoreTester.java
package enum_;
final class Score extends Enum
{
private Score(String s, int i)
{
super(s, i);
}
public static Score[] values()
{
Score ascore[];
int i;
Score ascore1[];
System.arraycopy(ascore = ENUM$VALUES, 0, ascore1 = new Score[i = ascore.length], 0, i);
return ascore1;
}
public static Score valueOf(String s)
{
return (Score)Enum.valueOf(enum_/Score, s);
}
public static final Score EXCELLENT;
public static final Score QUALIFIED;
public static final Score FAILED;
public static final Score ADD;
private static final Score ENUM$VALUES[];
static
{
EXCELLENT = new Score("EXCELLENT", 0);
QUALIFIED = new Score("QUALIFIED", 1);
FAILED = new Score("FAILED", 2);
ADD = new Score("ADD", 3);
ENUM$VALUES = (new Score[] {
EXCELLENT, QUALIFIED, FAILED, ADD
});
}
}
枚举的特点：
- 枚举定义实际上是定义了一个类
- 所有枚举类型都隐含继承（扩展）自Java.lang.Enum,因此枚举类型不能再继承任何其它类
- 枚举类型中的类可以包括方法和变量
- 枚举类型的构造方法必须是包内私有或者私有的。定义在枚举开头的常量会自动创建，不能显示地调用枚举类的构造方法。
枚举类型的默认方法
- 静态的values()方法用于获得枚举类型的枚举值的数组
- toString方法返回枚举值的字符串描述
- valueOf方法将以字符串形式表示的枚举值转化为枚举类型的对象
- Ordinal方法获得对象在枚举类型中的位置索引
最佳实践 （该部分内容为《编写高质量代码之java》学习笔记）
下边贴一下jdk中Enum的源码
/*
* Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.lang;
import java.io.Serializable;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectStreamException;
/**
* This is the common base class of all Java language enumeration types.
*
* More information about enums, including descriptions of the
* implicitly declared methods synthesized by the compiler, can be
* found in section 8.9 of
* <cite>The Java™ Language Specification</cite>.
*
* <p> Note that when using an enumeration type as the type of a set
* or as the type of the keys in a map, specialized and efficient
* {@linkplain java.util.EnumSet set} and {@linkplain
* java.util.EnumMap map} implementations are available.
*
* @param <E> The enum type subclass
* @author Josh Bloch
* @author Neal Gafter
* @see Class#getEnumConstants()
* @see java.util.EnumSet
* @see java.util.EnumMap
* @since 1.5
*/
public abstract class Enum<E extends Enum<E>>
implements Comparable<E>, Serializable {
/**
* The name of this enum constant, as declared in the enum declaration.
* Most programmers should use the {@link #toString} method rather than
* accessing this field.
*/
private final String name;
/**
* Returns the name of this enum constant, exactly as declared in its
* enum declaration.
*
* <b>Most programmers should use the {@link #toString} method in
* preference to this one, as the toString method may return
* a more user-friendly name.</b> This method is designed primarily for
* use in specialized situations where correctness depends on getting the
* exact name, which will not vary from release to release.
*
* @return the name of this enum constant
*/
public final String name() {
return name;
}
/**
* The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this field. It is designed
* for use by sophisticated enum-based data structures, such as
* {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*/
private final int ordinal;
/**
* Returns the ordinal of this enumeration constant (its position
* in its enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this method. It is
* designed for use by sophisticated enum-based data structures, such
* as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*
* @return the ordinal of this enumeration constant
*/
public final int ordinal() {
return ordinal;
}
/**
* Sole constructor. Programmers cannot invoke this constructor.
* It is for use by code emitted by the compiler in response to
* enum type declarations.
*
* @param name - The name of this enum constant, which is the identifier
* used to declare it.
* @param ordinal - The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*/
protected Enum(String name, int ordinal) {
this.name = name;
this.ordinal = ordinal;
}
/**
* Returns the name of this enum constant, as contained in the
* declaration. This method may be overridden, though it typically
* isn't necessary or desirable. An enum type should override this
* method when a more "programmer-friendly" string form exists.
*
* @return the name of this enum constant
*/
public String toString() {
return name;
}
/**
* Returns true if the specified object is equal to this
* enum constant.
*
* @param other the object to be compared for equality with this object.
* @return true if the specified object is equal to this
* enum constant.
*/
public final boolean equals(Object other) {
return this==other;
}
/**
* Returns a hash code for this enum constant.
*
* @return a hash code for this enum constant.
*/
public final int hashCode() {
return super.hashCode();
}
/**
* Throws CloneNotSupportedException. This guarantees that enums
* are never cloned, which is necessary to preserve their "singleton"
* status.
*
* @return (never returns)
*/
protected final Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}
/**
* Compares this enum with the specified object for order. Returns a
* negative integer, zero, or a positive integer as this object is less
* than, equal to, or greater than the specified object.
*
* Enum constants are only comparable to other enum constants of the
* same enum type. The natural order implemented by this
* method is the order in which the constants are declared.
*/
public final int compareTo(E o) {
Enum<?> other = (Enum<?>)o;
Enum<E> self = this;
if (self.getClass() != other.getClass() && // optimization
self.getDeclaringClass() != other.getDeclaringClass())
throw new ClassCastException();
return self.ordinal - other.ordinal;
}
/**
* Returns the Class object corresponding to this enum constant's
* enum type. Two enum constants e1 and e2 are of the
* same enum type if and only if
* e1.getDeclaringClass() == e2.getDeclaringClass().
* (The value returned by this method may differ from the one returned
* by the {@link Object#getClass} method for enum constants with
* constant-specific class bodies.)
*
* @return the Class object corresponding to this enum constant's
* enum type
*/
@SuppressWarnings("unchecked")
public final Class<E> getDeclaringClass() {
Class<?> clazz = getClass();
Class<?> zuper = clazz.getSuperclass();
return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper;
}
/**
* Returns the enum constant of the specified enum type with the
* specified name. The name must match exactly an identifier used
* to declare an enum constant in this type. (Extraneous whitespace
* characters are not permitted.)
*
* <p>Note that for a particular enum type {@code T}, the
* implicitly declared {@code public static T valueOf(String)}
* method on that enum may be used instead of this method to map
* from a name to the corresponding enum constant. All the
* constants of an enum type can be obtained by calling the
* implicit {@code public static T[] values()} method of that
* type.
*
* @param <T> The enum type whose constant is to be returned
* @param enumType the {@code Class} object of the enum type from which
* to return a constant
* @param name the name of the constant to return
* @return the enum constant of the specified enum type with the
* specified name
* @throws IllegalArgumentException if the specified enum type has
* no constant with the specified name, or the specified
* class object does not represent an enum type
* @throws NullPointerException if {@code enumType} or {@code name}
* is null
* @since 1.5
*/
public static <T extends Enum<T>> T valueOf(Class<T> enumType,
String name) {
T result = enumType.enumConstantDirectory().get(name);
if (result != null)
return result;
if (name == null)
throw new NullPointerException("Name is null");
throw new IllegalArgumentException(
"No enum constant " + enumType.getCanonicalName() + "." + name);
}
/**
* enum classes cannot have finalize methods.
*/
protected final void finalize() { }
/**
* prevent default deserialization
*/
private void readObject(ObjectInputStream in) throws IOException,
ClassNotFoundException {
throw new InvalidObjectException("can't deserialize enum");
}
private void readObjectNoData() throws ObjectStreamException {
throw new InvalidObjectException("can't deserialize enum");
}
}
推荐使用枚举定义常量
常量声明是每一个项目都不可或缺的，在Java 1.5之前，
我们只有两种方式的声明：类常量和接口常量，若在项目中
使用的是Java 1.5之前的版本基本上都是如此定义的。不过，
在1.5版以后有了改进，即新增了一种常量声明方式： 枚举声明常量。
枚举声明常量的优点
**（1）**枚举常量更简单
**（2）**枚举常量属于稳态型
**（3）**枚举具有内置方法
是java.lang.Enum的子类，该基类提供了诸如获得排序值的ordinal方法、compareTo比较方法等，大大简化了常量的访问。
**（4）**枚举可以自定义方法
举常量不仅可以定义静态方法，还可以定义非静态方法，而且还能够从根本上杜绝常量类被实例化。
package enumtest;
/**
* 枚举的优势
*
*/
public class TestSeason {
public static void main(String[] args) {
// 通过values方法获得所有的枚举项
for (Season s : Season.values()) {
System.out.println(s);
}
}
}
enum Season{
Spring,Summer,Autumn,Winter;
}
interface SeasonIntece{
int Spring=0;
int Summer=1;
int Autumn=2;
int Winter=3;
}
使用构造函数协助描述枚举项
枚举的属性：排序号，其默认值为0,1,2,3…
枚举描述：含义通过枚举的构造函数,声明每个枚举项（也就是枚举的实例）
必须具有属性和行为，这是对枚举的描述和补充，目的是使，枚举项表述的意义更加清晰准确
enum Season {
Spring("春"), Summer("夏"), Autumn("秋"), Winter("冬");
private String desc;
Season(String desc) {
this.desc = desc;
}
// 自定义方法
public static Season getComfortableSeason() {
return Spring;
}
//获取枚举描述
public String getDesc() {
return desc;
}
}
小心switch带来的空值异常，处理方法增加空判断
对于枚举类型很多的情况下，在default中添加: throw
new AssertionError(“没有该类型”);
使用valueOf前必须进行校验
package enumtest;
import java.util.Arrays;
import java.util.List;
public class ValueOfTest {
public static void main(String[] args) {
List<String> params = Arrays.asList("EXCELLENT", "aDD");
for (String name : params) {
// 查找直面值与name相同的枚举 项
// 添加代码
if(Score.contains(name)){
Score s = Score.valueOf(name);
if (s != null) {
// 有该枚举项时
System.out.println(s);
} else {
// 没有该枚举项时
System.out.println("无相关枚举项");
}
}
// Score s = Score.valueOf(name);
// if (s != null) {
// // 有该枚举项时
// System.out.println(s);
// } else {
// // 没有该枚举项时
// System.out.println("无相关枚举项");
// }
}
}
enum Score {
EXCELLENT, QUALIFIED, FAILED, ADD;
// 是否包含枚举项
public static boolean contains(String name) {
// 所有的枚举值
Score[] scores = values();
// 遍历查找
for (Score s : scores) {
if (s.name().equals(name))
return true;
}
return false;
}
}
}
/*
* Exception in thread "main" java.lang.IllegalArgumentException: No enum
* constant enumtest.ValueOfTest.Score.aDD
*
* public static＜T extends Enum＜T＞＞T valueOf（Class＜T＞enumType， String name）{
* //通过反射，从常量列表中查找 T result=enumType.enumConstantDirectory（）.get（name）；
* if（result！=null） return result； if（name==null） throw new
* NullPointerException（"Name is null"）； //最后排除无效参数异常 throw new
* IllegalArgumentException（"No enum const"+enumType+"."+name）； }
*/
（1） 枚举非静态方法实现工厂方法模式
用枚举实现工厂方法模式更简洁
package enumtest;
/**
* 工厂方法模式（Factory Method Pattern）是“创建对象的接口，让子类决定实例化哪一个类，并使一个类的实例化延迟到其子类”。
* 工厂方法模式在我们的开发工作中经常会用到。
* 这是最原始的工厂方法模式，有两个产品：福特汽车和别克汽车，然后通过工厂方法模式来生产。有了工厂方法模式，我们就不用关心一辆车具体是怎么生成的了
* ，只要告诉工厂“给我生产一辆福特汽车”就可以了
*/
// 抽象产品
interface Car {
};
// 具体产品类
class FordCar implements Car {
};
// 具体产品类
class BuickCar implements Car {
};
// 工厂类
public class CarFactory {
public static Car createCar(Class<? extends Car> c) {
try {
return c.newInstance();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
return null;
}
public static void main(String[] args) {
// 生产车辆
Car cat =CarFactory.createCar(FordCar.class);
}
}
package enumtest;
//枚举非静态方法实现工厂方法模式
public class CarFactoryEnum {
public static void main(String[] args) {
// 生产汽车
Car car = CarFactoryEnum1.BuickCar.create();
}
}
enum CarFactoryEnum1 {
// 定义工厂类能生产汽车的类型
FordCar, BuickCar;
// 生产汽车
public Car create() {
switch (this) {
case FordCar:
return new FordCar();
case BuickCar:
return new BuickCar();
default:
throw new AssertionError("无效参数");
}
}
}
（2）通过抽象方法生成产品
enum CarFactory3{
FordCar{
public Car create(){
return new FordCar();
}
},BuickCar{
public Car create(){
return new BuickCar();
}
};
// 首先定义一个抽象制造方法create，然后每个枚举项自行实现。
public abstract Car create();
}
用枚举类型的工厂方法模式有以下三个优点：
（1）避免错误调用的发生
（2）性能好，使用便捷
枚举类型的计算是以int类型的计算为基础的，这是最基本的操作，性能当
然会快，至于使用便捷，注意看客户端的调用，代码的字面意思就是“汽车
工厂，我要一辆别克汽车，赶快生产”。
（3）降低类间耦合
不管生产方法接收的是Class、String还是int的参数，都会成为客户端类的
负担，这些类并不是客户端需要的，而是因为工厂方法的限制必须输入的，
例如Class参数，对客户端main方法来说，它需要传递一个FordCar.class
参数才能生产一辆福特汽车，除了在create方法中传递该参数外，业务类
不需要改Car的实现类。这严重违背了迪米特原则（Law of Demeter，简称为LoD），也就是最少知识原则：一个对象应该对其他对象有最少的了解。
示例代码
/**
* 支付类型
*/
public enum PayType{
typeUnknow("未知支付类型" , -1), typeAlipay("支付宝支付" , 4), typeWechatPay("微信支付" , 5);
private String name;
private int value;
// 构造方法
private PayType(String name, int value) {
this.name = name;
this.value = value;
}
}
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