不要讓這個世界的複雜性阻礙你的前進。 要成爲一個行動主義者，將解決人類的不平等視爲己任。 它將成爲你生命中最重要的經歷之一。
——比爾·蓋茨在哈佛大學的演講
聲明枚舉
[public]enum 枚舉類型名稱[implements 接口名稱列表]
{
枚舉值；
變量成員聲明及初始化；
方法聲明及方法體；
}
簡單的例子：
/**
* 簡單的枚舉類型舉例
* @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;
}
/*
* 輸出 Excellent
* */
通過jad反編譯一下：
// 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
});
}
}
枚舉的特點：
- 枚舉定義實際上是定義了一個類
- 所有枚舉類型都隱含繼承（擴展）自Java.lang.Enum,因此枚舉類型不能再繼承任何其它類
- 枚舉類型中的類可以包括方法和變量
- 枚舉類型的構造方法必須是包內私有或者私有的。定義在枚舉開頭的常量會自動創建，不能顯示地調用枚舉類的構造方法。
枚舉類型的默認方法
- 靜態的values()方法用於獲得枚舉類型的枚舉值的數組
- toString方法返回枚舉值的字符串描述
- valueOf方法將以字符串形式表示的枚舉值轉化爲枚舉類型的對象
- Ordinal方法獲得對象在枚舉類型中的位置索引
最佳實踐 （該部分內容爲《編寫高質量代碼之java》學習筆記）
下邊貼一下jdk中Enum的源碼
/*
* 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");
}
}
推薦使用枚舉定義常量
常量聲明是每一個項目都不可或缺的，在Java 1.5之前，
我們只有兩種方式的聲明：類常量和接口常量，若在項目中
使用的是Java 1.5之前的版本基本上都是如此定義的。不過，
在1.5版以後有了改進，即新增了一種常量聲明方式： 枚舉聲明常量。
枚舉聲明常量的優點
**（1）**枚舉常量更簡單
**（2）**枚舉常量屬於穩態型
**（3）**枚舉具有內置方法
是java.lang.Enum的子類，該基類提供了諸如獲得排序值的ordinal方法、compareTo比較方法等，大大簡化了常量的訪問。
**（4）**枚舉可以自定義方法
舉常量不僅可以定義靜態方法，還可以定義非靜態方法，而且還能夠從根本上杜絕常量類被實例化。
package enumtest;
/**
* 枚舉的優勢
*
*/
public class TestSeason {
public static void main(String[] args) {
// 通過values方法獲得所有的枚舉項
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;
}
使用構造函數協助描述枚舉項
枚舉的屬性：排序號，其默認值爲0,1,2,3…
枚舉描述：含義通過枚舉的構造函數,聲明每個枚舉項（也就是枚舉的實例）
必須具有屬性和行爲，這是對枚舉的描述和補充，目的是使，枚舉項表述的意義更加清晰準確
enum Season {
Spring("春"), Summer("夏"), Autumn("秋"), Winter("冬");
private String desc;
Season(String desc) {
this.desc = desc;
}
// 自定義方法
public static Season getComfortableSeason() {
return Spring;
}
//獲取枚舉描述
public String getDesc() {
return desc;
}
}
小心switch帶來的空值異常，處理方法增加空判斷
對於枚舉類型很多的情況下，在default中添加: throw
new AssertionError(“沒有該類型”);
使用valueOf前必須進行校驗
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) {
// 查找直面值與name相同的枚舉 項
// 添加代碼
if(Score.contains(name)){
Score s = Score.valueOf(name);
if (s != null) {
// 有該枚舉項時
System.out.println(s);
} else {
// 沒有該枚舉項時
System.out.println("無相關枚舉項");
}
}
// Score s = Score.valueOf(name);
// if (s != null) {
// // 有該枚舉項時
// System.out.println(s);
// } else {
// // 沒有該枚舉項時
// System.out.println("無相關枚舉項");
// }
}
}
enum Score {
EXCELLENT, QUALIFIED, FAILED, ADD;
// 是否包含枚舉項
public static boolean contains(String name) {
// 所有的枚舉值
Score[] scores = values();
// 遍歷查找
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）{
* //通過反射，從常量列表中查找 T result=enumType.enumConstantDirectory（）.get（name）；
* if（result！=null） return result； if（name==null） throw new
* NullPointerException（"Name is null"）； //最後排除無效參數異常 throw new
* IllegalArgumentException（"No enum const"+enumType+"."+name）； }
*/
（1） 枚舉非靜態方法實現工廠方法模式
用枚舉實現工廠方法模式更簡潔
package enumtest;
/**
* 工廠方法模式（Factory Method Pattern）是“創建對象的接口，讓子類決定實例化哪一個類，並使一個類的實例化延遲到其子類”。
* 工廠方法模式在我們的開發工作中經常會用到。
* 這是最原始的工廠方法模式，有兩個產品：福特汽車和別克汽車，然後通過工廠方法模式來生產。有了工廠方法模式，我們就不用關心一輛車具體是怎麼生成的了
* ，只要告訴工廠“給我生產一輛福特汽車”就可以了
*/
// 抽象產品
interface Car {
};
// 具體產品類
class FordCar implements Car {
};
// 具體產品類
class BuickCar implements Car {
};
// 工廠類
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) {
// 生產車輛
Car cat =CarFactory.createCar(FordCar.class);
}
}
package enumtest;
//枚舉非靜態方法實現工廠方法模式
public class CarFactoryEnum {
public static void main(String[] args) {
// 生產汽車
Car car = CarFactoryEnum1.BuickCar.create();
}
}
enum CarFactoryEnum1 {
// 定義工廠類能生產汽車的類型
FordCar, BuickCar;
// 生產汽車
public Car create() {
switch (this) {
case FordCar:
return new FordCar();
case BuickCar:
return new BuickCar();
default:
throw new AssertionError("無效參數");
}
}
}
（2）通過抽象方法生成產品
enum CarFactory3{
FordCar{
public Car create(){
return new FordCar();
}
},BuickCar{
public Car create(){
return new BuickCar();
}
};
// 首先定義一個抽象製造方法create，然後每個枚舉項自行實現。
public abstract Car create();
}
用枚舉類型的工廠方法模式有以下三個優點：
（1）避免錯誤調用的發生
（2）性能好，使用便捷
枚舉類型的計算是以int類型的計算爲基礎的，這是最基本的操作，性能當
然會快，至於使用便捷，注意看客戶端的調用，代碼的字面意思就是“汽車
工廠，我要一輛別克汽車，趕快生產”。
（3）降低類間耦合
不管生產方法接收的是Class、String還是int的參數，都會成爲客戶端類的
負擔，這些類並不是客戶端需要的，而是因爲工廠方法的限制必須輸入的，
例如Class參數，對客戶端main方法來說，它需要傳遞一個FordCar.class
參數才能生產一輛福特汽車，除了在create方法中傳遞該參數外，業務類
不需要改Car的實現類。這嚴重違背了迪米特原則（Law of Demeter，簡稱爲LoD），也就是最少知識原則：一個對象應該對其他對象有最少的瞭解。
示例代碼
/**
* 支付類型
*/
public enum PayType{
typeUnknow("未知支付類型" , -1), typeAlipay("支付寶支付" , 4), typeWechatPay("微信支付" , 5);
private String name;
private int value;
// 構造方法
private PayType(String name, int value) {
this.name = name;
this.value = value;
}
}
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