java枚舉詳解

時間 2020-05-19

標籤 java 枚舉詳解欄目 Java 简体版

原文原文鏈接

1、簡介java

public enum Day { MONDAY,TUESDAY,WEDNESDAY,THURSDAY,FRIDAY,SATURDAY,SUNDAY; }

一個簡單的測試類：web

public class TestEnum { public static void main(String[] args) { Day today=Day.FRIDAY; switch(today) { case MONDAY: System.out.println("today is monday"); break; case TUESDAY: System.out.println("today is tuesday"); break; case WEDNESDAY: System.out.println("today is webnesday"); break; case THURSDAY: System.out.println("today is thursday"); break; case FRIDAY: System.out.println("today is firday"); break; case SATURDAY: System.out.println("today is saturday"); break; case SUNDAY: System.out.println("today is sunday"); break; } } }

測試結果：安全

today is firday

Day枚舉的本質就是一個類，編譯器會自動爲咱們生成Day類，經過反編譯獲得該類以下：ide

final class Day extends Enum { //編譯器爲咱們添加的靜態的values()方法
    public static Day[] values() { return (Day[])$VALUES.clone(); } //編譯器爲咱們添加的靜態的valueOf()方法，注意間接調用了Enum也類的valueOf方法
    public static Day valueOf(String s) { return (Day)Enum.valueOf(com/zejian/enumdemo/Day, s); } //私有構造函數
    private Day(String s, int i) { super(s, i); } //前面定義的7種枚舉實例
    public static final Day MONDAY; public static final Day TUESDAY; public static final Day WEDNESDAY; public static final Day THURSDAY; public static final Day FRIDAY; public static final Day SATURDAY; public static final Day SUNDAY; private static final Day $VALUES[]; static { //實例化枚舉實例
        MONDAY = new Day("MONDAY", 0); TUESDAY = new Day("TUESDAY", 1); WEDNESDAY = new Day("WEDNESDAY", 2); THURSDAY = new Day("THURSDAY", 3); FRIDAY = new Day("FRIDAY", 4); SATURDAY = new Day("SATURDAY", 5); SUNDAY = new Day("SUNDAY", 6); $VALUES = (new Day[] { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }); } }

能夠清楚地看出每一個枚舉類型即星期數就是該Day類的一個實例對象，該構成方式和單例模式有些相似，故能夠用只有一個枚舉類型的枚舉做爲單例模式，並且枚舉的構造器由編譯器管理安全性十分高，既能夠防止反射破解也能夠防止反序列破解。函數

Day類繼承了Enum類，下面看下Enum代碼，能夠更好理解枚舉的方法：測試

public abstract class Enum<E extends Enum<E>> implements Comparable<E>, Serializable { private final String name; //枚舉字符串名稱

    public final String name() { return name; } private final int ordinal;//枚舉順序值

    public final int ordinal() { return ordinal; } //枚舉的構造方法，只能由編譯器調用
    protected Enum(String name, int ordinal) { this.name = name; this.ordinal = ordinal; } public String toString() { return name; } public final boolean equals(Object other) { return this==other; } //比較的是ordinal值
    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;//根據ordinal值比較大小
 } @SuppressWarnings("unchecked") public final Class<E> getDeclaringClass() { //獲取class對象引用，getClass()是Object的方法
        Class<?> clazz = getClass(); //獲取父類Class對象引用
        Class<?> zuper = clazz.getSuperclass(); return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper; } public static <T extends Enum<T>> T valueOf(Class<T> enumType, String name) { //enumType.enumConstantDirectory()獲取到的是一個map集合，key值就是name，value則是枚舉變量值 //enumConstantDirectory是class對象內部的方法，根據class對象獲取一個map集合的值 
        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); } //.....省略其餘沒用的方法
}

2、7種常見的用法this

用法一：常量
　　在JDK1.5 以前，咱們定義常量都是： public static fianl.... 。如今好了，有了枚舉，能夠把相關的常量分組到一個枚舉類型裏，並且枚舉提供了比常量更多的方法。spa

1 public enum Color { 2 RED, GREEN, BLANK, YELLOW 3 }

用法二：switchcode

1 enum Signal { 2 GREEN, YELLOW, RED 3 } 4 public class TrafficLight { 5     Signal color = Signal.RED; 6     public void change() { 7         switch (color) { 8         case RED: 9             color = Signal.GREEN; 10             break; 11         case YELLOW: 12             color = Signal.RED; 13             break; 14         case GREEN: 15             color = Signal.YELLOW; 16             break; 17 } 18 } 19 }

用法三：向枚舉中添加新方法對象

1 public enum Color { 2     RED("紅色", 1), GREEN("綠色", 2), BLANK("白色", 3), YELLO("黃色", 4); 3     // 成員變量 
 4     private String name; 5     private int index; 6     // 構造方法 
 7     private Color(String name, int index) { 8         this.name = name; 9         this.index = index; 10 } 11     // 普通方法 
12     public static String getName(int index) { 13         for (Color c : Color.values()) { 14             if (c.getIndex() == index) { 15                 return c.name; 16 } 17 } 18         return null; 19 } 20     // get set 方法 
21     public String getName() { 22         return name; 23 } 24     public void setName(String name) { 25         this.name = name; 26 } 27     public int getIndex() { 28         return index; 29 } 30     public void setIndex(int index) { 31         this.index = index; 32 } 33 }

用法四：覆蓋枚舉的方法

下面給出一個toString()方法覆蓋的例子。

1 public enum Color { 2     RED("紅色", 1), GREEN("綠色", 2), BLANK("白色", 3), YELLO("黃色", 4); 3     // 成員變量 
 4     private String name; 5     private int index; 6     // 構造方法 
 7     private Color(String name, int index) { 8         this.name = name; 9         this.index = index; 10 } 11     //覆蓋方法 
12 @Override 13     public String toString() { 14         return this.index+"_"+this.name; 15 } 16 }

用法五：實現接口

　　全部的枚舉都繼承自java.lang.Enum類。因爲Java 不支持多繼承，因此枚舉對象不能再繼承其餘類。

1 public interface Behaviour { 2     void print(); 3 String getInfo(); 4 } 5 public enum Color implements Behaviour{ 6     RED("紅色", 1), GREEN("綠色", 2), BLANK("白色", 3), YELLO("黃色", 4); 7     // 成員變量 
 8     private String name; 9     private int index; 10     // 構造方法 
11     private Color(String name, int index) { 12         this.name = name; 13         this.index = index; 14 } 15 //接口方法 
16 @Override 17     public String getInfo() { 18         return this.name; 19 } 20     //接口方法 
21 @Override 22     public void print() { 23         System.out.println(this.index+":"+this.name); 24 } 25 }

用法六：使用接口組織枚舉

1 public interface Food { 2     enum Coffee implements Food{ 3 BLACK_COFFEE,DECAF_COFFEE,LATTE,CAPPUCCINO 4 } 5     enum Dessert implements Food{ 6 FRUIT, CAKE, GELATO 7 } 8 }