Enum列舉詳解和EnumSet、EnumMap使用
阿新 • • 發佈：2019-02-06
一、Enum列舉
public enum Day {
MONDAY,TUESDAY,WEDNESDAY,THURSDAY,FRIDAY,SATURDAY,SUNDAY;
}一個簡單的測試類：
public class TestEnum {
public static void main(String[] args) {
Day today=Day.FRIDAY;
switch(today)
{
case MONDAY:
System.out.println("today is monday" 測試結果：
today is firdayDay列舉的本質就是一個類，編譯器會自動為我們生成Day類，通過反編譯得到該類如下：
final class Day extends Enum
{
//編譯器為我們新增的靜態的values()方法
public static Day[] values()
{
return (Day[])$VALUES.clone();
}
//編譯器為我們新增的靜態的valueOf()方法，注意間接呼叫了Enum也類的valueOf方法
public static Day valueOf(String s)
{
return (Day)Enum.valueOf(com/zejian/enumdemo/Day, s);
}
//私有建構函式
private Day(String s, int i)
{
super(s, i);
}
//前面定義的7種列舉例項
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
{
//例項化列舉例項
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
});
}
}可以清楚地看出每個列舉型別即星期數就是該Day類的一個例項物件，該構成方式和單例模式有些類似，故可以用只有一個列舉型別的列舉作為單例模式，而且列舉的構造器由編譯器管理安全性十分高，既可以防止反射破解也可以防止反序列破解。
Day類繼承了Enum類，下面看下Enum程式碼，可以更好理解列舉的方法：
public abstract class Enum<E extends Enum<E>>
implements Comparable<E>, Serializable {
private final String name; //列舉字串名稱
public final String name() {
return name;
}
private final int ordinal;//列舉順序值
public final int ordinal() {
return ordinal;
}
//列舉的構造方法，只能由編譯器呼叫
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物件引用，getClass()是Object的方法
Class<?> clazz = getClass();
//獲取父類Class物件引用
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則是列舉變數值
//enumConstantDirectory是class物件內部的方法，根據class物件獲取一個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);
}
//.....省略其他沒用的方法
}二、EnumMap
public class TestEnumMap {
public static void main(String[] args) {
Day today=Day.FRIDAY;
EnumMap enummap=new EnumMap(Day.class); //對應Day類的EnumMap
enummap.put(Day.MONDAY, "work work"); //第一個值為key不能為null，第二個值為values可以為null
enummap.put(Day.TUESDAY, "work work"); //第一個值填入列舉例項，第二個值為該列舉例項的記錄資訊
enummap.put(Day.WEDNESDAY, "work work");
enummap.put(Day.THURSDAY, "work work");
enummap.put(Day.FRIDAY, "work work");
enummap.put(Day.SATURDAY, "have fun");
enummap.put(Day.SUNDAY, "have fun");
System.out.println(enummap);
String job=(String) enummap.get(today); //用get方法輸入key值獲得今天的工作資訊
System.out.println("job is:"+job);
}
}
輸出結果為：
{MONDAY=work work, TUESDAY=work work, WEDNESDAY=work work, THURSDAY=work work, FRIDAY=work work, SATURDAY=have fun, SUNDAY=have fun}
job is:work work這個例子很清晰展示了EnumMap的基本用法
三、EnumSet
1.noneOf建立空佇列
public class TestEnumSet {
public static void main(String[] args) {
EnumSet enumset=EnumSet.noneOf(Day.class);//建立一個空的Day列舉型別的佇列
System.out.println("Before:"+enumset); //新增元素前狀態
enumset.add(Day.MONDAY); //新增三個元素
enumset.add(Day.TUESDAY);
enumset.add(Day.WEDNESDAY);
System.out.println("After:"+enumset); //新增元素後狀態
}
}執行結果：
Before:[]
After:[MONDAY, TUESDAY, WEDNESDAY]2.allOf建立滿佇列
public class TestEnumSet {
public static void main(String[] args) {
EnumSet enumset=EnumSet.allOf(Day.class);
System.out.println(enumset);
}
}執行結果：
[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY]3.range建立指定範圍佇列
public class TestEnumSet {
public static void main(String[] args) {
EnumSet enumset=EnumSet.range(Day.MONDAY, Day.FRIDAY);
System.out.println(enumset);
}
}執行結果：
[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY]4.complementOf補集建立佇列
public class TestEnumSet {
public static void main(String[] args) {
EnumSet enumset1=EnumSet.range(Day.MONDAY, Day.FRIDAY);
System.out.println("enumset1:"+enumset1);
EnumSet enumset2=EnumSet.complementOf(enumset1);//建立enumset1的補集enumset2
System.out.println("enumset2:"+enumset2);
}
}
執行結果：
enumset1:[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY]
enumset2:[SATURDAY, SUNDAY]5.copyOf複製建立
public class TestEnumSet {
public static void main(String[] args) {
EnumSet enumset1=EnumSet.range(Day.MONDAY, Day.FRIDAY);
System.out.println("enumset1:"+enumset1);
EnumSet enumset2=EnumSet.copyOf(enumset1);//複製enumset1建立enumset2
System.out.println("enumset2:"+enumset2);
}
}執行結果：
enumset1:[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY]
enumset2:[MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY]6.copyOf複製ArrayList建立
public class TestEnumSet {
public static void main(String[] args) {
ArrayList<Day> list=new ArrayList<Day>();
list.add(Day.MONDAY);
list.add(Day.TUESDAY);
list.add(Day.WEDNESDAY);
list.add(Day.WEDNESDAY);
System.out.println("list:"+list);
EnumSet enumset=EnumSet.copyOf(list);
System.out.println("enumset:"+enumset);
}
}
執行結果：
list:[MONDAY, TUESDAY, WEDNESDAY, WEDNESDAY]
enumset:[MONDAY, TUESDAY, WEDNESDAY]通過結果可以看出ArrayList內放置的元素可以重複，而EnumSet內放置的元素不重複，畢竟是列舉列嘛