枚举

使用 `enum` 定义枚举类

JDK 1.5 新增的 enum 关键字用于定义枚举类
枚举类和普通类的区别

使用 enum 定义的枚举类默认继承了 java.lang.Enum 类
枚举类的构造器只能使用 private 访问控制符
枚举类的所有实例必须在枚举类中显式列出(,分隔;结尾). 列出的实例系统会自动添加 public static final 修饰
所有的枚举类都提供了一个 values 方法, 该方法可以很方便地遍历所有的枚举值
JDK 1.5 中可以在 switch 表达式中使用枚举类的对象作为表达式, case 子句可以直接使用枚举值的名字, 无需添加枚举类作为限定
若枚举只有一个成员, 则可以作为一种单子模式的实现方式

枚举类的属性

枚举类对象的属性不应允许被改动, 所以应该使用 private final 修饰
枚举类使用 private final 修饰的属性应该在构造器中为其赋值
若枚举类显式的定义了带参数的构造器, 则在列出枚举值时也必须对应的传入参数

public enum Week {

	MONDAY("MON"), TUESDAY("TUES"), WEDNESDAY("WED"), THURSDAY("THUR"), FRIDAY("FRI"), SATURDAY("SAT"), SUNDAY("SUN");
	// 必须要先定义枚举类型，才能定义方法

	private String abbr;
	// 声明的一个字符串，用于保存缩写信息

	private Week(String abbr) {
		this.abbr = abbr;
	}
	// 声明的一个私有构造方法，该方法只能在构造枚举常量时被调用，可以使枚举元素具有指定的缩写信息

	public String getAbbr() {
		return abbr;
	}
	// 通过该方法获得枚举元素的缩写
}

public class EnumPractice {
	public static void main(String[] args) {
		for (Week week : Week.values()) {
			System.out.println("The order of " + week + " is " + week.ordinal());
			// 使用ordinal()方法查看枚举元素的顺序

			System.out.println("Compare to MONDAY : " + week.compareTo(Week.MONDAY));
			// 用compareTo()方法来将当前枚举元素与Week.MONDAY比较

			System.out.println("Equal to MONDAY ? " + week.equals(Week.MONDAY));
			// 用equals()方法来将当前枚举元素与Week.MONDAY比较

			System.out.println("Equal to MONDAY by == ? " + (week == Week.MONDAY));
			// 用比较运算符==来将当前枚举元素与Week.MONDAY比较
			// 从这里你应该可以看到，可以直接使用==来比较两个枚举元素

			System.out.println("Name : " + week.name());
			// 使用name()方法获得枚举元素的名称

			System.out.println("Abbreviation : " + week.getAbbr());
			// 使用自定义的getAbbr()方法输出星期的缩写

			System.out.println("-------------------");
			// 在每次循环快要结束时，打印一条分割线用于区分
		}
	}
}

点击查看结果

``` The order of MONDAY is 0 Compare to MONDAY : 0 Equal to MONDAY ? true Equal to MONDAY by == ? true Name : MONDAY Abbreviation : MON ------------------- The order of TUESDAY is 1 Compare to MONDAY : 1 Equal to MONDAY ? false Equal to MONDAY by == ? false Name : TUESDAY Abbreviation : TUES ------------------- The order of WEDNESDAY is 2 Compare to MONDAY : 2 Equal to MONDAY ? false Equal to MONDAY by == ? false Name : WEDNESDAY Abbreviation : WED ------------------- The order of THURSDAY is 3 Compare to MONDAY : 3 Equal to MONDAY ? false Equal to MONDAY by == ? false Name : THURSDAY Abbreviation : THUR ------------------- The order of FRIDAY is 4 Compare to MONDAY : 4 Equal to MONDAY ? false Equal to MONDAY by == ? false Name : FRIDAY Abbreviation : FRI ------------------- The order of SATURDAY is 5 Compare to MONDAY : 5 Equal to MONDAY ? false Equal to MONDAY by == ? false Name : SATURDAY Abbreviation : SAT ------------------- The order of SUNDAY is 6 Compare to MONDAY : 6 Equal to MONDAY ? false Equal to MONDAY by == ? false Name : SUNDAY Abbreviation : SUN ------------------- ```

public class JavaSwitchEnumExample {
	public enum Day {
		Sun, Mon, Tue, Wed, Thu, Fri, Sat
	}
	public static void main(String args[]) {
		Day[] DayNow = Day.values();
		for (Day Now : DayNow) {
			switch (Now) {
			case Sun:
				System.out.println("Sunday");
				break;
			case Mon:
				System.out.println("Monday");
				break;
			case Tue:
				System.out.println("Tuesday");
				break;
			case Wed:
				System.out.println("Wednesday");
				break;
			case Thu:
				System.out.println("Thursday");
				break;
			case Fri:
				System.out.println("Friday");
				break;
			case Sat:
				System.out.println("Saturday");
				break;
			}
		}
	}
}

点击查看结果

``` Sunday Monday Tuesday Wednesday Thursday Friday Saturday ```

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

public class EnumTest {
	Day day;

	public EnumTest(Day day) {
		this.day = day;
	}

	public void tellItLikeItIs() {
		switch (day) {
		case MONDAY:
			System.out.println("Mondays are bad.");
			break;

		case FRIDAY:
			System.out.println("Fridays are better.");
			break;

		case SATURDAY:
		case SUNDAY:
			System.out.println("Weekends are best.");
			break;

		default:
			System.out.println("Midweek days are so-so.");
			break;
		}
	}

	public static void main(String[] args) {
		EnumTest firstDay = new EnumTest(Day.MONDAY);
		firstDay.tellItLikeItIs();
		EnumTest thirdDay = new EnumTest(Day.WEDNESDAY);
		thirdDay.tellItLikeItIs();
		EnumTest fifthDay = new EnumTest(Day.FRIDAY);
		fifthDay.tellItLikeItIs();
		EnumTest sixthDay = new EnumTest(Day.SATURDAY);
		sixthDay.tellItLikeItIs();
		EnumTest seventhDay = new EnumTest(Day.SUNDAY);
		seventhDay.tellItLikeItIs();
	}
}

点击查看结果

``` Mondays are bad. Midweek days are so-so. Fridays are better. Weekends are best. Weekends are best. ```

参考资料：
https://docs.oracle.com/javase/8/docs/api/java/lang/Enum.html

枚举

枚举

使用 enum 定义枚举类

枚举类的属性

使用 `enum` 定义枚举类