【达内课程】Enum(枚举)的使用

本文主要是介绍【达内课程】Enum(枚举)的使用，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

常规用法

在Java中，被 enum （enumeration）关键字修饰的类型就是枚举类型，是一组固定选项，代替整数代码常量选项，提供类型安全的选项，例如：

public enum TypeEnum {TYPE_IMAGE,TYPE_VIDEO,
}

如果枚举不添加任何方法，枚举值默认为从0开始的有序数值。以 TypeEnum 枚举类型举例，它的枚举常量依次为

TYPE_IMAGE：0，TYPE_VIDEO：1

每个选项，本质是 TypeEnum 类型的实例，所以刚才的枚举的代码等价于调用了两次Enum(String name, int ordinal)），即

new Enum<TypeEnum>("TYPE_IMAGE", 0);
new Enum<TypeEnum>("TYPE_VIDEO", 1);

遍历枚举

for(TypeEnum t:TypeEnum.values()){Log.d("TypeEnum",""+t+":"+t.ordinal()+","+t.name());
}

查看运行结果

作为switch的判断条件

使用枚举作为 switch 语句判断条件能让我们的代码可读性更强：

TypeEnum type = TypeEnum.TYPE_IMAGE;switch (type) {case TYPE_IMAGE:Log.d("TypeEnum", "图片类型");break;case TYPE_VIDEO:Log.d("TypeEnum", "视频类型");break;default:Log.d("TypeEnum", "类型错误");break;}

运行结果

给 enum 自定义属性和方法

枚举类型的构造函数是私有的 private，所以不能通过 New 来创建枚举类型，正因如此才保证了枚举数据类型的安全，不会被更改

public enum Color {RED("red",1),YELLOW("yellow",2),BLUE("blue",3);private String name;private int index;Color(String name,int index){this.name = name;this.index = index;}public String getName() {return name;}public int getIndex() {return index;}
}

		Color color = Color.RED;switch (color) {case RED:Log.d("Color", "红色象征热情"+color.getIndex());break;case YELLOW:Log.d("Color", "黄色象征活力"+color.getIndex());break;case BLUE:Log.d("Color", "蓝色象征深沉"+color.getIndex());break;default:Log.d("Color", "不存在的颜色");break;}

再举个栗子

public enum LanguageEnum {LANGUAGE("language"),LANGUAGE_zh("zh"),LANGUAGE_en("en");private String language;//自定义属性/**构造函数，枚举类型只能为私有*/LanguageEnum(String language) {this.language = language;}public String getLanguage(){return language;}
}

Log.d("TTT","语言类型为"+LanguageEnum.LANGUAGE_zh.getLanguage());

日志输出

语言类型为zh

替代方案

大佬们说枚举很占用内存

其实 Android 为我们提供了枚举的替代方案 —— 注解方式（@IntDef、@StringDef、@LongDef），其性能远远高于枚举，下面我们就来说说如何使用

首先添加依赖

implementation 'com.android.support:support-annotations:28.0.0'

public class MainActivity extends AppCompatActivity {//先定义 常量public static final int SUNDAY = 0;public static final int MONDAY = 1;public static final int TUESDAY = 2;public static final int WEDNESDAY = 3;public static final int THURSDAY = 4;public static final int FRIDAY = 5;public static final int SATURDAY = 6;//用 @IntDef "包住" 常量；// @Retention 定义策略// 声明构造器@IntDef({SUNDAY, MONDAY,TUESDAY,WEDNESDAY,THURSDAY,FRIDAY,SATURDAY})@Retention(RetentionPolicy.SOURCE)public @interface WeekDays {}@WeekDays int currentDay = SUNDAY;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);setCurrentDay(WEDNESDAY);//声明变量@WeekDays int today = getCurrentDay();switch (today){case SUNDAY:Log.d("TTT","SUNDAY");break;case MONDAY:Log.d("TTT","MONDAY");break;case TUESDAY:Log.d("TTT","TUESDAY");break;case WEDNESDAY:Log.d("TTT","WEDNESDAY");break;case THURSDAY:Log.d("TTT","THURSDAY");break;case FRIDAY:Log.d("TTT","FRIDAY");break;case SATURDAY:Log.d("TTT","SATURDAY");break;default:break;}}public void setCurrentDay(@WeekDays int currentDay) {this.currentDay = currentDay;}@WeekDayspublic int getCurrentDay() {return currentDay;}
}

我们可以换种写法，差不多，只不过是在初始化注解类的时候给成员赋值

public class SecondActivity extends AppCompatActivity {//注解类的成员默认就是 public static final 修饰的@IntDef({WeekDays.SUNDAY, WeekDays.MONDAY, WeekDays.TUESDAY, WeekDays.WEDNESDAY, WeekDays.THURSDAY, WeekDays.FRIDAY, WeekDays.SATURDAY})@Retention(RetentionPolicy.SOURCE)public @interface WeekDays {int SUNDAY = 0;int MONDAY = 1;int TUESDAY = 2;int WEDNESDAY = 3;int THURSDAY = 4;int FRIDAY = 5;int SATURDAY = 6;}@WeekDaysint currentDay = WeekDays.SUNDAY;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_second);setCurrentDay(WeekDays.WEDNESDAY);int today = getCurrentDay();switch (today) {case WeekDays.SUNDAY:Log.d("TTT", "SUNDAY");break;case WeekDays.MONDAY:Log.d("TTT", "MONDAY");break;case WeekDays.TUESDAY:Log.d("TTT", "TUESDAY");break;case WeekDays.WEDNESDAY:Log.d("TTT", "WEDNESDAY");break;case WeekDays.THURSDAY:Log.d("TTT", "THURSDAY");break;case WeekDays.FRIDAY:Log.d("TTT", "FRIDAY");break;case WeekDays.SATURDAY:Log.d("TTT", "SATURDAY");break;}}public void setCurrentDay(@WeekDays int currentDay) {this.currentDay = currentDay;}@WeekDayspublic int getCurrentDay() {return currentDay;}
}

@IntDef用于简化从枚举到整型值的转换，实现 String 类型的效果时需用@StringDef，还有 @LongDef

Java枚举：小小enum，优雅而干净

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