Android中Enum(枚举)的使用

常规用法

在Java中，被 enum 关键字修饰的类型就是枚举类型

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

所以刚才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

大佬们说枚举肥肠占用内存

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;

    @Override
    protected 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;
    }

    @WeekDays
    public 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;
    }

    @WeekDays
    int currentDay = WeekDays.SUNDAY;

    @Override
    protected 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;
    }

    @WeekDays
    public 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