@IntDef的使用（替代枚举）

安卓开发应避免使用Enum（枚举类），因为相比于静态常量Enum会花费两倍以上的内存。使用@IntDef注解来代替枚举是个不错的选择。

添加android注解依赖：

compile 'com.android.support:support-annotations:25.1.0'

使用：

public class MainActivity extends Activity {
 
    //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:
                break;
            case MONDAY:
                break;
            case TUESDAY:
                break;
            case WEDNESDAY:
                break;
            case THURSDAY:
                break;
            case FRIDAY:
                break;
            case SATURDAY:
                break;
            default:
                break;
        }
 
    }
 
    public void setCurrentDay(@WeekDays int currentDay) {
        this.currentDay = currentDay;
    }
 
    @WeekDays
    public int getCurrentDay() {
        return currentDay;
    }
}

public class Toast {
    static final String TAG = "Toast";
    static final boolean localLOGV = false;

    /** @hide */
    /*定义部分*/
    @IntDef({LENGTH_SHORT, LENGTH_LONG})
    @Retention(RetentionPolicy.SOURCE)
    public @interface Duration {}

    public static final int LENGTH_SHORT = 0;
    public static final int LENGTH_LONG = 1;

    ...

    /*作为类型使用时*/
     /**
     * Set how long to show the view for.
     * @see #LENGTH_SHORT
     * @see #LENGTH_LONG
     */
    public void setDuration(@Duration int duration) {
        mDuration = duration;
    }

    /*做为返回值时*/
    /**
     * Return the duration.
     * @see #setDuration
     */
    @Duration
    public int getDuration() {
        return mDuration;
    }

}

public class MainActivity extends AppCompatActivity{

  public static final int STATE_NONE = -1;
  public static final int STATE_LOADING = 0;
  public static final int STATE_SUCCESS = 1;
  public static final int STATE_ERROR = 2;
  public static final int STATE_EMPTY = 3;

  private @State int state;

  public void setState(@State int state){
      this.state = state;
  } 

  @State
  public int getState() {
    return this.state;
  }

  @IntDef({STATE_EMPTY, STATE_ERROR, STATE_LOADING, STATE_NONE, STATE_SUCCESS})
  @Retention(RetentionPolicy.SOURCE)
  public @interface State {
  }
}

 @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
  }