Calendar的时间比对问题

发现

Calendar的比较时基于毫秒的，但是calendar使用set方法只能精确到秒，如果用Calendar进行同一秒的比较，可能会有…

会有如下代码：

public class MainActivity extends AppCompatActivity{
    private static final String TAG = "MainActivity";
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        Calendar calStart = Calendar.getInstance();
        calStart.set(2008,0,1,0,0,0);//2008.01.01 00:00:00
        
        setOnClickListener(v->{
            Calendar calEnd = Calendar.getInstance();
            calEnd.set(2008,0,1,0,0,0);//2008.01.01 00:00:00
            Log.i(TAG,"is startTime before endTime?"+isNotBefore(calStart,calEnd));
        },R.id.tv_main_comparative)
    }
    
    private boolean isNotBefore(Calendar calStart,Calendar calEnd){
        return !calStart.before(calEnd);
    }
    
    private void setOnClickListener(OnClickListener listener,@ResId int ... ids){
        for(int id:ids){
            findViewById(id).setOnClickListener(listener)
        }
    }
}
//日志结果
//首次点击：MainActivity:is startTime before endTime?false
//再次点击：MainActivity:is startTime before endTime?true
//多次点击：...或false或true

每次点击时的结果都是不尽相同的。为什么？

探索

我们尝试新增如下日志

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    setOnClickListener(v->{
        ...
        Log.i(TAG,"startTime.getTimeInMillis():"+calStart.getTimeInMillis()+"--->endTime.getTimeInMillis():"+calEnd.getTimeInMillis());
    },R.id.tv_main_comparative);
}

可以看到打出的日志（这里就不贴日志了，可以自行尝试）有毫秒级的差别，也就是：

1. calStart的真实时间是2008.01.01 00:00:00.xxx
   （注：xxx是Calendar calStart = Calendar.getInstance();时的毫秒数）
2. calEnd的真实时间的毫秒数则是点击瞬间的毫秒数
3. 由于毫秒数的不确定性，导致时而calStart大于、时而小于、时而等于calEnd

解决方案

解决方案有两种：

1. 在使用到Calendar的类中定义一个基础Calendar

   Calendar calBase = Calendar.getInstance();
   
   Calendar calStart = Calendar.getInstance();
   calStart.setTime(calBase.getTime());
   calStart.set(2008,0,1,0,0,0);
   
   Calendar calEnd = Calendar.getInstance();
   calEnd.setTime(calBase.getTime());
   calEnd.set(2008,0,1,0,0,0);

   千万别写成如下：

   //那我还得和你解释数据是怎么存储在堆和栈中... pass
   Calendar calStart = calBase;
   calStart.set(2008,0,1,0,0,0);
   
   Calendar calEnd = calBase;
   calStart.set(2008,0,1,0,0,0);

   这种写法其实都是在操作calBase，都不能算是另外操作calStart和calEnd.可尝试如下代码看看效果：

   public void initTry() {
       Calendar calBase = Calendar.getInstance();
       Calendar calStart = calBase;
       calBase.set(2008,1,1,0,0,0);
       Calendar calEnd = calBase;
       calEnd.set(2009,1,1,0,0,0);
       Log.i(TAG,"log\nCalendar0-->"+calStart.getTime().toString()+"\nCalendar1-->"+calEnd.getTime().toString());
   }
   
   //日志
   //log
   //    Calendar0-->Sun Feb 01 00:00:00 GMT+08:00 2009
   //    Calendar1-->Sun Feb 01 00:00:00 GMT+08:00 2009

2. 比较时根据精确度进行比较：

   public class CalendarUtils{
       public boolean isNotBefore(Calendar calStart, Calendar calEnd, @NotNull CalendarType type) {
   //        int startYear = calStart.get(Calendar.YEAR);
   //        int endYear = calEnd.get(Calendar.YEAR);
   //        if (startYear <= endYear) return false;
           if (ifBefore(calStart,calEnd,Calendar.YEAR)) return false;
           if (CalendarType.YEAR == type) return true;
   
   //        int startMonth = calStart.get(Calendar.MONTH);
   //        int endMonth = calEnd.get(Calendar.MONTH);
   //        if (startMonth <= endMonth) return false;
           if (ifBefore(calStart,calEnd,Calendar.MONTH)) return false;
           if (CalendarType.MONTH == type) return true;
   
   //        int startDay = calStart.get(Calendar.DAY_OF_MONTH);
   //        int endDay = calEnd.get(Calendar.DAY_OF_MONTH);
   //        if (startDay <= endDay) return false;
           if (ifBefore(calStart,calEnd,Calendar.DAY_OF_MONTH)) return false;
           if (CalendarType.DAY == type) return true;
   
   //        int startHour = calStart.get(Calendar.HOUR_OF_DAY);
   //        int endHour = calEnd.get(Calendar.HOUR_OF_DAY);
   //        if (startHour <= endHour) return false;
           if (ifBefore(calStart,calEnd,Calendar.HOUR_OF_DAY)) return false;
           if (CalendarType.HOUR == type) return true;
   
   //        int startMinute = calStart.get(Calendar.MINUTE);
   //        int endMinute = calEnd.get(Calendar.MINUTE);
   //        if (startMinute <= endMinute) return false;
           if (ifBefore(calStart,calEnd,Calendar.MINUTE)) return false;
           if (CalendarType.MINUTE == type) return true;
   
   //        int startSecond = calStart.get(Calendar.SECOND);
   //        int endSecond = calEnd.get(Calendar.SECOND);
   //        if (startSecond <= endSecond) return false;
           if (ifBefore(calStart,calEnd,Calendar.SECOND)) return false;
           if (CalendarType.SECOND == type) return true;
   
           return calStart.getTimeInMillis() >= calEnd.getTimeInMillis();
       }
       
       //后续做了变更，封装此方法，方便调整代码
       private boolean isBefore(Calendar calStart,Calendar calEnd,int calendarField){
           return calStart.get(calendarField)<=calEnd.get(calendarField);
       }
   
       public enum CalendarType {
           YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, MILLIS_SECOND
       }
   }

结语

写代码时务必要注重细节~

另：（免责声明）以上代码未在编辑器上校验过，纯手打~

如有问题，请根据实际情况修改代码

如有疑问，评论区见~