java - 如何从 Date 对象中修剪分钟、小时和秒？

Question

我需要制作以日期为键的 map 。如果 2 个日期对象具有相同的 getTime() 方法值，则它们是相等的。

我只对年月日感兴趣。我如何修剪不必要的小时和分钟以获得“明确”的日期？

Answer 1

您可以创建一个 trim 方法:

public static Date trim(Date date) {
      Calendar cal = Calendar.getInstance();
      cal.clear(); // as per BalusC comment.
      cal.setTime( date );
      cal.set(Calendar.HOUR_OF_DAY, 0);
      cal.set(Calendar.MINUTE, 0);
      cal.set(Calendar.SECOND, 0);
      cal.set(Calendar.MILLISECOND, 0);
      return cal.getTime();
 }

像这样使用它:

 map.put( trim( aDate ), xyz() );

...

 map.get( trim( otherDate ));

这是一个完整的工作示例:

import java.util.Calendar;
import java.util.Date;
import static java.util.Calendar.*;
import static java.lang.System.out;


public class DateTest {
    public static void main( String [] args )  throws InterruptedException {
            Date date = new Date();
            Thread.sleep(1);
            Date other = new Date();
            out.printf("equals? = %s, hashCode? = %s %n", (date.equals(other)), (date.hashCode() == other.hashCode()));

            Date todayeOne = trim( date );
            Date todayTwo  = trim( date );


            out.printf("equals? = %s, hashCode? = %s %n", (todayeOne.equals(todayTwo)), (todayeOne.hashCode() == todayTwo.hashCode()));

    }

    public static Date trim(Date date) {
          Calendar cal = Calendar.getInstance();
          cal.setTime( date );
          cal.set(HOUR_OF_DAY, 0);
          cal.set(MINUTE, 0);
          cal.set(SECOND, 0);
          cal.set(MILLISECOND, 0);
          return cal.getTime();
     }

}

输出:

$ java DateTest 
equals? = false, hashCode? = false 
equals? = true, hashCode? = true