eg1：

Date date = new Date();
Calendar calendar = new GregorianCalendar();
calendar.setTime(date);
calendar.add(Calendar.DATE, 1);
date = calendar.getTime();
System.out.println(date);

eg2:

LocalDate localDate = LocalDate.now();
System.out.println(localDate);
localDate = localDate.minusDays
 
(-1);
System.out.println(localDate);

Calendar單例還是多例

package com.czxy;

import java.util.Calendar;

public class LeakTest {

   /**
    * @param args
    */
   public static void main(String[] args) {
       Calendar c1 = Calendar.getInstance();
       Calendar c2 = Calendar.getInstance();
       Runtime run1 =
 
 Runtime.getRuntime();
       Runtime run2 = Runtime.getRuntime();
       System.out.println("c1.equals(c2)  " + c1.equals(c2));
       System.out.println("c1 == c2 is " + (c1 == c2));
       System.out.println("run1.equals(run2) is " + run1.equals(run2));
       System.out.println("run1 == run2 is "
 
 + (run1 == run2));
   }
}

結果

單例模式表示返回的永遠是同一個物件，即物件的引用是相同的。
c1 == c2 false
說明Calendar 不是單例模式。從實際情況分析也應該不是單例，如果c1、c2是同一個物件，那麼你修改c1時會影響到c2，時間就沒辦法比較了。