java Calendar類 一個奇怪的現象

import java.util.Calendar;

import org.junit.Test;
public class CalendarDemo1 {

@Test
public void testCalendarSetMethod() throws InterruptedException {
Calendar cal1 = Calendar.getInstance();
cal1.set(2000, 7, 31, 0, 0, 0);// 2000-8-31
cal1.set(Calendar.MONTH, Calendar.SEPTEMBER);
cal1.set(Calendar.DAY_OF_MONTH, 30);
System.out.println(cal1.getTime());
}
}

輸出：Sat Sep 30 00:00:00 CST 2000

import java.util.Calendar;
import org.junit.Test;

public class CalendarDemo1 {

@Test
public void testCalendarSetMethod() throws InterruptedException {

Calendar cal1 = Calendar.getInstance();
cal1.set(2000, 7, 31, 0, 0, 0);// 2000-8-31
cal1.set(Calendar.MONTH, Calendar.SEPTEMBER);
cal1.getTime();
cal1.set(Calendar.DAY_OF_MONTH, 30);
System.out.println(cal1.getTime());
}
}

輸出：Mon Oct 30 00:00:00 CST 2000

根據以上理解java Calendar的set（）延緩計算的。查看getTime（）的原始碼可以看出，對時間進行了更新。

  public final Date getTime() {
        return new Date(getTimeInMillis());
    }

    public long getTimeInMillis() {
        if (!isTimeSet) {
   updateTime();
}
        return time;
    }

 private void updateTime() {
        computeTime();
// The areFieldsSet and areAllFieldsSet values are no longer
// controlled here (as of 1.5).
        isTimeSet = true;
    }