Java中的 util.date 类方法和示例

util.date 类方法
以下是一些重要的日期类方法：

.toString() : Java.util.Date.tostring()方法是Java.util.Date 类方法。它显示当前日期和时间。
这里 Date 对象被转换为字符串并表示为：
```
day mon dd hh:mm:ss zz yyyy 
```
日：星期几
星期一：月份
dd : 一个月中的一天
hh：小时
毫米：分钟
ss：第二
zz : 时区
yyyy : 年份到小数点后 4 位
```
Syntax:
public String toString()
Return:
a string representation of the given date.
```
.setTime() ： Java.util.Date.setTime()方法是Java.util.Date 类方法。将此 Date 对象设置为表示 1970 年 1 月 1 日 00:00:00 GMT 之后的时间毫秒的时间点。
```
Syntax:
public void setTime(long time)
Parameters:
time : the number of milliseconds.
```

.hashCode() ： Java.util.Date.hashCode()方法是Java.util.Date 类方法。返回 Date 对象的哈希码值。结果是 getTime() 方法返回的原始 long 值的两半的异或。

Syntax:
public int hashCode()
Return:
a hash code value for the Date object.

Java代码来说明 .toString()、setTime()、hashCode() 方法的使用。

// Java Program explaining util.date class methods//
// use of .toString(), setTime(), hashCode() methods
import java.util.*;  // class having access to Date class methods
  
public class NewClass
{
    public static void main(String[] args)
    {
        Date mydate = new Date();
  
        // Displaying the current date and time
        System.out.println("System date : "+ mydate.toString() );
  
        // Is used to set time by milliseconds. Adds 15680 
        // milliseconds to January 1, 1970 to get new time.
        mydate.setTime(15680);
  
        System.out.println("Time after setting:  " + mydate.toString());
  
        int d = mydate.hashCode();
        System.out.println("Amount (in ms) by which time" + 
                           " is shifted :  " + d);
    }
}

Java代码的输出：

System date : Tue Nov 01 02:37:18 IST 2016
Time after setting:  Thu Jan 01 05:30:15 IST 1970
Amount (in milliseconds)  by which time is shifted :  15680

.after( ) : Java.util.Date.after()方法测试当前日期是否在给定日期之后。

Syntax:
public boolean after(Date d)
Parameters:
d : date
Return:
true if and only if the instant represented by this Date object is strictly later
than the instant represented by 'when'; else false
Exception:
NullPointerException - if Date object is null.

.clone() : Java.util.Date.clone()方法返回传递的 Date 对象的副本。
```
Syntax:
public Object clone()
Return:
a clone of this instance.
```

.before() : Java.util.Date.after()方法测试当前日期是否早于给定日期。

Syntax:
public boolean before(Date d)
Parameters:
d : date
Return:
true if and only if the instant represented by this Date object is strictly earlier
than the instant represented by 'when'; else false
Exception:
NullPointerException - if when is null.

Java代码说明 after()、clone()、before() 方法的使用。

// JAVA program explaining Date class methods
// after(), clone(), before()
import java.util.Date;
public class NewClass
{
    public static void main(String[] args)
    {
        // create 2 dates
        Date date1 = new Date(2016, 11, 18);
        Date date2 = new Date(1997, 10, 27);
  
        // Use of after() to check date2 is after date1
        boolean a = date2.after(date1);
        System.out.println("Is date2 is after date1 : " + a);
  
        // Use of after() to check date2 is after date1
        a = date1.after(date2);
        System.out.println("Is date1 is after date2 : " + a);
        System.out.println("");
  
        // Use of clone() method
        Object date3 = date1.clone();
        System.out.println("Cloned date3 :" + date3.toString());
        System.out.println("");
  
        // Use of before() to check date2 is after date1
        boolean b = date2.before(date1);
        System.out.println("Is date2 is before date1 : " + a);
    }
}

输出：

Is date2 is after date1 : false
Is date1 is after date2 : true

Cloned date3 :Mon Dec 18 00:00:00 IST 3916

Is date2 is before date1 : true

.compareTo() : Java.util.Date.compareTo()方法比较两个日期并根据比较结果为 -1、0 或 1。

Syntax:
public int compareTo(Date argDate)
Parameters:
argDate : another date to compare with
Result:
0  : if the argumented date = given date.
-1 : if the argumented date > given date.
1  : if the argumented date < given date.

.equals() : Java.util.Date.equals()方法根据它们的毫秒差检查两个日期是否相等。

Syntax:
public boolean equals(Object argDate)
Parameters:
argDate : another date to compare with
Result:
true if both the date are equal; else false.

.getTime() : Java.util.Date.getTime()方法产生参数日期的毫秒计数，参考 1970 年 1 月 1 日 00:00:00 GMT。

Syntax:
public long getTime()
Result:
milliseconds of the argumented date, referencing January 1, 1970, 00:00:00 GMT.

Java代码说明 compareTo()、getTime()、equals() 方法的使用。

// Java program explaining Date class methods
// compareTo(), getTime(), equals()
import java.util.*;
public class NewClass
{
    public static void main(String[] args)
    {
        Date d1 = new Date(97, 10, 27);
        Date d2 = new Date(97, 6, 12);
  
        // Use of compareto() method
        int comparison = d1.compareTo(d2);    // d1 > d2
        int comparison2 = d2.compareTo(d1);   // d2 > d1
        int comparison3 = d1.compareTo(d1);   // d1 = d1
  
        System.out.println("d1 > d2 : " + comparison);
        System.out.println("d1 < d2 : " + comparison2);
        System.out.println("d1 = d1 : " + comparison3);
        System.out.println("");
  
        // Use of equal() method
        boolean r1 = d1.equals(d2);
        System.out.println("Result of equal() r1 : " + r1);
  
        boolean r2 = d1.equals(d1);
        System.out.println("Result of equal() r2 : " + r2);
        System.out.println("");
  
  
        // Use of getTime() method
        long count1 = d1.getTime();
        long count2 = d1.getTime();
        System.out.println("Milliseconds of d1 : " + count1);
        System.out.println("Milliseconds of d2 : " + count2);
    }
}

输出：

d1 > d2 : 1
d1 < d2 : -1
d1 = d1 : 0

Result of equal() r1 : false
Result of equal() r2 : true

Milliseconds of d1 : 880569000000
Milliseconds of d2 : 880569000000