java基础系列--Date类
   unique    发表于:2017-07-07 07:45:08
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原文: 唯一浩哥

1、Date类概述

  Date类是从JDK1.1就开始存在的老类,其提供了针对日期进行操作的诸多方法,但其却一直饱受诟病,不同的起始编号,国际化的低支持,JDK官方也认识到这个问题,后台提出使用Calendar类进行日期操作,日期的格式化交给DateFormat,虽然我们已经不再使用Date类中的大多数方法,但是还有一部分保留的内容指的我们一谈。

2、构造器

Date类之前有6大构造器,其中四个已经标注弃用,我们我不再看他,我们重点看另外两个:

/**
 2      * Allocates a <code>Date</code> object and initializes it so that
 3      * it represents the time at which it was allocated, measured to the
 4      * nearest millisecond.
 5      *
 6      * @see     java.lang.System#currentTimeMillis()
 7      */
 8     public Date() {
 9         this(System.currentTimeMillis());
10     }
11 
12     /**
13      * Allocates a <code>Date</code> object and initializes it to
14      * represent the specified number of milliseconds since the
15      * standard base time known as "the epoch", namely January 1,
16      * 1970, 00:00:00 GMT.
17      *
18      * @param   date   the milliseconds since January 1, 1970, 00:00:00 GMT.
19      * @see     java.lang.System#currentTimeMillis()
20      */
21     public Date(long date) {
22         fastTime = date;
23     }

第一个构造器是无参构造器,通过调用System的currentTimeMillis()方法来获取当前时间戳,这个时间戳是从1970年到当前时间的毫秒级数据,第二个构造器,可以将一个毫秒级的数据定义为Date格式的日期。

3、常用方法

  Date中定义了诸多的日期操作方法,但是大多数都已弃用,只剩余为数不多的几个方法:

/**
     * Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT
     * represented by this <tt>Date</tt> object.
     *
     * @return  the number of milliseconds since January 1, 1970, 00:00:00 GMT
     *          represented by this date.
     */
    public long getTime() {
        return getTimeImpl();
    }

    /**
     * Sets this <code>Date</code> object to represent a point in time that is
     * <code>time</code> milliseconds after January 1, 1970 00:00:00 GMT.
     *
     * @param   time   the number of milliseconds.
     */    
    public void setTime(long time) {
        fastTime = time;
        cdate = null;
    }
    /**
     * Tests if this date is before the specified date.
     *
     * @param   when   a date.
     * @return  <code>true</code> if and only if the instant of time
     *            represented by this <tt>Date</tt> object is strictly
     *            earlier than the instant represented by <tt>when</tt>;
     *          <code>false</code> otherwise.
     * @exception NullPointerException if <code>when</code> is null.
     */
    public boolean before(Date when) {
        return getMillisOf(this) < getMillisOf(when);
    }
    /**
     * Tests if this date is after the specified date.
     *
     * @param   when   a date.
     * @return  <code>true</code> if and only if the instant represented
     *          by this <tt>Date</tt> object is strictly later than the
     *          instant represented by <tt>when</tt>;
     *          <code>false</code> otherwise.
     * @exception NullPointerException if <code>when</code> is null.
     */
    public boolean after(Date when) {
        return getMillisOf(this) > getMillisOf(when);
    }

上面显示的四个方法是Date类中现在还在使用的几个常用方法:

    long getTime()方法:返回从1970年00:00:00到Date对象所代表时间的毫秒级数据

    void setTime(long time)方法:设置一个Date对象用来代表从1970年00:00:00开始的一段毫秒级数据后所代表的时间点

    boolean before(Date when)方法:判断Date对象所代表的时间点是否在when所代表的时间点之前

    boolean after(Date when)方法:判断Date对象所代表的时间点是否在when所代表的时间点之后

4、其他

  Date类实现了java.io.Serializable接口,可以执行序列化与反序列化操作,在Date类中定义了writeObject(ObjectOutputStream s)方法和readObject(ObjectInputStream s)方法,分别用于在Date对象进行序列化和反序列化操作时将对象所代表的时间戳(long型数据)进行保存与获取,因为fastTime字段采用transient修饰,其内容会被序列化机制过滤掉,而这个字段内保存的是Date对象所代表时间的时间戳(long型)

有关内容详情见《Java常用API解析——序列化API(一)

1     private transient long fastTime;
 2 
 3     /**
 4      * Save the state of this object to a stream (i.e., serialize it).
 5      *
 6      * @serialData The value returned by <code>getTime()</code>
 7      *             is emitted (long).  This represents the offset from
 8      *             January 1, 1970, 00:00:00 GMT in milliseconds.
 9      */
10     private void writeObject(ObjectOutputStream s)
11          throws IOException
12     {
13         s.writeLong(getTimeImpl());
14     }
15 
16     /**
17      * Reconstitute this object from a stream (i.e., deserialize it).
18      */
19     private void readObject(ObjectInputStream s)
20          throws IOException, ClassNotFoundException
21     {
22         fastTime = s.readLong();
23     }

5、实例解析

 public static void main(String[] args) {
 2         Date now = new Date();//获取当前时间
 3         Date when = new Date(10201020097865L);//根据时间戳定义指定时间点
 4         boolean b1 = now.after(when);
 5         boolean b2 = now.before(when);
 6         Long d1 = now.getTime();
 7         Long d2 = when.getTime();
 8         
 9         System.out.println("now值为:"+now);
10         System.out.println("when值为:"+when);
11         System.out.println("b1值为:"+b1);
12         System.out.println("b2值为:"+b2);
13         System.out.println("d1值为:"+d1);
14         System.out.println("d2值为:"+d2);
15         
16     }

结果为:

now值为:Thu Jul 06 13:39:12 CST 2017
when值为:Tue Apr 04 16:41:37 CST 2293
b1值为:false
b2值为:true
d1值为:1499319552116
d2值为:10201020097865

6、总结

  Date类现在并不推荐使用,Java推荐了Calendar和DateFormat,甚至SimpleDateFormat来替代它,Date中仅剩的几个方法仍然还很实用,尤其是before与after方法,可以很方便的判断两个时间点的先后,当然判断的条件是将你的时间转换成Date格式,使用Date剩余的两个构造器实现即可,当然也可以使用推荐的SimpleDateFormat方法进行简单的格式化日期格式字符串的方式得到Date格式的时间点,这些会在稍后了解到!

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