Source reading: new Date Tour

Wind from the Java Date

That day asked a colleague Youmie have to understand the source code Date, colleagues said no to find out about. As usual date tools inside players Date, Zhefan answer probably will not be satisfied, then we are going to do some interviews with the athlete, let us re-recognize the Date class java.util package provides it.

Rain

public static void main(String[] args) {
    Date date = new Date();
    System.out.println(date);
}

Let us look at the print results

Tue Jan 14 14:51:54 CST 2020

Should not output class Date do? For the hair so it seems we have to continue in-depth Date veteran, opened its layers of fog.

Fog to

 /**
     * Allocates a <code>Date</code> object and initializes it so that
     * it represents the time at which it was allocated, measured to the
     * nearest millisecond.
     *
     * @see     java.lang.System#currentTimeMillis()
     */
    public Date() {
        this(System.currentTimeMillis());
    }

    /**
     * Allocates a <code>Date</code> object and initializes it to
     * represent the specified number of milliseconds since the
     * standard base time known as "the epoch", namely January 1,
     * 1970, 00:00:00 GMT.
     *
     * @param   date   the milliseconds since January 1, 1970, 00:00:00 GMT.
     * @see     java.lang.System#currentTimeMillis()
     */
    public Date(long date) {
        fastTime = date;
    }

At this point, we see the Date constructor is called System.currentTimeMillis () system time, and finally had to fastTime assignment would be finished, but why the result is a string it, since Date itself is no problem, then it is System.out.println () which do the things we do not know, it seems that the source of the road long way to continue it.

	/**
     * Prints an Object and then terminate the line.  This method calls
     * at first String.valueOf(x) to get the printed object's string value,
     * then behaves as
     * though it invokes <code>{@link #print(String)}</code> and then
     * <code>{@link #println()}</code>.
     *
     * @param x  The <code>Object</code> to be printed.
     */
    public void println(Object x) {
        String s = String.valueOf(x);
        synchronized (this) {
            print(s);
            newLine();
        }
    }

The truth seems about to emerge, let us look String.valueOf (x) did what it

	/**
     * Returns the string representation of the {@code Object} argument.
     *
     * @param   obj   an {@code Object}.
     * @return  if the argument is {@code null}, then a string equal to
     *          {@code "null"}; otherwise, the value of
     *          {@code obj.toString()} is returned.
     * @see     java.lang.Object#toString()
     */
    public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }

	/**
     * Converts this <code>Date</code> object to a <code>String</code>
     * of the form:
     * <blockquote><pre>
     * dow mon dd hh:mm:ss zzz yyyy</pre></blockquote>
     * where:<ul>
     * <li><tt>dow</tt> is the day of the week (<tt>Sun, Mon, Tue, Wed,
     *     Thu, Fri, Sat</tt>).
     * <li><tt>mon</tt> is the month (<tt>Jan, Feb, Mar, Apr, May, Jun,
     *     Jul, Aug, Sep, Oct, Nov, Dec</tt>).
     * <li><tt>dd</tt> is the day of the month (<tt>01</tt> through
     *     <tt>31</tt>), as two decimal digits.
     * <li><tt>hh</tt> is the hour of the day (<tt>00</tt> through
     *     <tt>23</tt>), as two decimal digits.
     * <li><tt>mm</tt> is the minute within the hour (<tt>00</tt> through
     *     <tt>59</tt>), as two decimal digits.
     * <li><tt>ss</tt> is the second within the minute (<tt>00</tt> through
     *     <tt>61</tt>, as two decimal digits.
     * <li><tt>zzz</tt> is the time zone (and may reflect daylight saving
     *     time). Standard time zone abbreviations include those
     *     recognized by the method <tt>parse</tt>. If time zone
     *     information is not available, then <tt>zzz</tt> is empty -
     *     that is, it consists of no characters at all.
     * <li><tt>yyyy</tt> is the year, as four decimal digits.
     * </ul>
     *
     * @return  a string representation of this date.
     * @see     java.util.Date#toLocaleString()
     * @see     java.util.Date#toGMTString()
     */
    public String toString() {
        // "EEE MMM dd HH:mm:ss zzz yyyy";
        BaseCalendar.Date date = normalize();
        StringBuilder sb = new StringBuilder(28);
        int index = date.getDayOfWeek();
        if (index == BaseCalendar.SUNDAY) {
            index = 8;
        }
        convertToAbbr(sb, wtb[index]).append(' ');                        // EEE
        convertToAbbr(sb, wtb[date.getMonth() - 1 + 2 + 7]).append(' ');  // MMM
        CalendarUtils.sprintf0d(sb, date.getDayOfMonth(), 2).append(' '); // dd

        CalendarUtils.sprintf0d(sb, date.getHours(), 2).append(':');   // HH
        CalendarUtils.sprintf0d(sb, date.getMinutes(), 2).append(':'); // mm
        CalendarUtils.sprintf0d(sb, date.getSeconds(), 2).append(' '); // ss
        TimeZone zi = date.getZone();
        if (zi != null) {
            sb.append(zi.getDisplayName(date.isDaylightTime(), TimeZone.SHORT, Locale.US)); // zzz
        } else {
            sb.append("GMT");
        }
        sb.append(' ').append(date.getYear());  // yyyy
        return sb.toString();
    }

Scattered clouds, fog open

At this point telling the truth, will automatically call the toString method, there have been the end result when you print a class object. Of course, at the same time our Secret Date, but also inseparable from fastTime and help cdate two partners.

    private transient long fastTime;

    /*
     * If cdate is null, then fastTime indicates the time in millis.
     * If cdate.isNormalized() is true, then fastTime and cdate are in
     * synch. Otherwise, fastTime is ignored, and cdate indicates the
     * time.
     */
    private transient BaseCalendar.Date cdate;

Special Mention guest star from BaseCalendar sun.util.calendar of.

public abstract class BaseCalendar extends AbstractCalendar {
    public static final int JANUARY = 1;
    public static final int FEBRUARY = 2;
    public static final int MARCH = 3;
    public static final int APRIL = 4;
    public static final int MAY = 5;
    public static final int JUNE = 6;
    public static final int JULY = 7;
    public static final int AUGUST = 8;
    public static final int SEPTEMBER = 9;
    public static final int OCTOBER = 10;
    public static final int NOVEMBER = 11;
    public static final int DECEMBER = 12;
    public static final int SUNDAY = 1;
    public static final int MONDAY = 2;
    public static final int TUESDAY = 3;
    public static final int WEDNESDAY = 4;
    public static final int THURSDAY = 5;
    public static final int FRIDAY = 6;
    public static final int SATURDAY = 7;
    private static final int BASE_YEAR = 1970;
    ...
    }

On medical knowledge often said that "pain is unreasonable, General does not hurt," knowledge is also true, not General unknown, the saying goes through a physical and 100 Liming.

Extra

We found the road debug initial time is January 1, 1970, why this happen, it is not only Java, almost all the time from this moment language is counting. Because java originated in UNIX systems, and UNIX think that at 0:00 on January 1, 1970 is the time era. With all the results we see.