java::util::Date Class Reference

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Collaboration diagram for java::util::Date:

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List of all members.


Detailed Description

The class Date represents a specific instant in time, with millisecond precision.

Prior to DK 1.1, the class Date had two additional functions. It allowed the interpretation of dates as year, month, day, hour, minute, and second values. It also allowed the formatting and parsing of date strings. Unfortunately, the API for these functions was not amenable to internationalization. As of DK 1.1, the Calendar class should be used to convert between dates and time fields and the DateFormat class should be used to format and parse date strings. The corresponding methods in Date are deprecated.

Although the Date class is intended to reflect coordinated universal time (UTC), it may not do so exactly, depending on the host environment of the Java Virtual Machine. Nearly all modern operating systems assume that 1 day = 24  60  60 = 86400 seconds in all cases. In UTC, however, about once every year or two there is an extra second, called a "leap second." The leap second is always added as the last second of the day, and always on December 31 or June 30. For example, the last minute of the year 1995 was 61 seconds long, thanks to an added leap second. Most computer clocks are not accurate enough to be able to reflect the leap-second distinction.

Some computer standards are defined in terms of Greenwich mean time (GMT), which is equivalent to universal time (UT). GMT is the "civil" name for the standard; UT is the "scientific" name for the same standard. The distinction between UTC and UT is that UTC is based on an atomic clock and UT is based on astronomical observations, which for all practical purposes is an invisibly fine hair to split. Because the earth's rotation is not uniform (it slows down and speeds up in complicated ways), UT does not always flow uniformly. Leap seconds are introduced as needed into UTC so as to keep UTC within 0.9 seconds of UT1, which is a version of UT with certain corrections applied. There are other time and date systems as well; for example, the time scale used by the satellite-based global positioning system (GPS) is synchronized to UTC but is not adjusted for leap seconds. An interesting source of further information is the U.S. Naval Observatory, particularly the Directorate of Time at: <blockquote>

     http://tycho.usno.navy.mil
 
</blockquote>

and their definitions of "Systems of Time" at: <blockquote>

     http://tycho.usno.navy.mil/systime.html
 
</blockquote>

In all methods of class Date that accept or return year, month, date, hours, minutes, and seconds values, the following representations are used:

In all cases, arguments given to methods for these purposes need not fall within the indicated ranges; for example, a date may be specified as January 32 and is interpreted as meaning February 1.

Author:
James Gosling

Arthur van Hoff

Alan Liu

Version:
1.69, 02/09/01
See also:
java.text.DateFormat

java.util.Calendar

java.util.TimeZone

Since:
DK1.0

Public Member Functions

 Date ()
 Allocates a Date object and initializes it so that it represents the time at which it was allocated, measured to the nearest millisecond.
 Date (jlong date)
 Allocates a Date 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.
virtual jlong getTime () const
 Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this Date object.
virtual void setTime (jlong time)
 Sets this Date object to represent a point in time that is time milliseconds after January 1, 1970 00:00:00 GMT.
virtual jboolean before (Date when) const
 Tests if this date is before the specified date.
virtual jboolean after (Date when) const
 Tests if this date is after the specified date.
virtual jboolean equals (const Object &obj) const
 Compares two dates for equality.
virtual jint compareTo (const Object &o) const
 Compares this Date to another Object.
virtual jint hashCode () const
 Returns a hash code value for this object.
virtual String toString () const
 Return a string representation of this Date.

Constructor & Destructor Documentation

java::util::Date::Date (  ) 

Allocates a Date object and initializes it so that it represents the time at which it was allocated, measured to the nearest millisecond.

See also:
java.lang.System::currentTimeMillis()

java::util::Date::Date ( jlong  date  ) 

Allocates a Date 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.

Parameters:
date the milliseconds since January 1, 1970, 00:00:00 GMT.
See also:
java.lang.System::currentTimeMillis()


Member Function Documentation

virtual jlong java::util::Date::getTime (  )  const [virtual]

Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this Date object.

Returns:
the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this date.

virtual void java::util::Date::setTime ( jlong  time  )  [virtual]

Sets this Date object to represent a point in time that is time milliseconds after January 1, 1970 00:00:00 GMT.

Parameters:
time the number of milliseconds.

virtual jboolean java::util::Date::before ( Date  when  )  const [virtual]

Tests if this date is before the specified date.

Parameters:
when a date.
Returns:
true if and only if the instant of time represented by this Date object is strictly earlier than the instant represented by when; false otherwise.

virtual jboolean java::util::Date::after ( Date  when  )  const [virtual]

Tests if this date is after the specified date.

Parameters:
when a date.
Returns:
true if and only if the instant represented by this Date object is strictly later than the instant represented by when; false otherwise.

virtual jboolean java::util::Date::equals ( const Object obj  )  const [virtual]

Compares two dates for equality.

The result is true if and only if the argument is not null and is a Date object that represents the same point in time, to the millisecond, as this object.

Thus, two Date objects are equal if and only if the getTime method returns the same long value for both.

Parameters:
obj the object to compare with.
Returns:
true if the objects are the same; false otherwise.
See also:
java.util.Date::getTime()

Reimplemented from java::lang::Object.

virtual jint java::util::Date::compareTo ( const Object o  )  const [virtual]

Compares this Date to another Object.

If the Object is a Date, this function behaves like compareTo(Date). Otherwise, it throws a ClassCastException (as Dates are comparable only to other Dates).

Parameters:
o the Object to be compared.
Returns:
the value 0 if the argument is a Date equal to this Date; a value less than 0 if the argument is a Date after this Date; and a value greater than 0 if the argument is a Date before this Date.
Exceptions:
ClassCastException if the argument is not a Date.
See also:
java.lang.Comparable
Since:
1.2

Reimplemented from java::lang::Comparable.

virtual jint java::util::Date::hashCode (  )  const [virtual]

Returns a hash code value for this object.

The result is the exclusive OR of the two halves of the primitive long value returned by the Date#getTime method. That is, the hash code is the value of the expression: <blockquote>

 (int)(this.getTime()^(this.getTime() >>> 32))
</blockquote>

Returns:
a hash code value for this object.

Reimplemented from java::lang::Object.

virtual String java::util::Date::toString (  )  const [virtual]

Return a string representation of this Date.

This method is intended to be used only for debugging purposes, and the format of the returned string may vary between implementations. The returned string may be empty but may not be null.

Returns:
a string representation of this date.

Reimplemented from java::lang::Object.


The documentation for this class was generated from the following file:
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