java.util
Class Date

java.lang.Object
  |
  +--java.util.Date

All Implemented Interfaces:

Cloneable, Comparable, Serializable

public class Date

extends Object

implements Serializable, Cloneable, Comparable

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

Prior to JDK 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 JDK 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:

     http://tycho.usno.navy.mil
 

and their definitions of "Systems of Time" at:

     http://tycho.usno.navy.mil/systime.html
 

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

• A year y is represented by the integer y - 1900.
• A month is represented by an integer form 0 to 11; 0 is January, 1 is February, and so forth; thus 11 is December.
• A date (day of month) is represented by an integer from 1 to 31 in the usual manner.
• An hour is represented by an integer from 0 to 23. Thus, the hour from midnight to 1 a.m. is hour 0, and the hour from noon to 1 p.m. is hour 12.
• A minute is represented by an integer from 0 to 59 in the usual manner.
• A second is represented by an integer from 0 to 61; the values 60 and 61 occur only for leap seconds and even then only in Java implementations that actually track leap seconds correctly. Because of the manner in which leap seconds are currently introduced, it is extremely unlikely that two leap seconds will occur in the same minute, but this specification follows the date and time conventions for ISO C.

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.

Since:

JDK1.0

See Also:

DateFormat, Calendar, TimeZone, Serialized Form

Constructor Summary

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(long 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.

Method Summary

boolean	after(Date when) Tests if this date is after the specified date.
boolean	before(Date when) Tests if this date is before the specified date.
Object	clone() Return a copy of this object.
int	compareTo(Date anotherDate) Compares two Dates for ordering.
int	compareTo(Object o) Compares this Date to another Object.
boolean	equals(Object obj) Compares two dates for equality.
long	getTime() Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this Date object.
int	hashCode() Returns a hash code value for this object.
void	setTime(long time) Sets this Date object to represent a point in time that is time milliseconds after January 1, 1970 00:00:00 GMT.
String	toString() Converts this Date object to a String of the form:

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

Date

public 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:

System.currentTimeMillis()

Date

public Date(long 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:

System.currentTimeMillis()

Method Detail

clone

public Object clone()

Return a copy of this object.

Overrides:

clone in class Object

Returns:

a clone of this instance.

See Also:

Cloneable

getTime

public long getTime()

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.

setTime

public void setTime(long time)

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.

before

public boolean before(Date when)

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.

after

public boolean after(Date when)

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.

equals

public boolean equals(Object obj)

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.

Overrides:

equals in class Object

Parameters:

obj - the object to compare with.

Returns:

true if the objects are the same; false otherwise.

See Also:

getTime()

compareTo

public int compareTo(Date anotherDate)

Compares two Dates for ordering.

Parameters:

anotherDate - the Date to be compared.

Returns:

the value 0 if the argument Date is equal to this Date; a value less than 0 if this Date is before the Date argument; and a value greater than 0 if this Date is after the Date argument.

Since:

1.2

compareTo

public int compareTo(Object o)

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).

Specified by:

compareTo in interface Comparable

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.

Throws:

ClassCastException - if the argument is not a Date.

Since:

1.2

See Also:

Comparable

hashCode

public int hashCode()

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 getTime() method. That is, the hash code is the value of the expression:

 (int)(this.getTime()^(this.getTime() >>> 32))

Overrides:

hashCode in class Object

Returns:

a hash code value for this object.

See Also:

Object.equals(java.lang.Object), Hashtable

toString

public String toString()

Converts this Date object to a String of the form:

 dow mon dd hh:mm:ss zzz yyyy

where:

• dow is the day of the week (Sun, Mon, Tue, Wed, Thu, Fri, Sat).
• mon is the month (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec).
• dd is the day of the month (01 through 31), as two decimal digits.
• hh is the hour of the day (00 through 23), as two decimal digits.
• mm is the minute within the hour (00 through 59), as two decimal digits.
• ss is the second within the minute (00 through 61, as two decimal digits.
• zzz is the time zone (and may reflect daylight savings time). Standard time zone abbreviations include those recognized by the method parse. If time zone informationi is not available, then zzz is empty - that is, it consists of no characters at all.
• yyyy is the year, as four decimal digits.

Overrides:

toString in class Object

Returns:

a string representation of this date.