public final class System extends Object
System
class contains several useful class fields
and methods. It cannot be instantiated.
Among the facilities provided by the System
class
are standard input, standard output, and error output streams;
access to externally defined properties and environment
variables; a means of loading files and libraries; and a utility
method for quickly copying a portion of an array.
Modifier and Type | Field and Description |
---|---|
static PrintStream |
err
The "standard" error output stream.
|
static PrintStream |
out
The "standard" output stream.
|
Modifier and Type | Method and Description |
---|---|
static void |
arraycopy(Object src,
int srcPos,
Object dest,
int destPos,
int length)
Copies an array from the specified source array, beginning at the
specified position, to the specified position of the destination array.
|
static long |
currentTimeMillis()
Returns the current time in milliseconds.
|
static void |
exit(int status)
Terminates the currently running Java Virtual Machine.
|
static void |
gc()
Runs the garbage collector.
|
static String |
getProperty(String key)
Gets the system property indicated by the specified key.
|
static String |
getProperty(String key,
String def)
Gets the system property indicated by the specified key.
|
static SecurityManager |
getSecurityManager()
Gets the system security interface.
|
static int |
identityHashCode(Object x)
Returns the same hash code for the given object as
would be returned by the default method hashCode(),
whether or not the given object's class overrides
hashCode().
|
static long |
nanoTime()
Returns the current value of the running Java Virtual Machine's
high-resolution time source, in nanoseconds.
|
public static final PrintStream err
Typically this stream corresponds to display output or another
output destination specified by the host environment or user. By
convention, this output stream is used to display error messages
or other information that should come to the immediate attention
of a user even if the principal output stream, the value of the
variable out
, has been redirected to a file or other
destination that is typically not continuously monitored.
public static final PrintStream out
For simple stand-alone Java applications, a typical way to write a line of output data is:
System.out.println(data)
See the println
methods in class PrintStream
.
PrintStream.println()
,
PrintStream.println(boolean)
,
PrintStream.println(char)
,
PrintStream.println(char[])
,
PrintStream.println(double)
,
PrintStream.println(float)
,
PrintStream.println(int)
,
PrintStream.println(long)
,
PrintStream.println(java.lang.Object)
,
PrintStream.println(java.lang.String)
public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
src
to the destination array
referenced by dest
. The number of components copied is
equal to the length
argument. The components at
positions srcPos
through
srcPos+length-1
in the source array are copied into
positions destPos
through
destPos+length-1
, respectively, of the destination
array.
If the src
and dest
arguments refer to the
same array object, then the copying is performed as if the
components at positions srcPos
through
srcPos+length-1
were first copied to a temporary
array with length
components and then the contents of
the temporary array were copied into positions
destPos
through destPos+length-1
of the
destination array.
If dest
is null
, then a
NullPointerException
is thrown.
If src
is null
, then a
NullPointerException
is thrown and the destination
array is not modified.
Otherwise, if any of the following is true, an
ArrayStoreException
is thrown and the destination is
not modified:
src
argument refers to an object that is not an
array.
dest
argument refers to an object that is not an
array.
src
argument and dest
argument refer
to arrays whose component types are different primitive types.
src
argument refers to an array with a primitive
component type and the dest
argument refers to an array
with a reference component type.
src
argument refers to an array with a reference
component type and the dest
argument refers to an array
with a primitive component type.
Otherwise, if any of the following is true, an
IndexOutOfBoundsException
is
thrown and the destination is not modified:
srcPos
argument is negative.
destPos
argument is negative.
length
argument is negative.
srcPos+length
is greater than
src.length
, the length of the source array.
destPos+length
is greater than
dest.length
, the length of the destination array.
Otherwise, if any actual component of the source array from
position srcPos
through
srcPos+length-1
cannot be converted to the component
type of the destination array by assignment conversion, an
ArrayStoreException
is thrown. In this case, let
k be the smallest nonnegative integer less than
length such that src[srcPos+
k]
cannot be converted to the component type of the destination
array; when the exception is thrown, source array components from
positions srcPos
through
srcPos+
k-1
will already have been copied to destination array positions
destPos
through
destPos+
k-1
and no other
positions of the destination array will have been modified.
(Because of the restrictions already itemized, this
paragraph effectively applies only to the situation where both
arrays have component types that are reference types.)
src
- the source array.srcPos
- starting position in the source array.dest
- the destination array.destPos
- starting position in the destination data.length
- the number of array elements to be copied.IndexOutOfBoundsException
- if copying would cause
access of data outside array bounds.ArrayStoreException
- if an element in the src
array could not be stored into the dest
array
because of a type mismatch.NullPointerException
- if either src
or
dest
is null
.public static long currentTimeMillis()
See the description of the class Date
for
a discussion of slight discrepancies that may arise between
"computer time" and coordinated universal time (UTC).
Date
public static void exit(int status)
This method calls the exit
method in class
Runtime
. This method never returns normally.
The call System.exit(n)
is effectively equivalent to
the call:
Runtime.getRuntime().exit(n)
status
- exit status.SecurityException
- if a security manager exists and its checkExit
method doesn't allow exit with the specified status.Runtime.exit(int)
public static void gc()
Calling the gc
method suggests that the Java Virtual
Machine expend effort toward recycling unused objects in order to
make the memory they currently occupy available for quick reuse.
When control returns from the method call, the Java Virtual
Machine has made a best effort to reclaim space from all discarded
objects.
The call System.gc()
is effectively equivalent to the
call:
Runtime.getRuntime().gc()
Runtime.gc()
public static String getProperty(String key)
If there is a security manager, its
checkPropertyAccess
method is called with the key as
its argument. This may result in a SecurityException.
key
- the name of the system property.null
if there is no property with that key.SecurityException
- if a security manager exists and its
checkPropertyAccess
method doesn't allow
access to the specified system property.NullPointerException
- if key
is
null
.IllegalArgumentException
- if key
is empty.SecurityException
,
SecurityManager.checkPropertyAccess(java.lang.String)
public static String getProperty(String key, String def)
First, if there is a security manager, its
checkPropertyAccess
method is called with the
key
as its argument.
If there is no current set of system properties, a set of system
properties is first created and initialized in the same manner as
for the getProperties
method.
key
- the name of the system property.def
- a default value.SecurityException
- if a security manager exists and its
checkPropertyAccess
method doesn't allow
access to the specified system property.NullPointerException
- if key
is
null
.IllegalArgumentException
- if key
is empty.SecurityManager.checkPropertyAccess(java.lang.String)
public static SecurityManager getSecurityManager()
null
is returned.public static int identityHashCode(Object x)
x
- object for which the hashCode is to be calculatedpublic static long nanoTime()
This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. The value returned represents nanoseconds since some fixed but arbitrary origin time (perhaps in the future, so values may be negative). The same origin is used by all invocations of this method in an instance of a Java virtual machine; other virtual machine instances are likely to use a different origin.
This method provides nanosecond precision, but not necessarily
nanosecond resolution (that is, how frequently the value changes)
- no guarantees are made except that the resolution is at least as
good as that of currentTimeMillis()
.
Differences in successive calls that span greater than approximately 292 years (263 nanoseconds) will not correctly compute elapsed time due to numerical overflow.
The values returned by this method become meaningful only when the difference between two such values, obtained within the same instance of a Java virtual machine, is computed.
For example, to measure how long some code takes to execute:
long startTime = System.nanoTime();
// ... the code being measured ...
long estimatedTime = System.nanoTime() - startTime;
To compare two nanoTime values
long t0 = System.nanoTime();
...
long t1 = System.nanoTime();
one should use t1 - t0 < 0
, not t1 < t0
,
because of the possibility of numerical overflow.Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved. Use of this specification is subject to license terms.