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java.lang.Object | +----java.lang.System
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"; a means of
loading files and libraries; and a utility method for quickly
copying a portion of an array.
err
in
out
arraycopy(Object, int, Object, int, int)
currentTimeMillis()
exit(int)
gc()
getenv(String)
getProperties()
getProperty(String)
getProperty(String, String)
getSecurityManager()
identityHashCode(Object)
load(String)
loadLibrary(String)
libname
argument.
runFinalization()
runFinalizersOnExit(boolean)
setErr(PrintStream)
setIn(InputStream)
setOut(PrintStream)
setProperties(Properties)
Properties
argument.
setSecurityManager(SecurityManager)
in
public static final InputStream in
out
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.
err
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.
setIn
public static void setIn(InputStream in)
setOut
public static void setOut(PrintStream out)
setErr
public static void setErr(PrintStream err)
setSecurityManager
public static void setSecurityManager(SecurityManager s)
SecurityException
is thrown. Otherwise, the argument is established as the current
security manager. If the argument is null and no
security manager has been established, then no action is taken and
the method simply returns.
getSecurityManager
public static SecurityManager getSecurityManager()
null is returned.
currentTimeMillis
public static native 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).
arraycopy
public static native void arraycopy(Object src,
int src_position,
Object dst,
int dst_position,
int length)
src to the destination array
referenced by dst. The number of components copied is
equal to the length argument. The components at
positions srcOffset through
srcOffset+length-1 in the source array are copied into
positions dstOffset through
dstOffset+length-1, respectively, of the destination
array.
If the src and dst arguments refer to the
same array object, then the copying is performed as if the
components at positions srcOffset through
srcOffset+length-1 were first copied to a temporary
array with length components and then the contents of
the temporary array were copied into positions
dstOffset through dstOffset+length-1 of the
argument array.
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.
dst argument refers to an object that is not an
array.
src argument and dst argument refer to
arrays whose component types are different primitive types.
src argument refers to an array with a primitive
component type and the dst argument refers to an array
with a reference component type.
src argument refers to an array with a reference
component type and the dst argument refers to an array
with a primitive component type.
Otherwise, if any of the following is true, an
ArrayIndexOutOfBoundsException is
thrown and the destination is not modified:
srcOffset argument is negative.
dstOffset argument is negative.
length argument is negative.
srcOffset+length is greater than
src.length, the length of the source array.
dstOffset+length is greater than
dst.length, the length of the destination array.
Otherwise, if any actual component of the source array from
position srcOffset through
srcOffset+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[srcOffset+k]
cannot be converted to the component type of the destination
array; when the exception is thrown, source array components from
positions srcOffset through
srcOffset+k-1
will already have been copied to destination array positions
dstOffset through
dstOffset+k-1 and no other
positions of the destination array will have been modified.
src
array could not be stored into the dest array
because of a type mismatch.
identityHashCode
public static native int identityHashCode(Object x)
getProperties
public static Properties getProperties()
If there is a security manager, its
checkPropertiesAccess method is called with no
arguments. This may result in a security exception.
The current set of system properties is returned as a
Properties object. If there is no current set of
system properties, a set of system properties is first created and
initialized.
This set of system properties always includes values for the following keys:
| Key | Description of Associated Value |
|---|---|
java.version |
Java version number |
java.vendor |
Java vendor-specific string |
java.vendor.url |
Java vendor URL |
java.home |
Java installation directory |
java.class.version |
Java class format version number |
java.class.path |
Java class path |
os.name |
Operating system name |
os.arch |
Operating system architecture |
os.version |
Operating system version |
file.separator |
File separator ("/" on UNIX) |
path.separator |
Path separator (":" on UNIX) |
line.separator |
Line separator ("\n" on UNIX) |
user.name |
User's account name |
user.home |
User's home directory |
user.dir |
User's current working directory |
setProperties
public static void setProperties(Properties props)
Properties
argument.
First, if there is a security manager, its
checkPropertiesAccess method is called with no
arguments. This may result in a security exception.
The argument becomes the current set of system properties for use
by the getProperty method. If the argument is
null, then the current set of system properties is
forgotten.
getProperty
public static String getProperty(String key)
First, if there is a security manager, its
checkPropertyAccess method is called with the key as
its argument. This may result in a system exception.
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.
null if there is no property with that key.
getProperty
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.
getenv
public static String getenv(String name)
exit
public static void exit(int status)
This method calls the exit method in class
Runtime. This method never returns normally.
gc
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 unused
objects.
runFinalization
public static void runFinalization()
Calling this method suggests that the Java Virtual Machine expend
effort toward running the finalize methods of objects
that have been found to be discarded but whose finalize
methods have not yet been run. When control returns from the
method call, the Java Virtual Machine has made a best effort to
complete all outstanding finalizations.
runFinalizersOnExit
public static void runFinalizersOnExit(boolean value)
load
public static void load(String filename)
This method calls the load method in class
Runtime.
loadLibrary
public static void loadLibrary(String libname)
libname
argument. The manner in which a library name is mapped to the
actual system library is system dependent.
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