java.lang System

java.lang
System

Declaration
public final class System
java.lang.Object
|
+--java.lang.System
Description
The System class contains several useful class fields and methods. It cannot be instantiated.

Fields
static
java.io.PrintStream
err
static
java.io.PrintStream
out
Methods
static void arraycopy(Object src, int srcOffset, Object dst, int
dstOffset, int length)
static long currentTimeMillis()
static void exit(int status)
static void gc()
static String getProperty(String key)
static int identityHashCode(Object x)

Methods inherited from class Object
equals(Object), getClass(), hashCode(), notify(), notifyAll(), toString(), wait(),
wait(), wait()

Fields
out
Declaration:
public static final java.io.PrintStream out

Description:
The “standard” output stream. This stream is already open and ready to accept output data. Typically this
stream corresponds to display output or another output destination specified by the host environment or
user.
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.
See Also: java.io.PrintStream.println(),
java.io.PrintStream.println(boolean),
java.io.PrintStream.println(char),
java.io.PrintStream.println(char[]), java.io.PrintStream.println(int),
java.io.PrintStream.println(long),
java.io.PrintStream.println(Object),
java.io.PrintStream.println(String)
err
Declaration:
public static final java.io.PrintStream err
Description:
The “standard” error output stream. This stream is already open and ready to accept output data.
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.


Methods
currentTimeMillis()
Declaration:
public static long currentTimeMillis()
Description:
Returns the current time in milliseconds.
Returns: the difference, measured in milliseconds, between the current time and midnight, January 1,
1970 UTC.
arraycopy(Object, int, Object, int, int)
Declaration:
public static void arraycopy(java.lang.Object src, int srcOffset, java.lang.Object dst,
int dstOffset, int length)
Description:
Copies an array from the specified source array, beginning at the specified position, to the specified position
of the destination array. A subsequence of array components are copied from the source array referenced by
src to the destination array referenced by dst. The number of components copied is equal to the length

arraycopy(Object, int, Object, int, int)

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 destination array.
If dst 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:
• The src argument refers to an object that is not an array.
• The dst argument refers to an object that is not an array.
• The src argument and dst argument refer to arrays whose component types are different primitive
types.
• The src argument refers to an array with a primitive component type and the dst argument refers to
an array with a reference component type.
• The 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 IndexOutOfBoundsException is thrown and the
destination is not modified:
• The srcOffset argument is negative.
• The dstOffset argument is negative.
• The 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. (Because of the restrictions already itemized, this paragraph effectively applies only to the
situation where both arrays have component types that are reference types.)
Parameters:
src - the source array.
srcOffset - start position in the source array.
dst - the destination array.
dstOffset - start position in the destination data.
length - the number of array elements to be copied.

Throws:
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 dst is null.
identityHashCode(Object)
Declaration:
public static int identityHashCode(java.lang.Object x)
Description:
Returns the same hashcode for the given object as would be returned by the default method hashCode(),
whether or not the given object’s class overrides hashCode(). The hashcode for the null reference is zero.
Parameters:
x - object for which the hashCode is to be calculated
Returns: the hashCode
Since: JDK1.1
getProperty(String)
Declaration:
public static java.lang.String getProperty(java.lang.String key)
Description:
Gets the system property indicated by the specified key.
Parameters:
key - the name of the system property.
Returns: the string value of the system property, or null if there is no property with that key.
Throws:
NullPointerException - if key is null.
IllegalArgumentException - if key is empty.
exit(int)
Declaration:
public static void exit(int status)
Description:
Terminates the currently running Java application. The argument serves as a status code; by convention, a
nonzero status code indicates abnormal termination.
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)
Parameters:
status - exit status.
See Also: Runtime.exit(int)

gc()
Declaration:
public static void gc()
Description:
Runs the garbage collector.
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()
See Also: Runtime.gc()