java.lang.System
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java.lang.Object
The System class contains several useful class fields
and methods. It cannot be instantiated.
| Field Summary | |
static PrintStream |
err
The "standard" error output stream. |
static PrintStream |
out
The "standard" output stream. |
| Method Summary | |
static void |
arraycopy(Object src,
int src_position,
Object dst,
int dst_position,
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 application. |
static void |
gc()
Runs the garbage collector. |
static String |
getProperty(String key)
Gets the system property indicated by the specified key. |
static int |
identityHashCode(Object x)
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(). |
| Methods inherited from class java.lang.Object |
equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
| Field Detail |
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(int),
PrintStream.println(long),
PrintStream.println(java.lang.Object),
PrintStream.println(java.lang.String)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.
| Method Detail |
public static long currentTimeMillis()
public static 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
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:
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
IndexOutOfBoundsException 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.
(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.src_position - start position in the source array.dst - the destination array.dst_position - pos start 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
dst is null.public static int identityHashCode(Object x)
x - object for which the hashCode is to be calculated
public static String getProperty(String key)
key - the name of the system property.
null if there is no property with that key.
NullPointerException - if key is
null.
IllegalArgumentException - if key is empty.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.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()
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