java.lang
Class System
java.lang.Object
java.lang.System
public final class System
- extends Object
The System class contains several useful class fields and methods. It cannot be instantiated.
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Method Summary |
static void |
arraycopy(Object src,
int srcOffset,
Object dst,
int dstOffset,
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(). |
out
public static final PrintStream out
- 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.
err
public static final PrintStream err
- 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.
arraycopy
public static void arraycopy(Object src,
int srcOffset,
Object dst,
int dstOffset,
int length)
- 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 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.)
currentTimeMillis
public static long currentTimeMillis()
- Returns the current time in milliseconds.
- Returns:
- the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.
exit
public static void exit(int status)
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)
See important notes in Runtime.exit(int)
gc
public static void gc()
- 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()
getProperty
public static String getProperty(String key)
- Gets the system property indicated by the specified key.
identityHashCode
public 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(). The hashcode for the null reference is zero.