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# Class java.lang.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.

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
  The "standard" error output stream.
in
  The "standard" input stream.
out
  The "standard" output stream.

##

arraycopy(Object, int, Object, int, int)
  Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array.
currentTimeMillis()
  Returns the current time in milliseconds.
exit(int)
  Terminates the currently running Java Virtual Machine.
gc()
  Runs the garbage collector.
getenv(String)
  Gets an environment variable. Deprecated.
getProperties()
  Determines the current system properties.
getProperty(String)
  Gets the system property indicated by the specified key.
getProperty(String, String)
  Gets the system property indicated by the specified key.
getSecurityManager()
  Gets the system security interface.
identityHashCode(Object)
  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().
load(String)
  Loads the specified filename as a dynamic library.
loadLibrary(String)
  Loads the system library specified by the `libname` argument.
runFinalization()
  Runs the finalization methods of any objects pending finalization.
runFinalizersOnExit(boolean)
  Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits.
setErr(PrintStream)
  Reassigns the "standard" error output stream.
setIn(InputStream)
  Reassigns the "standard" input stream.
setOut(PrintStream)
  Reassigns the "standard" output stream.
setProperties(Properties)
  Sets the system properties to the `Properties` argument.
setSecurityManager(SecurityManager)
  Sets the System security.

##

in

      public final static InputStream in

  The "standard" input stream. This stream is already open and ready to supply input data. Typically this stream corresponds to keyboard input or another input source specified by the host environment or user.

out

      public final static 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`.

  See Also:
    println, println, println, println, println, println, println, println, println, println

err

      public final static 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.

##

setIn

      public static void setIn(InputStream in)

  Reassigns the "standard" input stream.

setOut

      public static void setOut(PrintStream out)

  Reassigns the "standard" output stream.

setErr

      public static void setErr(PrintStream err)

  Reassigns the "standard" error output stream.

setSecurityManager

      public static void setSecurityManager(SecurityManager s)

  Sets the System security. If a security manager has already been established for the currently running Java application, a `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.

  Parameters:
    s - the security manager.
  Throws: SecurityException
    if the security manager has already been set.

getSecurityManager

      public static SecurityManager getSecurityManager()

  Gets the system security interface.

  Returns:
    if a security manager has already been established for the current application, then that security manager is returned; otherwise, `null` is returned.

currentTimeMillis

      public static long currentTimeMillis()

  Returns the current time in milliseconds.

  See the description of the class `Date` for a discussion of slight discrepancies that may arise between "computer time" and coordinated universal time (UTC).

  Returns:
    the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.
  See Also:
    Date

arraycopy

      public 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. 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 argument array.

  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 `ArrayIndexOutOfBoundsException` 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.

  Parameters:
    src: - the source array.
    srcpos - start position in the source array.
    dest - the destination array.
    destpos - start position in the destination data.
    length - the number of array elements to be copied.
  Throws: ArrayIndexOutOfBoundsException
    if copying would cause access of data outside array bounds.
  Throws: ArrayStoreException
    if an element in the `src` array could not be stored into the `dest` array because of a type mismatch.

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.

getProperties

      public static Properties getProperties()

  Determines the current system properties.

  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

  Throws: SecurityException
    if the current thread cannot access the system properties.
  See Also:
    SecurityException, checkPropertiesAccess, Properties

setProperties

      public static void setProperties(Properties props)

  Sets the system properties to the `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.

  Parameters:
    props - the new system properties.
  Throws: SecurityException
    if the current thread cannot set the system properties.
  See Also:
    SecurityException, checkPropertiesAccess

getProperty

      public static String getProperty(String key)

  Gets the system property indicated by the specified 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.

  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: SecurityException
    if the current thread cannot access the system properties or the specified property.
  See Also:
    SecurityException, checkPropertyAccess, getProperties

getProperty

      public static String getProperty(String key,
                                       String def)

  Gets the system property indicated by the specified key.

  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.

  Parameters:
    key - the name of the system property.
    def - a default value.
  Returns:
    the string value of the system property, or the default value if there is no property with that key.
  Throws: SecurityException
    if the current thread cannot access the system properties or the specified property.
  See Also:
    checkPropertyAccess, getProperties

getenv

      public static String getenv(String name)

  Note: getenv() is deprecated.

  Gets an environment variable. An environment variable is a system dependent external variable that has a string value.

  Parameters:
    the - name of the environment variable.
  Returns:
    the value of the variable, or null if the variable is not defined.

exit

      public static void exit(int status)

  Terminates the currently running Java Virtual Machine. 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.

  Parameters:
    status - exit status.
  Throws: SecurityException
    if the current thread cannot exit with the specified status.
  See Also:
    exit

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 unused objects.

  See Also:
    gc

runFinalization

      public static void runFinalization()

  Runs the finalization methods of any objects pending finalization.

  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.

  See Also:
    runFinalization

runFinalizersOnExit

      public static void runFinalizersOnExit(boolean value)

  Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits. By default, finalization on exit is disabled.

  See Also:
    exit, gc

load

      public static void load(String filename)

  Loads the specified filename as a dynamic library. The filename argument must be a complete pathname.

  This method calls the `load` method in class `Runtime.`

  Parameters:
    filename - the file to load.
  Throws: SecurityException
    if the current thread cannot load the specified dynamic library.
  Throws: UnsatisfiedLinkError
    if the file does not exist.
  See Also:
    load

loadLibrary

      public static void loadLibrary(String libname)

  Loads the system library specified by the `libname` argument. The manner in which a library name is mapped to the actual system library is system dependent.

  Parameters:
    libname - the name of the library.
  Throws: SecurityException
    if the current thread cannot load the specified dynamic library.
  Throws: UnsatisfiedLinkError
    if the library does not exist.
  See Also:
    loadLibrary

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