Package java.lang

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Class System

public  final  class  java.lang.System
    extends  java.lang.Object  
{
        // Fields
    public static PrintStream err;	
    public static InputStream in;	
    public static PrintStream out;	

        // Methods
    public static void	
        arraycopy(Object  src, int  src_position,
                Object  dst, int  dst_position, int  length);
    public static long currentTimeMillis();	
    public static void exit(int  status);	
    public static void gc();	
    public static Properties getProperties();	
    public static String getProperty(String  key);	
    public static String getProperty(String  key, String  def);	
    public static SecurityManager getSecurityManager();	
    public static void load(String  filename);	
    public static void loadLibrary(String  libname);	
    public static void runFinalization();	
    public static void setProperties(Properties  props);	
    public static void setSecurityManager(SecurityManager  s);	
}

The System class contains a number of 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.

Fields

err

public static PrintStream err 

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. 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.

in

public 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 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 standalone Java applications, a typical way to write a line of output data is

	System.out.println(data) 

See the println methods in class PrintStream.

Methods

arraycopy

public static void
arraycopy(Object  src, int  src_position, Object  dst,
                    int  dst_position, int  length) 

A subsequence of array components arecopied 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 ArrayIndex-Out-Of-Bounds-Exception 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.

Parameter	Description
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

Throw:

ArrayIndexOutOfBoundsException

If copy would cause access of data outside array bounds.

Throw:

ArrayStoreException

If an element in the src array could could not be stored into the dest array due to a type mismatch

currentTimeMillis

public static long currentTimeMillis() 

Gets the current time.

See the description of the class Date for a discussion of slight discrepancies that may arise between "computer time" and UTC (Coordinated Universal Time).

Return Value:

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 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.

Parameter	Description
status	exit status

Throw:

SecurityException

If the current thread cannot exit with the specified status.

gc

public static void gc() 

This method alls the gc method in class Runtime.

Calling this 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.

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 classpath 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 account name user.home User home directory user.dir User's current working directory

Throw:

SecurityException

If the current thread cannot access the system properties.

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 .

Return Value:

Returns the string value of the system property, or null if there is no property with that key.

Parameter	Description
key	the name of the system property

Throw:

SecurityException

If the current thread cannot access the system properties or the specified property.

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 .

Return Value:

Returns the string value of the system property, or the default value if there is no property with that key.

Parameter	Description
key	the name of the system property
def	a default value

Throw:

SecurityException

If the current thread cannot access the system properties or the specified property.

getSecurityManager

public static SecurityManager getSecurityManager() 

Return Value:

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

load

public static void load(String  filename) 

Loads the given filename as a dynamic library. The filename argument must be a complete path name.

This method calls the load method in class Runtime.

Parameter	Description
filename	the file to load

Throw:

UnsatisfiedLinkError

If the file does not exist.

Throw:

SecurityException

If the current thread cannot load the specified dynamic library.

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.

Parameter	Description
libname	the name of the library

Throw:

UnsatisfiedLinkError

If the library does not exist.

Throw:

SecurityException

If the current thread cannot load the specified dynamic library.

runFinalization

public static void runFinalization() 

This method calls the runFinalization method in class Runtime.

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.

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.

Parameter	Description
props	the new system properties.

Throw:

SecurityException

If the current thread cannot set the system properties.

setSecurityManager

public static void setSecurityManager(SecurityManager  s) 

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.

Parameter	Description
s	the security manager

Throw:

SecurityException

If the security manager has already been set.