Java.lang.System class in Java

Among the facilities provided by the System class are standard input, standard output, and error output streams; access to externally defined properties and environment variables; a means of loading files and libraries; and a utility method for quickly copying a portion of an array. It extends class Object. 

Fields: 

1. public static final 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.
2. 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.
3. 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.

Methods: 

1. static void arraycopy(Object source, int sourceStart, Object Target, int targetStart, int size): Copies an array. The array to be copied is passed in source, and the index at which point the copy will begin within source is passed in sourceStart. The array that will receive the copy is passed in target, and the index which point the copy will begin within the target is passed in targetStart. Size is the number of elements that are copied. 

Syntax: public static void arraycopy(Object source, 
int sourceStart, Object Target, int targetStart, int size)
Returns: NA.
Exception: 
IndexOutOfBoundsException - if copying would cause access of data 
outside array bounds.
ArrayStoreException - if an element in the source array could not 
be stored into the target array because of a type mismatch.
NullPointerException - if either source or target is null.

Java

// Java code illustrating arraycopy() method
import java.lang.*;
import java.util.Arrays;
class SystemDemo
{
    public static void main(String args[])
    {
        int[] a = {1, 2, 3, 4, 5};
        int[] b = {6, 7, 8, 9, 10};

        System.arraycopy(a, 0, b, 2, 2);

        // array b after arraycopy operation
        System.out.println(Arrays.toString(b));

    }
}

Output: 

[6, 7, 1, 2, 10]

2. static String clearProperty(String key): Removes the system property indicated by the specified key. 

Syntax: public static String clearProperty(String key)
Returns: the previous string value 
of the system property, or null if there was no property 
with that key.
Exception: 
SecurityException - if a security manager exists and its 
checkPropertyAccess method doesn't allow 
access to the specified system property.
NullPointerException - if key is null.
IllegalArgumentException - if key is empty.

3. static String getProperty(String key): Gets the system property indicated by the specified key. 

Syntax: public static String getProperty(String key)
Returns: the string value of the system 
property, or null if there is no property with that key.
Exception: 
SecurityException - if a security manager exists and its 
checkPropertyAccess method doesn't allow access to the 
specified system property.
NullPointerException - if key is null.
IllegalArgumentException - if key is empty.

4. static String getProperty(String key, String def): Gets the system property indicated by the specified key. 

Syntax: public static String getProperty(String key, String def)
Returns: the string value of the system property,
 or the default value if there is no property with that key.
Exception: 
SecurityException - if a security manager exists and its 
checkPropertyAccess method doesn't allow access to the 
specified system property.
NullPointerException - if key is null.
IllegalArgumentException - if key is empty.

5. static String setProperty(String key, String value): Sets the system property indicated by the specified key. 

Syntax: public static String setProperty(String key, String value)
Returns: the previous value of the system 
property, or null if it did not have one.
Exception: 
SecurityException - if a security manager exists and its checkPermission 
method doesn't allow setting of the specified property.
NullPointerException - if key or value is null.
IllegalArgumentException - if key is empty.

Java

// Java code illustrating clearProperty(), getProperty()
// and setProperty() methods
import java.lang.*;
import static java.lang.System.clearProperty;
import static java.lang.System.setProperty;
import java.util.Arrays;
class SystemDemo
{
    public static void main(String args[])
    {
        // checking specific property
        System.out.println(System.getProperty("user.home"));

        // clearing this property
        clearProperty("user.home");

        System.out.println(System.getProperty("user.home"));

        // setting specific property
        setProperty("user.country", "US");

        // checking property
        System.out.println(System.getProperty("user.country"));

        // checking property other than system property
        // illustrating getProperty(String key, String def)
        System.out.println(System.getProperty("user.password",
              "none of your business"));
    }
}

Output: 

/Users/abhishekverma
null
US
none of your business

6. static Console console(): Returns the unique Console object associated with the current Java virtual machine, if any. 

Syntax: public static Console console()
Returns: The system console, if any, otherwise null.
Exception: NA

Java

// Java code illustrating console() method
import java.io.Console;
import java.lang.*;
import java.util.Currency;
import java.util.Locale;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {

        Console c = System.console();
        if(c != null)
        {
           Currency currency = Currency.getInstance(Locale.ITALY);
           c.printf(currency.getSymbol());
            c.flush();
        }
        else
            System.out.println("No console attached");
    }
}

Output: 

No console attached

7. static long currentTimeMillis(): Returns the current time in milliseconds. Note that while the unit of time of the return value is a millisecond, the granularity of the value depends on the underlying operating system and may be larger. For example, many operating systems measure time in units of tens of milliseconds. 

Syntax: public static long currentTimeMillis()
Returns: the difference, measured in milliseconds,
 between the current time and midnight, January 1, 1970 UTC.
Exception: NA.

8. static long nanoTime(): Returns the current value of the running Java Virtual Machine’s high-resolution time source, in nanoseconds. 

Syntax: public static long nanoTime()
Returns: the current value of the running Java
 Virtual Machine's high-resolution time source, in nanoseconds
Exception: NA

Java

// Java code illustrating currentTimeMillis() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {
        System.out.println("difference between the "
                + "current time and midnight,"
                + " January 1, 1970 UTC is: " +
                System.currentTimeMillis());
        System.out.println("current time in "
                + "nano sec: " +
                System.nanoTime());
    }
}

Output: 

difference between the current time 
and midnight, January 1, 1970 UTC is: 
1499520649545
current time in nano sec: 29976939759226

9. 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 an 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) 

Syntax: public static void exit(int status)
Returns: NA
Exception: 
SecurityException - if a security manager exists and its 
checkExit method doesn't allow exit with the specified status.

Java

// Java code illustrating exit() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {
        System.gc();
        System.out.println("Garbage collector executed ");

        System.out.println(System.getProperty("os.name"));

        System.exit(1);

        // this line will not execute as JVM terminated
        System.out.println("JVM terminated");
    }
}

Output: 

Garbage collector executed 
Mac OS X

10. static void gc(): Runs the garbage collector. Calling the gc method suggests that the Java Virtual Machine expand 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. 

Syntax: public static void gc()
Returns: NA
Exception: NA

Java

// Java code illustrating gc() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[])
    {
        Runtime gfg = Runtime.getRuntime();
        long memory1, memory2;
        Integer integer[] = new Integer[1000];

        // checking the total memory
        System.out.println("Total memory is: "
                           + gfg.totalMemory());

        // checking free memory
        memory1 = gfg.freeMemory();
        System.out.println("Initial free memory: "
                                      + memory1);

        // calling the garbage collector on demand
        System.gc();

        memory1 = gfg.freeMemory();

        System.out.println("Free memory after garbage "
                           + "collection: " + memory1);

        // allocating integers
        for (int i = 0; i < 1000; i++)
            integer[i] = new Integer(i);

        memory2 = gfg.freeMemory();
        System.out.println("Free memory after allocation: "
                           + memory2);

        System.out.println("Memory used by allocation: " +
                                    (memory1 - memory2));

        // discard integers
        for (int i = 0; i < 1000; i++)
            integer[i] = null;

        System.gc();

        memory2 = gfg.freeMemory();
        System.out.println("Free memory after  "
            + "collecting discarded Integers: " + memory2);
    }
}

Output: 

Total memory is: 128974848
Initial free memory: 126929976
Free memory after garbage collection: 128632160
Free memory after allocation: 127950520
Memory used by allocation: 681640
Free memory after  collecting discarded Integers: 128643472

11. static Map getenv(): Returns an unmodifiable string map view of the current system environment. The environment is a system-dependent mapping from names to values which is passed from parent to child processes. 
If the system does not support environment variables, an empty map is returned. 

Syntax: public static Map getenv()
Returns: the environment as a map of variable names to values.
Exception: 
SecurityException - if a security manager exists and its 
checkPermission method doesn't allow access to the process 
environment

12. static String getenv(String name): Gets the value of the specified environment variable. An environment variable is a system-dependent external named value. 
System properties and environment variables are both conceptually mappings between names and values. Both mechanisms can be used to pass user-defined information to a Java process. Environment variables have a more global effect, because they are visible to all descendants of the process which defines them, not just the immediate Java subprocess. They can have subtly different semantics, such as case insensitivity, on different operating systems. For these reasons, environment variables are more likely to have unintended side effects. It is best to use system properties where possible. Environment variables should be used when a global effect is desired, or when an external system interface requires an environment variable (such as PATH). 

Syntax: public static String getenv(String name)
Returns: the string value of the variable,
 or null if the variable is not defined in the system environment.
Exception: 
NullPointerException - if name is null
SecurityException - if a security manager exists and 
its checkPermission method doesn't allow access to the
 environment variable name.

Java

// Java code illustrating getenv() method
import java.lang.*;
import java.util.Map;
import java.util.Set;
class SystemDemo
{
    public static void main(String args[])
    {
        Map<String, String> gfg = System.getenv();
        Set<String> keySet = gfg.keySet();
        for(String key : keySet)
        {
            System.out.println("key= " + key);
        }

        // checking specific environment variable
        System.out.println(System.getenv("PATH"));
    }
}

Output: 

key= JAVA_MAIN_CLASS_5396
key= PATH
key= J2D_PIXMAPS
key= SHELL
key= USER
key= TMPDIR
key= SSH_AUTH_SOCK
key= XPC_FLAGS
key= LD_LIBRARY_PATH
key= __CF_USER_TEXT_ENCODING
key= Apple_PubSub_Socket_Render
key= LOGNAME
key= LC_CTYPE
key= XPC_SERVICE_NAME
key= PWD
key= JAVA_MAIN_CLASS_2336
key= SHLVL
key= HOME
key= _
/usr/bin:/bin:/usr/sbin:/sbin

13. static Properties getProperties(): Determines the current system properties. 

Syntax: public static Properties getProperties()
Returns: the system properties.
Exception: 
SecurityException - if a security manager exists and 
its checkPropertiesAccess method doesn't allow access 
to the system properties.

Java

// Java code illustrating getProperties() method
import java.lang.*;
import java.util.Properties;
import java.util.Set;
class SystemDemo
{
    public static void main(String args[])
    {
        Properties gfg = System.getProperties();
        Set<Object> keySet = gfg.keySet();
        for(Object key : keySet)
        {
            System.out.println("key= " + key);
        }
    }
}

Output: 

key= java.runtime.name
key= sun.boot.library.path
key= java.vm.version
key= user.country.format
key= gopherProxySet
key= java.vm.vendor
key= java.vendor.url
key= path.separator
key= java.vm.name
key= file.encoding.pkg
key= user.country
key= sun.java.launcher
key= sun.os.patch.level
key= java.vm.specification.name
key= user.dir
key= java.runtime.version
key= java.awt.graphicsenv
key= java.endorsed.dirs
key= os.arch
key= java.io.tmpdir
key= line.separator
key= java.vm.specification.vendor
key= os.name
key= sun.jnu.encoding
key= java.library.path
key= java.specification.name
key= java.class.version
key= sun.management.compiler
key= os.version
key= http.nonProxyHosts
key= user.home
key= user.timezone
key= java.awt.printerjob
key= file.encoding
key= java.specification.version
key= java.class.path
key= user.name
key= java.vm.specification.version
key= sun.java.command
key= java.home
key= sun.arch.data.model
key= user.language
key= java.specification.vendor
key= awt.toolkit
key= java.vm.info
key= java.version
key= java.ext.dirs
key= sun.boot.class.path
key= java.vendor
key= file.separator
key= java.vendor.url.bug
key= sun.io.unicode.encoding
key= sun.cpu.endian
key= socksNonProxyHosts
key= ftp.nonProxyHosts
key= sun.cpu.isalist

14. static SecurityManager getSecurityManager(): Gets the system security interface. 

Syntax: static SecurityManager getSecurityManager()
Returns: if a security manager has 
already been established for the current application,
 then that security manager is returned; otherwise, 
null is returned.
Exception: NA

15. static void setSecurityManager(SecurityManager s): Sets the System security. 

Syntax: public static void setSecurityManager(SecurityManager s)
Returns: NA.
Exception: 
SecurityException - if the security manager has 
already been set and its checkPermission method 
doesn't allow it to be replaced.

Java

// Java code illustrating setSecurityManager()
// and getSecurityManager() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[])
    {
        SecurityManager gfg = new SecurityManager();

        // setting the security manager
        System.setSecurityManager(gfg);

        gfg = System.getSecurityManager();
        if(gfg != null)
            System.out.println("Security manager is configured");
    }
}

Output: 

Security manager is configured

16. static void setErr(PrintStream err): Reassigns the “standard” error output stream. 

Syntax: public static void setErr(PrintStream err)
Returns: NA
Exception: 
SecurityException - if a security manager exists and its
 checkPermission method doesn't allow reassigning of the
 standard error output stream.

17. static void setIn(InputStream in): Reassigns the “standard” input stream. 

Syntax: public static void setIn(InputStream in)
Returns: NA.
Exception: 
SecurityException - if a security manager exists and its
 checkPermission method doesn't allow reassigning of the
 standard input stream.

18. static void setOut(PrintStream out): Reassigns the “standard” output stream. 

Syntax: public void setOut(PrintStream out)
Returns: NA
Exception: 
SecurityException - if a security manager exists and its
 checkPermission method doesn't allow reassigning of the
 standard output stream.

Java

// Java code illustrating setOut(), setIn() and setErr() method
import java.lang.*;
import java.util.Properties;
import java.io.*;
class SystemDemo
{
    public static void main(String args[])  throws IOException
    {
        FileInputStream IN = new FileInputStream("input.txt");
        FileOutputStream OUT = new FileOutputStream("system.txt");

        // set input stream
        System.setIn(IN);
        char c = (char) System.in.read();
        System.out.print(c);

        // set output stream
        System.setOut(new PrintStream(OUT));
        System.out.write("Hi Abhishek\n".getBytes());

        // set error stream
        System.setErr(new PrintStream(OUT));
        System.err.write("Exception message\n".getBytes());
    }
}

Output: Output of above java code depends on content in “input.txt” file. 
Create your own “input.txt” then run the code and check the output. 

19. static void load(String filename): Loads a code file with the specified filename from the local file system as a dynamic library. The filename argument must be a complete path name. 

Syntax: public static void load(String filename)
Returns: NA
Exception: 
SecurityException - if a security manager exists and
 its checkLink method doesn't allow loading of the specified
 dynamic library
UnsatisfiedLinkError - if the file does not exist.
NullPointerException - if filename is null

20. 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. 

Syntax: public static void loadLibrary(String libname)
Returns: NA
Exception: 
SecurityException - if a security manager exists and its 
checkLink method doesn't allow loading of the specified dynamic
 library
UnsatisfiedLinkError - if the library does not exist.
NullPointerException - if libname is null

21. static String mapLibraryName(String libname): Maps a library name into a platform-specific string representing a native library. 

Syntax: public static String mapLibraryName(String libname)
Returns: a platform-dependent native library name.
Exception: NullPointerException - if libname is null

22. static void runFinalization(): Runs the finalization methods of any objects pending finalization. Calling this method suggests that the Java Virtual Machine expand 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. 

Syntax: public static void runFinalization()
Returns: NA
Exception: NA.

Java

// Java code illustrating runFinalization(), load()
// loadLibrary() and mapLibraryName() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {

        // map library name
        String libName = System.mapLibraryName("os.name");
        System.out.println("os.name library= " + libName);

        //load external libraries
        System.load("lixXYZ.so");
        System.loadLibrary("libos.name.dylib");

        //run finalization
        System.runFinalization();


    }
}

Output: 

os.name library= libos.name.dylib

23. static int identityHashCode(Object x): Returns the same hash code for the given object as would be returned by the default method hashCode(), whether or not the given object’s class overrides hashCode(). The hash code for the null reference is zero. 

Syntax: public static int identityHashCode(Object x)
Returns: the hashCode.
Exception: NA.

24. static Channel inheritedChannel(): Returns the channel inherited from the entity that created this Java virtual machine. 

Syntax: public static Channel inheritedChannel().
Returns:  inherited channel, if any, otherwise null.
Exception: 
IOException - If an I/O error occurs
SecurityException - If a security manager is present and
 it does not permit access to the channel.

25. static String lineSeparator(): Returns the system-dependent line separator string. It always returns the same value – the initial value of the system property line separator. 

Syntax: public static String lineSeparator()
Returns: On UNIX systems, it returns "\n";
 on Microsoft Windows systems it returns "\r\n".
Exception: NA

Java

// Java code illustrating lineSeparator(), inherentChannel()
// and identityHashCode() method
import java.io.IOException;
import java.lang.*;
import java.nio.channels.Channel;
class SystemDemo
{
    public static void main(String args[])
            throws NullPointerException,
            IOException
    {

        Integer x = 400;
        System.out.println(System.identityHashCode(x));

        Channel ch = System.inheritedChannel();
        System.out.println(ch);

        System.out.println(System.lineSeparator());

    }
}

Output: 

1735600054
null
"\r\n"

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