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.

Also see:

java.io.PrintStream#println()
java.io.PrintStream#println(boolean)
java.io.PrintStream#println(char)
java.io.PrintStream#println(char[])
java.io.PrintStream#println(double)
java.io.PrintStream#println(float)
java.io.PrintStream#println(int)
java.io.PrintStream#println(long)
java.io.PrintStream#println(java.lang.Object)
java.io.PrintStream#println(java.lang.String)

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. 

 public static native  void arraycopy(Object src,
    int srcPos,
    Object dest,
    int destPos,
    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 dest. The number of components copied is
equal to the length argument. The components at
positions srcPos through
srcPos+length-1 in the source array are copied into
positions destPos through
destPos+length-1, respectively, of the destination
array.

If the src and dest arguments refer to the
same array object, then the copying is performed as if the
components at positions srcPos through
srcPos+length-1 were first copied to a temporary
array with length components and then the contents of
the temporary array were copied into positions
destPos through destPos+length-1 of the
destination array.

If dest 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 dest argument refers to an object that is not an
    array.
The src argument and dest 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 dest argument refers to an array
    with a reference component type.
The src argument refers to an array with a reference
   component type and the dest 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 srcPos argument is negative.
The destPos argument is negative.
The length argument is negative.
srcPos+length is greater than
    src.length, the length of the source array.
destPos+length is greater than
    dest.length, the length of the destination array.


Otherwise, if any actual component of the source array from
position srcPos through
srcPos+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[srcPos+k]
cannot be converted to the component type of the destination
array; when the exception is thrown, source array components from
positions srcPos through
srcPos+k-1
will already have been copied to destination array positions
destPos through
destPos+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.)

 public static String clearProperty(String key) 
{
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new PropertyPermission(key, "write"));
        }
        return (String) props.remove(key);
}

Removes the system property indicated by the specified key.

First, if a security manager exists, its SecurityManager.checkPermission method is called with a PropertyPermission(key, "write") permission. This may result in a SecurityException being thrown. If no exception is thrown, the specified property is removed.

 public static Console console() 
{
         if (cons == null) {
             synchronized (System.class) {
                 cons = sun.misc.SharedSecrets.getJavaIOAccess().console();
             }
         }
         return cons;
}

Returns the unique Console object associated with the current Java virtual machine, if any.

 public static native 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.

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

 public static  void exit(int status) 
{
        Runtime.getRuntime().exit(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.

The call System.exit(n) is effectively equivalent to the call:

Runtime.getRuntime().exit(n)

 public static  void gc() 
{
        Runtime.getRuntime().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()

 static Class<?> getCallerClass() 
{
        // NOTE use of more generic Reflection.getCallerClass()
        return Reflection.getCallerClass(3);
}

 public static Properties getProperties() 
{
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertiesAccess();
        }
        return props;
}

Determines the current system properties.

First, 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 for use by the #getProperty(String) method 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 Runtime Environment version
java.vendor	Java Runtime Environment vendor
java.vendor.url	Java vendor URL
java.home	Java installation directory
java.vm.specification.version	Java Virtual Machine specification version
java.vm.specification.vendor	Java Virtual Machine specification vendor
java.vm.specification.name	Java Virtual Machine specification name
java.vm.version	Java Virtual Machine implementation version
java.vm.vendor	Java Virtual Machine implementation vendor
java.vm.name	Java Virtual Machine implementation name
java.specification.version	Java Runtime Environment specification version
java.specification.vendor	Java Runtime Environment specification vendor
java.specification.name	Java Runtime Environment specification name
java.class.version	Java class format version number
java.class.path	Java class path
java.library.path	List of paths to search when loading libraries
java.io.tmpdir	Default temp file path
java.compiler	Name of JIT compiler to use
java.ext.dirs	Path of extension directory or directories
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

Multiple paths in a system property value are separated by the path separator character of the platform.

Note that even if the security manager does not permit the getProperties operation, it may choose to permit the #getProperty(String) operation.

 public static String getProperty(String key) 
{
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertyAccess(key);
        }
        return props.getProperty(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 SecurityException.

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.

 public static String getProperty(String key,
    String def) 
{
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertyAccess(key);
        }
        return props.getProperty(key, 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.

 public static SecurityManager getSecurityManager() 
{
        return security;
}

Gets the system security interface.

 public static Map<String, String> getenv() 
{
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("getenv.*"));
        }
        return ProcessEnvironment.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.

The returned map will never contain null keys or values. Attempting to query the presence of a null key or value will throw a NullPointerException . Attempting to query the presence of a key or value which is not of type String will throw a ClassCastException .

The returned map and its collection views may not obey the general contract of the Object#equals and Object#hashCode methods.

The returned map is typically case-sensitive on all platforms.

If a security manager exists, its checkPermission method is called with a RuntimePermission ("getenv.*") permission. This may result in a SecurityException being thrown.

When passing information to a Java subprocess, system properties are generally preferred over environment variables.

 public static String getenv(String name) 
{
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("getenv."+name));
        }
        return ProcessEnvironment.getenv(name);
}

Gets the value of the specified environment variable. An environment variable is a system-dependent external named value.

If a security manager exists, its checkPermission method is called with a RuntimePermission ("getenv."+name) permission. This may result in a SecurityException being thrown. If no exception is thrown the value of the variable name is returned.

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).

On UNIX systems the alphabetic case of name is typically significant, while on Microsoft Windows systems it is typically not. For example, the expression System.getenv("FOO").equals(System.getenv("foo")) is likely to be true on Microsoft Windows.

 public static native 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.

 public static Channel inheritedChannel() throws IOException 
{
        return SelectorProvider.provider().inheritedChannel();
}

Returns the channel inherited from the entity that created this Java virtual machine.

This method returns the channel obtained by invoking the inheritedChannel method of the system-wide default java.nio.channels.spi.SelectorProvider object.

In addition to the network-oriented channels described in inheritedChannel , this method may return other kinds of channels in the future.

 public static String lineSeparator() 
{
        return lineSeparator;
}

Returns the system-dependent line separator string. It always returns the same value - the initial value of the {@linkplain #getProperty(String) system property} {@code line.separator}.

On UNIX systems, it returns {@code "\n"}; on Microsoft Windows systems it returns {@code "\r\n"}.

 public static  void load(String filename) 
{
        Runtime.getRuntime().load0(getCallerClass(), 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.

The call System.load(name) is effectively equivalent to the call:

Runtime.getRuntime().load(name)

 public static  void loadLibrary(String libname) 
{
        Runtime.getRuntime().loadLibrary0(getCallerClass(), 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.

The call System.loadLibrary(name) is effectively equivalent to the call

Runtime.getRuntime().loadLibrary(name)

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

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

This method can only be used to measure elapsed time and is
not related to any other notion of system or wall-clock time.
The value returned represents nanoseconds since some fixed but
arbitrary origin time (perhaps in the future, so values
may be negative).  The same origin is used by all invocations of
this method in an instance of a Java virtual machine; other
virtual machine instances are likely to use a different origin.

This method provides nanosecond precision, but not necessarily
nanosecond resolution (that is, how frequently the value changes)
- no guarantees are made except that the resolution is at least as
good as that of #currentTimeMillis() .

Differences in successive calls that span greater than
approximately 292 years (263 nanoseconds) will not
correctly compute elapsed time due to numerical overflow.

The values returned by this method become meaningful only when
the difference between two such values, obtained within the same
instance of a Java virtual machine, is computed.

 For example, to measure how long some code takes to execute:
 
 {@code
long startTime = System.nanoTime();
// ... the code being measured ...
long estimatedTime = System.nanoTime() - startTime;}

To compare two nanoTime values
 
 {@code
long t0 = System.nanoTime();
...
long t1 = System.nanoTime();}

one should use {@code t1 - t0 < 0}, not {@code t1 < t0},
because of the possibility of numerical overflow.

 public static  void runFinalization() 
{
        Runtime.getRuntime().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.

The call System.runFinalization() is effectively equivalent to the call:

Runtime.getRuntime().runFinalization()

 public static  void runFinalizersOnExit(boolean value) 
{
        Runtime.getRuntime().runFinalizersOnExit(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.

If there is a security manager, its checkExit method is first called with 0 as its argument to ensure the exit is allowed. This could result in a SecurityException.

 public static  void setErr(PrintStream err) 
{
        checkIO();
        setErr0(err);
}

Reassigns the "standard" error output stream.

First, if there is a security manager, its checkPermission method is called with a RuntimePermission("setIO") permission to see if it's ok to reassign the "standard" error output stream.

 public static  void setIn(InputStream in) 
{
        checkIO();
        setIn0(in);
}

Reassigns the "standard" input stream.

First, if there is a security manager, its checkPermission method is called with a RuntimePermission("setIO") permission to see if it's ok to reassign the "standard" input stream.

 public static  void setOut(PrintStream out) 
{
        checkIO();
        setOut0(out);
}

Reassigns the "standard" output stream.

First, if there is a security manager, its checkPermission method is called with a RuntimePermission("setIO") permission to see if it's ok to reassign the "standard" output stream.

 public static  void setProperties(Properties props) 
{
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPropertiesAccess();
        }
        if (props == null) {
            props = new Properties();
            initProperties(props);
        }
        System.props = 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(String) method. If the argument is null, then the current set of system properties is forgotten.

 public static String setProperty(String key,
    String value) 
{
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new PropertyPermission(key,
                SecurityConstants.PROPERTY_WRITE_ACTION));
        }
        return (String) props.setProperty(key, value);
}

Sets the system property indicated by the specified key.

First, if a security manager exists, its SecurityManager.checkPermission method is called with a PropertyPermission(key, "write") permission. This may result in a SecurityException being thrown. If no exception is thrown, the specified property is set to the given value.

 public static  void setSecurityManager(SecurityManager s) 
{
        try {
            s.checkPackageAccess("java.lang");
        } catch (Exception e) {
            // no-op
        }
        setSecurityManager0(s);
}

Sets the System security.

If there is a security manager already installed, this method first calls the security manager's checkPermission method with a RuntimePermission("setSecurityManager") permission to ensure it's ok to replace the existing security manager. This may result in throwing a SecurityException.

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.