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

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.)

src	the source array.
srcPos	starting position in the source array.
dest	the destination array.
destPos	starting position in the destination data.
length	the number of array elements to be copied.
Throws	IndexOutOfBoundsException: 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.
Throws	NullPointerException: if either src or dest is null.

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.

key	the name of the system property to be removed.
return	the previous string value of the system property, or null if there was no property with that key.
Throws	SecurityException: if a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property.
Throws	NullPointerException: if key is null.
Throws	IllegalArgumentException: if key is empty.
since	1.5
See also	getProperty, setProperty, java.util.Properties, java.lang.SecurityException, checkPropertiesAccess()

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

return	the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.
See also	java.util.Date

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)
 

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()
 

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.

return	The inherited channel, if any, otherwise null.
Throws	IOException: If an I/O error occurs
Throws	SecurityException: If a security manager is present and it does not permit access to the channel.
since	1.5

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

 Runtime.getRuntime().load(name)
 

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

 Runtime.getRuntime().loadLibrary(name)
 

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 time (perhaps in the future, so values may be negative). This method provides nanosecond precision, but not necessarily nanosecond accuracy. No guarantees are made about how frequently values change. Differences in successive calls that span greater than approximately 292 years (2⁶³ nanoseconds) will not accurately compute elapsed time due to numerical overflow.

For example, to measure how long some code takes to execute:

   long startTime = System.nanoTime();
   // ... the code being measured ...
   long estimatedTime = System.nanoTime() - startTime;
 

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()
 

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.

deprecated	This method is inherently unsafe. It may result in finalizers being called on live objects while other threads are concurrently manipulating those objects, resulting in erratic behavior or deadlock.
value	indicating enabling or disabling of finalization
Throws	SecurityException: if a security manager exists and its checkExit method doesn't allow the exit.
since	JDK1.1
See also	exit(int), gc(), checkExit(int)

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.

name	the name of the environment variable
return	the string value of the variable, or null if the variable is not defined in the system environment
Throws	NullPointerException: if name is null
Throws	SecurityException: if a security manager exists and its checkPermission method doesn't allow access to the environment variable name
See also	getenv(), environment()

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.

return	the environment as a map of variable names to values
Throws	SecurityException: if a security manager exists and its checkPermission method doesn't allow access to the process environment
since	1.5
See also	getenv(String), environment()

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.

err	the new standard error output stream.
Throws	SecurityException: if a security manager exists and its checkPermission method doesn't allow reassigning of the standard error output stream.
since	JDK1.1
See also	checkPermission, java.lang.RuntimePermission

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.

in	the new standard input stream.
Throws	SecurityException: if a security manager exists and its checkPermission method doesn't allow reassigning of the standard input stream.
since	JDK1.1
See also	checkPermission, java.lang.RuntimePermission

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.

out	the new standard output stream
Throws	SecurityException: if a security manager exists and its checkPermission method doesn't allow reassigning of the standard output stream.
since	JDK1.1
See also	checkPermission, java.lang.RuntimePermission

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.

props	the new system properties.
Throws	SecurityException: if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.
See also	getProperties, java.util.Properties, java.lang.SecurityException, checkPropertiesAccess()

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.

return	the system properties
Throws	SecurityException: if a security manager exists and its checkPropertiesAccess method doesn't allow access to the system properties.
See also	setProperties, java.lang.SecurityException, checkPropertiesAccess(), java.util.Properties

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.

key	the name of the system property.
value	the value of the system property.
return	the previous value of the system property, or null if it did not have one.
Throws	SecurityException: if a security manager exists and its checkPermission method doesn't allow setting of the specified property.
Throws	NullPointerException: if key or value is null.
Throws	IllegalArgumentException: if key is empty.
since	1.2
See also	getProperty, getProperty(java.lang.String), getProperty(java.lang.String, java.lang.String), java.util.PropertyPermission, checkPermission

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.

key	the name of the system property.
return	the string value of the system property, or null if there is no property with that key.
Throws	SecurityException: if a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property.
Throws	NullPointerException: if key is null.
Throws	IllegalArgumentException: if key is empty.
See also	setProperty, java.lang.SecurityException, checkPropertyAccess(java.lang.String), getProperties()

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.

key	the name of the system property.
def	a default value.
return	the string value of the system property, or the default value if there is no property with that key.
Throws	SecurityException: if a security manager exists and its checkPropertyAccess method doesn't allow access to the specified system property.
Throws	NullPointerException: if key is null.
Throws	IllegalArgumentException: if key is empty.
See also	setProperty, checkPropertyAccess(java.lang.String), getProperties()

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.

s	the security manager.
Throws	SecurityException: if the security manager has already been set and its checkPermission method doesn't allow it to be replaced.
See also	getSecurityManager, checkPermission, java.lang.RuntimePermission