public static final FileDescriptor in

A handle to the standard input stream. Usually, this file descriptor is not used directly, but rather via the input stream known as System.in.

public static final FileDescriptor out

A handle to the standard output stream. Usually, this file descriptor is not used directly, but rather via the output stream known as System.out.

public static final FileDescriptor err

A handle to the standard error stream. Usually, this file descriptor is not used directly, but rather via the output stream known as System.err.

public FileDescriptor()

Constructs an (invalid) FileDescriptor object. The fd or handle is set later.

public boolean valid()

Tests if this file descriptor object is valid.

public void sync() throws SyncFailedException

Force all system buffers to synchronize with the underlying device. This method returns after all modified data and attributes of this FileDescriptor have been written to the relevant device(s). In particular, if this FileDescriptor refers to a physical storage medium, such as a file in a file system, sync will not return until all in-memory modified copies of buffers associated with this FileDescriptor have been written to the physical medium. sync is meant to be used by code that requires physical storage (such as a file) to be in a known state For example, a class that provided a simple transaction facility might use sync to ensure that all changes to a file caused by a given transaction were recorded on a storage medium. sync only affects buffers downstream of this FileDescriptor. If any in-memory buffering is being done by the application (for example, by a BufferedOutputStream object), those buffers must be flushed into the FileDescriptor (for example, by invoking OutputStream.flush) before that data will be affected by sync.