The central abstractions of the NIO APIs are:

• Buffers, which are containers for data;

• Charsets and their associated decoders and encoders,
  which translate between bytes and Unicode characters;

• Channels of various types, which represent connections
  to entities capable of performing I/O operations; and

• Selectors and selection keys, which together with
  selectable channels define a multiplexed, non-blocking
  I/O facility.

The java.nio package defines the buffer classes, which are used throughout the NIO APIs. The charset API is defined in the {@link java.nio.charset} package, and the channel and selector APIs are defined in the {@link java.nio.channels} package. Each of these subpackages has its own service-provider (SPI) subpackage, the contents of which can be used to extend the platform's default implementations or to construct alternative implementations.

Buffers	Description
{@link java.nio.Buffer}	Position, limit, and capacity; clear, flip, rewind, and mark/reset
{@link java.nio.ByteBuffer}	Get/put, compact, views; allocate, wrap
{@link java.nio.MappedByteBuffer}	A byte buffer mapped to a file
{@link java.nio.CharBuffer}	Get/put, compact; allocate, wrap
{@link java.nio.DoubleBuffer}	' '
{@link java.nio.FloatBuffer}	' '
{@link java.nio.IntBuffer}	' '
{@link java.nio.LongBuffer}	' '
{@link java.nio.ShortBuffer}	' '
{@link java.nio.ByteOrder}	Typesafe enumeration for byte orders

A buffer is a container for a fixed amount of data of a specific primitive type. In addition to its content a buffer has a position, which is the index of the next element to be read or written, and a limit, which is the index of the first element that should not be read or written. The base {@link java.nio.Buffer} class defines these properties as well as methods for clearing, flipping, and rewinding, for marking the current position, and for resetting the position to the previous mark.

There is a buffer class for each non-boolean primitive type. Each class defines a family of get and put methods for moving data out of and in to a buffer, methods for compacting, duplicating, and slicing a buffer, and static methods for allocating a new buffer as well as for wrapping an existing array into a buffer.

Byte buffers are distinguished in that they can be used as the sources and targets of I/O operations. They also support several features not found in the other buffer classes:

• A byte buffer can be allocated as a direct buffer, in which case the Java virtual machine will make a best effort to perform native I/O operations directly upon it.

• A byte buffer can be created by {@link java.nio.channels.FileChannel#map mapping} a region of a file directly into memory, in which case a few additional file-related operations defined in the {@link java.nio.MappedByteBuffer} class are available.

• A byte buffer provides access to its content as either a heterogeneous or homogeneous sequence of binary data of any non-boolean primitive type, in either big-endian or little-endian byte order.

Unless otherwise noted, passing a null argument to a constructor or method in any class or interface in this package will cause a {@link java.lang.NullPointerException NullPointerException} to be thrown. @since 1.4 @author Mark Reinhold @author JSR-51 Expert Group