The central abstractions of the NIO APIs are:

• Buffers, which are containers for data;

• Charsets and their associateddecoders andencoders,
  which translate between bytes and Unicode characters;

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

• Selectors andselection keys, which together with
  selectable channels define amultiplexed, non-blocking
  I/O facility.

• Path, which together with theFiles class provides access to files.

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

Abuffer is a container for a fixed amount of data of a specific primitive type. In addition to its content a buffer has aposition, which is the index of the next element to be read or written, and alimit, which is the index of the first element that should not be read or written. The baseBuffer class defines these properties as well as methods forclearing,flipping, andrewinding, formarking the current position, and forresetting the position to the previous mark.

There is a buffer class for each non-boolean primitive type. Each class defines a family ofget andput methods for moving data out of and in to a buffer, methods forcompacting,duplicating, andslicing a buffer, and static methods forallocating a new buffer as well as forwrapping 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 adirect 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 bymapping a region of a file directly into memory, in which case a few additional file-related operations defined in theMappedByteBuffer class are available.

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

Unless otherwise noted, passing anull argument to a constructor or method in any class or interface in this package will cause aNullPointerException to be thrown.