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.

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

The java.nio package defines the buffer classes, which are used throughout the NIO APIs. The charset API is defined in the java.nio.charset package, the channel and selector APIs in the java.nio.channels package, and the files and path APIs in the java.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.

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 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 mapping a region of a file directly into memory, in which case a few additional file-related operations defined in the 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 NullPointerException to be thrown.