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Overview of I/O Streams

To bring in information, a program opens a stream on an information source (a file, memory, a socket) and reads the information sequentially, as shown here:

Similarly, a program can send information to an external destination by opening a stream to a destination and writing the information out sequentially, like this:

No matter where the data is coming from or going to and no matter what its type, the algorithms for sequentially reading and writing data are basically the same:

Reading	Writing
open a stream while more information read information close the stream	open a stream while more information write information close the stream

The java.io package contains a collection of stream classes that support these algorithms for reading and writing. To use these classes, a program needs to import the java.io package. The stream classes are divided into two class hierarchies, based on the data type (either characters or bytes) on which they operate.

Character Streams

Reader and Writer are the abstract superclasses for character streams in java.io. Reader provides the API and partial implementation for readers--streams that read 16-bit characters--and Writer provides the API and partial implementation for writers--streams that write 16-bit characters. Subclasses of Reader and Writer implement specialized streams and are divided into two categories: those that read from or write to data sinks (shown in gray in the following figures) and those that perform some sort of processing (shown in white). The figure shows the class hierarchies for the Reader and Writer classes.

Most programs should use readers and writers to read and write textual information. The reason is that they can handle any character in the Unicode character set, whereas the byte streams are limited to ISO-Latin-1 8-bit bytes.

Byte Streams

To read and write 8-bit bytes, programs should use the byte streams, descendants of InputStream and OutputStream. InputStream and OutputStream provide the API and partial implementation for input streams (streams that read 8-bit bytes) and output streams (streams that write 8-bit bytes). These streams are typically used to read and write binary data such as images and sounds. Two of the byte stream classes, ObjectInputStream and ObjectOutputStream, are used for object serialization. These classes are covered in Object Serialization.

As with Reader and Writer, subclasses of InputStream and OutputStream provide specialized I/O that falls into two categories, as shown in the following class hierarchy figure: data sink streams (shaded) and processing streams (unshaded).

Understanding the I/O Superclasses

Reader and InputStream define similar APIs but for different data types. For example, Reader contains these methods for reading characters and arrays of characters:

int read()
int read(char cbuf[])
int read(char cbuf[], int offset, int length)

InputStream defines the same methods but for reading bytes and arrays of bytes:

int read()
int read(byte cbuf[])
int read(byte cbuf[], int offset, int length)

Also, both Reader and InputStream provide methods for marking a location in the stream, skipping input, and resetting the current position.

Writer and OutputStream are similarly parallel. Writer defines these methods for writing characters and arrays of characters:

int write(int c)
int write(char cbuf[])
int write(char cbuf[], int offset, int length)

And OutputStream defines the same methods but for bytes:

int write(int c)
int write(byte cbuf[])
int write(byte cbuf[], int offset, int length)

All of the streams--readers, writers, input streams, and output streams--are automatically opened when created. You can close any stream explicitly by calling its close method. Or the garbage collector can implicitly close it, which occurs when the object is no longer referenced.

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