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Overview of I/O Streams

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.

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Most programs should use readers and writers to read and write information. This is because they both can handle any character in the Unicode character set (while the byte streams are limited to ISO-Latin-1 8-bit bytes).

Byte Streams

Programs should use the byte streams, descendants of InputStream and OutputStream, to read and write 8-bit bytes. InputStream and OutputStream provide the API and some 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.

As with Reader and Writer, subclasses of InputStream and OutputStream provide specialized I/O that falls into two categories: data sink streams and processing streams. Figure 56 shows the class hierarchies for the byte streams.

As mentioned, two of the byte stream classes, ObjectInputStream and ObjectOutputStream, are used for object serialization. These classes are fully covered in Object Serialization.

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.

Learn how to use a selected assortment of these two types of streams in the next two sections:

• Using the Data Sink Streams
• Using the Processing Streams

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