Java I/O Streams

Java I/O Streams

Contents

Overview
Character Streams
Byte Streams
Understanding the I/O Superclasses
Using the Streams

See Also

File streams
Filter streams
Pipe streams
Concatenate Files

Overview

To bring in information, a program opens a stream on an information source (a file, memory, a socket) and reads the information sequentially. Similarly, a program can send information to an external destination by opening a stream to a destination and writing the information out sequentially. 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).
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.
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.

Using the Streams

The following table lists java.io's streams and describes what they do. Note that many times, java.io contains character streams and byte streams that perform the same type of I/O but for different data types.

Type of I/O Streams Description

Memory CharArrayReader
CharArrayWriter
ByteArrayInputStream
ByteArrayOutputStream Read from and write to memory. Create on an existing array and then use the read and write methods to read from or write to the array.

Memory StringReader
StringWriter
StringBufferInputStream top>StringReader reads characters from a String in memory.
StringWriter writes to a String, collecting characters written to it in a StringBuffer, which can then be converted to a String.
StringBufferInputStream is similar to StringReader, except that it reads bytes from a StringBuffer.

Pipe PipedReader
PipedWriter
PipedInputStream
PipedOutputStream Implement the input and output components of a pipe. Pipes are used to channel the output from one thread into the input of another.

File FileReader
FileWriter
FileInputStream
FileOutputStream Collectively called file streams, these streams are used to read from or write to a file on the native file system.

Concatenation N/A
SequenceInputStream Concatenates multiple input streams into one input stream.

Object Serialization N/A
ObjectInputStream
ObjectOutputStream Used to serialize objects.

Data
Conversion N/A
DataInputStream
DataOutputStream Read or write primitive data types in a machine-independent format.

Counting LineNumberReader
LineNumberInputStream Keeps track of line numbers while reading.

Peeking Ahead PushbackReader
PushbackInputStream These input streams each have a pushback buffer. When reading data from a stream, it is sometimes useful to peek at the next few bytes or characters in the stream to decide what to do next.

Printing PrintWriter
PrintStream Contain convenient printing methods. These are the easiest streams to write to, so you will often see other writable streams wrapped in one of these.

Buffering BufferedReader
BufferedWriter
BufferedInputStream
BufferedOutputStream Buffer data while reading or writing, thereby reducing the number of accesses required on the original data source. Buffered streams are typically more efficient than similar nonbuffered streams and are often used with other streams.

Filters FilterReader
FilterWriter
FilterInputStream
FilterOutputStream These abstract classes define the interface for filter streams, which filter data as it's being read or written.

Convert InputStreamReader
OutputStreamWriter Convert byte streams to character streams, and vice-versa.
InputStreamReaders read bytes from InputStreams and convert them to characters. OutputStreamWriters convert characters to bytes and write to an OutputStream. Default character encodings are stored in System.getProperty("file.encoding").

Type of I/O	Streams	Description
Memory	CharArrayReader CharArrayWriter ByteArrayInputStream ByteArrayOutputStream	Read from and write to memory. Create on an existing array and then use the read and write methods to read from or write to the array.
Memory	StringReader StringWriter StringBufferInputStream	top>StringReader reads characters from a String in memory. StringWriter writes to a String, collecting characters written to it in a StringBuffer, which can then be converted to a String. StringBufferInputStream is similar to StringReader, except that it reads bytes from a StringBuffer.
Pipe	PipedReader PipedWriter PipedInputStream PipedOutputStream	Implement the input and output components of a pipe. Pipes are used to channel the output from one thread into the input of another.
File	FileReader FileWriter FileInputStream FileOutputStream	Collectively called file streams, these streams are used to read from or write to a file on the native file system.
Concatenation	N/A SequenceInputStream	Concatenates multiple input streams into one input stream.
Object Serialization	N/A ObjectInputStream ObjectOutputStream	Used to serialize objects.
Data Conversion	N/A DataInputStream DataOutputStream	Read or write primitive data types in a machine-independent format.
Counting	LineNumberReader LineNumberInputStream	Keeps track of line numbers while reading.
Peeking Ahead	PushbackReader PushbackInputStream	These input streams each have a pushback buffer. When reading data from a stream, it is sometimes useful to peek at the next few bytes or characters in the stream to decide what to do next.
Printing	PrintWriter PrintStream	Contain convenient printing methods. These are the easiest streams to write to, so you will often see other writable streams wrapped in one of these.
Buffering	BufferedReader BufferedWriter BufferedInputStream BufferedOutputStream	Buffer data while reading or writing, thereby reducing the number of accesses required on the original data source. Buffered streams are typically more efficient than similar nonbuffered streams and are often used with other streams.
Filters	FilterReader FilterWriter FilterInputStream FilterOutputStream	These abstract classes define the interface for filter streams, which filter data as it's being read or written.
Convert	InputStreamReader OutputStreamWriter	Convert byte streams to character streams, and vice-versa. InputStreamReaders read bytes from InputStreams and convert them to characters. OutputStreamWriters convert characters to bytes and write to an OutputStream. Default character encodings are stored in System.getProperty("file.encoding").