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Character streams (Introduced in JDK 1.1) - JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档

Character streams (Introduced in JDK 1.1)

Version 1.1 of the Java Development Kit introduced support for character streams to the java.io package.

Prior to JDK 1.1, the standard I/O facilities supported only byte streams, via the InputStream and OutputStream classes and their subclasses. Character streams are like byte streams, but they contain 16-bit Unicode characters rather than eight-bit bytes. They are implemented by the Reader and Writer classes and their subclasses. Readers and Writers support essentially the same operations as InputStreams and OutputStreams, except that where byte-stream methods operate on bytes or byte arrays, character-stream methods operate on characters, character arrays, or strings.

Most of the functionality available for byte streams is also provided for character streams. This is reflected in the name of each character-stream class, whose prefix is usually shared with the name of the corresponding byte-stream class. For example, there is a PushbackReader class that provides the same functionality for character streams that is provided by PushbackInputStream for byte streams.

Why use character streams?

The primary advantage of character streams is that they make it easy to write programs that are not dependent upon a specific character encoding, and are therefore easy to internationalize.

Java stores strings in Unicode, an international standard character encoding that is capable of representing most of the world's written languages. Typical user-readable text files, however, use encodings that are not necessarily related to Unicode, or even to ASCII, and there are many such encodings. Character streams hide the complexity of dealing with these encodings by providing two classes that serve as bridges between byte streams and character streams. The InputStreamReader class implements a character-input stream that reads bytes from a byte-input stream and converts them to characters according to a specified encoding. Similarly, the OutputStreamWriter class implements a character-output stream that converts characters into bytes according a specified encoding and writes them to a byte-output stream.

A second advantage of character streams is that they are potentially much more efficient than byte streams. The implementations of many of Java's original byte streams are oriented around byte-at-a-time read and write operations. The character-stream classes, in contrast, are oriented around buffer-at-a-time read and write operations. This difference, in combination with a more efficient locking scheme, allows the character stream classes to make up for the added overhead of encoding conversion in many cases.

API overview

The character-stream classes have been designed to parallel the existing byte-stream classes in the java.io package. As noted above, the name of each character-stream class ends in Reader or Writer, as appropriate, while its prefix is usually shared with the corresponding byte-stream class, if any. The following table summarizes the new classes; in the left column, indentation indicates subclass relationships.

Character-stream class	Description	Corresponding byte class
Reader	Abstract class for character-input streams	InputStream
BufferedReader	Buffers input, parses lines	BufferedInputStream
LineNumberReader	Keeps track of line numbers	LineNumberInputStream
CharArrayReader	Reads from a character array	ByteArrayInputStream
InputStreamReader	Translates a byte stream into a character stream	(none)
FileReader	Translates bytes from a file into a character stream	FileInputStream
FilterReader	Abstract class for filtered character input	FilterInputStream
PushbackReader	Allows characters to be pushed back	PushbackInputStream
PipedReader	Reads from a PipedWriter	PipedInputStream
StringReader	Reads from a String	StringBufferInputStream
Writer	Abstract class for character-output streams	OutputStream
BufferedWriter	Buffers output, uses platform's line separator	BufferedOutputStream
CharArrayWriter	Writes to a character array	ByteArrayOutputStream
FilterWriter	Abstract class for filtered character output	FilterOutputStream
OutputStreamWriter	Translates a character stream into a byte stream	(none)
FileWriter	Translates a character stream into a byte file	FileOutputStream
PrintWriter	Prints values and objects to a Writer	PrintStream
PipedWriter	Writes to a PipedReader	PipedOutputStream
StringWriter	Writes to a String	(none)

Related changes

PrintStream

The PrintStream class has been modified to use the platform's default character encoding and the platform's default line terminator. Thus each PrintStream incorporates an OutputStreamWriter, and it passes all characters through this writer to produce bytes for output. The println methods use the platform's default line terminator, which is defined by the system property line.separator and is not necessarily a single newline character ('\n'). Bytes and byte arrays written via the existing write methods are not passed through the writer.

The primary motivation for changing the PrintStream class is that it will make System.out and System.err more useful to people writing Java programs on platforms where the local encoding is something other than ASCII. PrintStream is, in other words, provided primarily for use in debugging and for compatibility with existing code. Code that produces textual output should use the new PrintWriter class, which allows the character encoding to be specified or the default encoding to be accepted. For convenience, the PrintWriter class provides constructors that take an OutputStream object and create an intermediate OutputStreamWriter object that uses the default encoding.

Other classes

The following constructors and methods have been deprecated because they do not properly convert between bytes and characters:

String	DataInputStream.readLine()
InputStream	Runtime.getLocalizedInputStream(InputStream)
OutputStream	Runtime.getLocalizedOutputStream(OutputStream)
	StreamTokenizer(InputStream)
	String(byte ascii[], int hibyte, int offset, int count)
	String(byte ascii[], int hibyte)
void	String.getBytes(int srcBegin, int srcEnd, byte dst[], int dstBegin)

Finally, the following constructor and methods have been added:

	StreamTokenizer(Reader)
byte[]	String.getBytes()
void	Throwable.printStackTrace(PrintWriter)

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