Prior to this release, 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.
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.
| 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) |
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. Therefore its two constructors have been deprecated:
| PrintStream(OutputStream) | |
| PrintStream(OutputStream, boolean) |
Deprecating the constructors rather than the entire class allows existing uses of System.out and System.err to be compiled without generating deprecation warnings. Thus programmers just learning Java, or programmers inserting System.err.println calls for debugging purposes, will not be bothered by such warnings. Programmers writing code that explicitly constructs a PrintStream, on the other hand, will see a deprecation warning when that code is compiled. 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.
Minor changes have been made to several existing classes. The following constructors and methods have been deprecated because they do not properly convert between bytes and characters:
Finally, the following constructor and methods have been added:
| StreamTokenizer(Reader) | |
| byte[] | String.getBytes() |
| void | Throwable.printStackTrace(PrintWriter) |
| Comments to: java-io@java.sun.com | Mark Reinhold / 4 February 1997 |