Java Network Programming, Second Edition

Filter Streams

InputStream and OutputStream are fairly raw classes. They allow you to read and write bytes, either singly or in groups, but that’s all. Deciding what those bytes mean—whether they’re integers or IEEE 754 floating point numbers or Unicode text—is completely up to the programmer and the code. However, there are certain data formats that are extremely common and can benefit from a solid implementation in the class library. For example, many integers passed as parts of network protocols are 32-bit big-endian integers. Much text sent over the Web is either 7-bit ASCII or 8-bit Latin-1. Many files transferred by ftp are stored in the zip format. Java provides a number of filter classes you can attach to raw streams to translate the raw bytes to and from these and other formats.

The filters come in two versions: the filter streams and the readers and writers. The filter streams still work primarily with raw data as bytes, for instance, by compressing the data or interpreting it as binary numbers. The readers and writers handle the special case of text in a variety of encodings such as UTF-8 and ISO 8859-1. Filter streams are placed on top of raw streams such as a TelnetInputStream or a FileOutputStream or other filter streams. Readers and writers can be layered on top of raw streams, filter streams, or other readers and writers. However, filter streams cannot be placed on top of a reader or a writer, so we’ll start here with filter streams and address readers and writers ...