FilterInputStream

FilterInputStream is a class in the Java I/O library that provides a simple framework for wrapping an existing InputStream to apply filtering or transformations to the data as it is read. It is a concrete subclass of InputStream and is designed to be extended by concrete filter implementations.

The core idea is delegation: FilterInputStream delegates most of its methods to the underlying stream, which is stored in a protected field commonly named in. The primary constructor FilterInputStream(InputStream in) initializes this wrapped stream. Subclasses can override read methods and other operations to alter the data stream, while the default implementation simply passes calls through to the wrapped stream.

By itself, FilterInputStream does not add significant processing; it serves as a base class for building more specialized filters. Examples of concrete filters include DataInputStream, which adds methods to read Java primitive types from the underlying stream, and BufferedInputStream, which provides buffering to improve efficiency. Many other streams such as InflaterInputStream and GZIPInputStream extend FilterInputStream to perform decompression or other transformations.

Key behavior is that most methods—read, skip, available, close, mark, and reset—delegate to the wrapped stream unless overridden. This design allows stacking multiple filters in a chain, each adding functionality while preserving a common interface to the underlying data source.

Related concepts include FilterOutputStream, which provides a counterpart for OutputStream, and the broader decorator design pattern. FilterInputStream is widely used to compose streaming behavior in a modular way.