FilterOutputStream (Java SE 23 & JDK 23)
Description: declaration: module: java.base, package: java.io, class: FilterOutputStream
Keywords: java.io.FilterOutputStream class

# Class FilterOutputStream

java.lang.Object
java.io.OutputStream
java.io.FilterOutputStream

All Implemented Interfaces:
  `Closeable`, `Flushable`, `AutoCloseable`

Direct Known Subclasses:
  `BufferedOutputStream`, `CheckedOutputStream`, `CipherOutputStream`, `DataOutputStream`, `DeflaterOutputStream`, `DigestOutputStream`, `InflaterOutputStream`, `PrintStream`

public class FilterOutputStream extends OutputStream
This class is the superclass of all classes that filter output streams. These streams sit on top of an already existing output stream (the underlying output stream) which it uses as its basic sink of data, but possibly transforming the data along the way or providing additional functionality.

The class `FilterOutputStream` itself simply overrides all methods of `OutputStream` with versions that pass all requests to the underlying output stream. Subclasses of `FilterOutputStream` may further override some of these methods as well as provide additional methods and fields.

Since:
  1.0

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `protected OutputStream`
  `out`
  The underlying output stream to be filtered.

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `FilterOutputStream(OutputStream out)`
  Creates an output stream filter built on top of the specified underlying output stream.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `close()`
  Closes this output stream and releases any system resources associated with the stream.
  `void`
  `flush()`
  Flushes this output stream and forces any buffered output bytes to be written out to the stream.
  `void`
  `write(byte[] b)`
  Writes `b.length` bytes to this output stream.
  `void`
  `write(byte[] b, int off, int len)`
  Writes `len` bytes from the specified `byte` array starting at offset `off` to this output stream.
  `void`
  `write(int b)`
  Writes the specified `byte` to this output stream.

  ### Methods declared in class java.io.OutputStream

  `nullOutputStream`

  ### Methods declared in class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Field Details

  - ### out

    protected OutputStream out
    The underlying output stream to be filtered.

- ## Constructor Details

  - ### FilterOutputStream

    public FilterOutputStream(OutputStream out)
    Creates an output stream filter built on top of the specified underlying output stream.
    Parameters:
      `out` - the underlying output stream to be assigned to the field `this.out` for later use, or `null` if this instance is to be created without an underlying stream.

- ## Method Details

  - ### write

    public void write(int b) throws IOException
    Writes the specified `byte` to this output stream.

    The `write` method of `FilterOutputStream` calls the `write` method of its underlying output stream, that is, it performs `out.write(b)`.

    Implements the abstract `write` method of `OutputStream`.

    Specified by:
      `write` in class `OutputStream`
    Parameters:
      `b` - the `byte`.
    Throws:
      `IOException` - if an I/O error occurs.

  - ### write

    public void write(byte[] b) throws IOException
    Writes `b.length` bytes to this output stream.
    Overrides:
      `write` in class `OutputStream`
    Implementation Requirements:
      The `write` method of `FilterOutputStream` calls its `write` method of three arguments with the arguments `b`, `0`, and `b.length`.
    Implementation Note:
      Note that this method does not call the one-argument `write` method of its underlying output stream with the single argument `b`.
    Parameters:
      `b` - the data to be written.
    Throws:
      `IOException` - if an I/O error occurs.
    See Also:
      - `write(byte[], int, int)`

  - ### write

    public void write(byte[] b, int off, int len) throws IOException
    Writes `len` bytes from the specified `byte` array starting at offset `off` to this output stream.
    Overrides:
      `write` in class `OutputStream`
    Implementation Requirements:
      The `write` method of `FilterOutputStream` calls the `write` method of one argument on each `byte` to output.
    Implementation Note:
      Note that this method does not call the `write` method of its underlying output stream with the same arguments. Subclasses of `FilterOutputStream` should provide a more efficient implementation of this method.
    Parameters:
      `b` - the data.
      `off` - the start offset in the data.
      `len` - the number of bytes to write.
    Throws:
      `IOException` - if an I/O error occurs.
      `IndexOutOfBoundsException` - If `off` is negative, `len` is negative, or `len` is greater than `b.length - off`
    See Also:
      - `write(int)`

  - ### flush

    public void flush() throws IOException
    Flushes this output stream and forces any buffered output bytes to be written out to the stream.
    Specified by:
      `flush` in interface `Flushable`
    Overrides:
      `flush` in class `OutputStream`
    Implementation Requirements:
      The `flush` method of `FilterOutputStream` calls the `flush` method of its underlying output stream.
    Throws:
      `IOException` - if an I/O error occurs.
    See Also:
      - `out`

  - ### close

    public void close() throws IOException
    Closes this output stream and releases any system resources associated with the stream.
    Specified by:
      `close` in interface `AutoCloseable`
    Specified by:
      `close` in interface `Closeable`
    Overrides:
      `close` in class `OutputStream`
    Implementation Requirements:
      When not already closed, the `close` method of `FilterOutputStream` calls its `flush` method, and then calls the `close` method of its underlying output stream.
    Throws:
      `IOException` - if an I/O error occurs.
    See Also:
      - `flush()`
      - `out`