The classes java.io.OutputStream and java.io.FilterOutputStream only require subclasses to implement the method write(byte b). Typically, uses of OutputStreams will not write single bytes, but an array via the write(byte[] b, int off, int len) method. The default implementation of this method, which you are not required to override, calls write(byte b) for each byte in the array. If this method involves I/O, such as accessing the network or disk, this is likely to incur significant overhead.

Always provide an implementation of the write(byte[] b, int off, int len) method.

The following example shows a subclass of OutputStream that simply wraps a DigestOutputStream to confirm that the data it writes to a file has the expected MD5 hash. Without an implementation of write(byte[] b, int off, int len) this will be very slow, because it makes a call to DigestOutputStream.write(byte b) and FileOutputStream.write(byte b) for each byte written.

The example can be updated to use a more efficient method. In this case, calls to write(byte[] b, int off, int len) are simply forwarded to DigestOutputStream.write(byte[] b, int off, int len).

Java Platform, Standard Edition 8, API Documentation: OutputStream.write(byte[] b, int off, int len), FilterOutputStream.write(byte[] b, int off, int len).