Java I/O

Subclassing OutputStream

OutputStream is an abstract class that mainly describes the operations available with any particular OutputStream object. Specific subclasses know how to write bytes to particular destinations. For instance, a FileOutputStream uses native code to write data in files. A ByteArrayOutputStream uses pure Java to write its output in a potentially expanding byte array.

Recall that there are three overloaded variants of the write() method in OutputStream, one abstract, two concrete:

public abstract void write(int b) throws IOException
public void write(byte[] data) throws IOException
public void write(byte[] data, int offset, int length) throws IOException

Subclasses must implement the abstract write(int b) method. They often choose to override the third variant, write(byte[], data int offset, int length), for reasons of performance. The implementation of the three-argument version of the write() method in OutputStream simply invokes write(int b) repeatedly; that is:

public void write(byte[] data, int offset, int length) throws IOException {
  for (int i = offset; i < offset+length; i++) write(data[i]);
}

Most subclasses can provide more efficient implementations of this method. The one-argument variant of write() merely invokes write(data, 0, data.length); if the three-argument variant has been overridden, this method will perform reasonably well. However, a few subclasses may override it anyway.

Example 2.3 is a simple program called NullOutputStream that mimics the ...