Although InputStream and OutputStream are abstract classes, many methods in the class library are only specified to return an InputStream or OutputStream, not the more-specific subclass.
For example, this is the signature for the openStream() method in java.net.URL:

public final InputStream openStream()
 throws IOException

In most implementations of Java, this method actually returns a sun.net.TelnetInputStream object, which is a subclass of java.io.InputStream; but you can't use the methods of that class. Thus you will often only be able to use the methods declared in these base classes.