java.io.Filter{In,Out}put stream constructors should allow null stream parameter. The constructor javadoc

"in - the underlying input stream, or null if this instance is to be created without an underlying stream."

and

"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."

override the package javadoc

"Unless otherwise noted, passing a null argument to a constructor or method in any class or interface in this package will cause a NullPointerException to be thrown."