A improved StringBuffer - Java.Lang

I would personally use the decorator pattern for this - a class that wraps the original StringBuilder and then provides extra methods to use as appropriate.

If you only want to add a couple more methods, you may also want to look at just using equivalent static methods that do the job.

(As others have mentioned, StringBuilder is the better, more efficient choice over StringBuffer, unless you really need the thread safety the latter provides.)

Simply copying and pasting the code isn't a great idea, you're likely breaking licensing laws and duplicating functionality (even if that functionality is in the original API.)

I would work with StringBuilder rather than StringBuffer.

It's unfortunate that you can't implement an interface representing StringBuilder (unless Appendable is suitable?). Have you tried creating an EnhancedStringBuilder class that contains your StringBuilder as a member. You can then delegate most(all?) calls to this, and provide your own.

An alternative is to simply take the OpenJDK source code for StringBuilder and repackage and enhance it. I think the licensing will permit this, but I'm not familiar with your circumstances and you should check this.

Potential licensing issues aside, I would not recommend copying and pasting the source code just to add a couple of methods. Instead, I'd suggest making those methods static and placing them into a helper class (call it StringBuilderUtils or something similar).

The methods would take an instance of StringBuilder or StringBuffer (or perhaps even Appendable if you can implement your functionality in terms of Appendable).

You could copy the AbstractStringBuilder also. But I don't know the dependencies of that one. My recommendation is to create a Wrapper class to StringBuilder, and encapsulate it.

Don't worry about the performance, delegate calls can really easy be inlined into classes using your StringBuilderWrapper.