http://www.exforsys.com/tutorials/oops/oops-tutorials.html
http://www.onesmartclick.com/programming/object-oriented-fundamentals.html
http://www.exforsys.com/tutorials/oops.html
http://www.codeproject.com/KB/architecture/OOP_Concepts_and_manymore.aspx
http://www.math.vu.nl/~eliens/documents/java/tutorial/java/concepts/index.html