OOP and class design

In the study in CSC 148, one important concept is object-oriented program. This method is more significant in programming than other methods and save much more time. In the learning process of OOP, I found class designing important and made the code shorter.
What I've learned:

• property(attributes)
• How to write exceptions
• How to write type contract and parameters together
• Sub-classes (inheritance)
• recursion 

What I found hard:

• exceptions
• inheritance
• recursion

Hope I can understand more about my weakness. I already found the lab and tutorial very useful. I've learned and understand how to design a class completely in it. And hope I can make more friends who can teach me on this! XD