I need to make sure that no object attribute is null and add default value in case if it is null. Is there any easy way to do this, or do I have to do it manually by checkin
I don't have enough context to give you a correct answer, but I'll suggest you to make you code immutable as much as possible. Use public final fields. No more getters or setters : every field has to be defined by the constructor. Your code is shorter, more readable and prevents you from writing code with side effects.
It doesn't prevent you from passing null arguments to your constructor though... You can still check every argument as suggested by @cletus, but I'll suggest you to throw IllegalArgumentException instead of NullPointerException that doesn't give no new hint about what you've done.
Anyway, that's what I do as much as I can and it improved my code (readability, stability) to a great extend. Everyone in my team does so and we are very happy with that. We learned that when we try to write some erlang code where everything is immutable.
Hope this helps.