There are many certification and exams available in Java world which check if you have acquired enough competency in this technology.  You may have taken may of those and cleared with high scores. But here is something which comes from my experience of Java developers around me. Ask these simple test questions to yourself and know if you really understand Java programming and can appreciate this technology.

If you ask anybody using Java (or someone even not using Java), what is special about Java? The answer will definitely have two things- platform independent language and garbage collection. There will be a long list of other items but these two will be on top the list.

These articles explain the object oriented concepts in detail. UML representation of most of the concept is provided in these articles. Also Java code examples are available to explain the concepts better.  Here is the list of articles.

Encapsulation is hiding information. Encapsulation hides implementation details from world and exposes only the contract. In case of a class, the methods exposed without private access modifier defines the contract of that class. How these methods are implemented is never known to the users of these methods.

Polymorphism means poly multiple forms. Many shapes. In object oriented programming, polymorphism means the type of object is determined as late as at runtime. What does it mean, and what do we achieve by this? Let us take an example. We have following classes.