Object-Oriented Programming: The Big Picture

Java's object-oriented design rests on four ideas, all of which you've already touched in Level 1 and will now go much deeper on:

• Encapsulation — hiding a class's internal state behind private fields and a controlled public interface.
• Inheritance — a class reusing and extending another class's fields and methods.
• Polymorphism — code written against a general type (a parent class or interface) automatically working with any specific subtype.
• Abstraction — modeling only the details that matter for the problem at hand, via abstract classes and interfaces.

The next several lessons take each of these in turn.