OOP
Why
How
What
In OOP, computer programs are designed by making them out of objects that interact with one another.
Abstraction(抽象), encapsulation(封装), polymorphism(多态), and inheritance(继承) are the four main theoretical principles of object-oriented programming. But Java also works with three further OOP concepts: association(关联), aggregation(聚合), and composition(组合).