组成意味着HAS A
继承IS A

Example：汽车有发动机，汽车是汽车

在编程中，它表示为：

class Engine {} // The Engine class.

class Automobile {} // Automobile class which is parent to Car class.

class Car extends Automobile { // Car is an Automobile, so Car class extends Automobile class.
  private Engine engine; // Car has an Engine so, Car class has an instance of Engine class as its member.
}