理解聚合，关联，组合

Association:
Uses the services of another class

Aggregation:
A class “owns” another class

Composition:
A class is composed of another class; refers to an aggregation within which the component parts and the larger encompassing whole share a lifetime

========
关联Association
========
public class A {
public A() {
}

/* This method is not a must method in class A
Class A only have association with B C when this method is called
*/

public void Method-need-BC(){
B b = new B();
C c = new C();

}
public class B{
}
public class C{
}

======
聚合Aggregation
======
public class A {
public A() {
}
/* This method is a must method in class A, but this method is called after the constructor function is finished. So A and B,C have the different life cycle. Class A could exist without B,C*/

public void MethodOfA(){
B b = new B();
C c = new C();

}
public class B{
}
public class C{
}

======
组合Composition
======
public class A {
//Constructor function must include B and C, all of them have the same life cycle.
public A() {
B b = new B();
C c = new C();
}
...
}
public class B{
}
public class C{
}

举一个简单的例子:

一个人和他的手的关系就是组合
一个人和他的衣服的关系就是聚合
一个人和他的邻居的关系就关联