在Java中 ‘is a relationship’ 与‘Has a relationship’的区别

今天在SCJP的试题中看见关于‘is a relationship’ ，‘has a relationship’的试题，一开始没搞清楚怎么回事，就google了一下，找到如下文章，觉得挺有用的。跟大家分享下啊。

what is 'is a' and 'has a' relationship:

The ‘is a relationship’ is expressed with inheritance and ‘has a relationship’ is expressed with composition.
Both inheritance and composition allow you to place sub-objects inside your new class. Two of the main techniques for code reuse are class inheritance and object composition.

e.g.,

is a --- House is a Building

class Building {

  .......
}

class House extends Building {

  .........
}

has a -- House has a bathroom

class House
{
  Bathroom room = new Bathroom() ;
  ....
  public void getTotMirrors()
  {
    room.getNoMirrors();
    ....
   }
}
Inheritance Vs Composition

  Inheritance is uni-directional. For example House is a Building. But Building is not a House. Inheritance uses
‘extends’ key word. Composition: is used when House has a Bathroom. It is incorrect to say House is a Bathroom.

  Composition simply means using instance variables that refer to other objects. The class House will have an instance variable, which refers to a Bathroom object.

就是类和成员的关系吧。