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.