大家在最初接触面向对象编程时,都难免会遇到描述类与类之间关系的三个词:Association、Aggregation和Composition。对于初学者而言,往往很难弄清楚它们之间的区别。现在,我就这三者之间的区别,做一个大致的说明。
从概念上来讲:
Association描述的是类与类之间一般的关联关系,而Aggregation和Composition都有整体和部分的关系。其中Aggregation中的部分脱离了整体,部分仍然有意义,是共享式的。而Composition中的部分脱离了整体,部分将没有任何意义,是独占式的。
从实现的角度上讲:
三者中以属性出现时,有着不同的实现方法。其中Association中作为属性出现时,不需要对其进行强制赋值,只要在使用时对其进行初始化即可。Aggregation中作为属性出现时,需要在构造器中通过传递参数来对其进行初始化。Composition作为属性出现时,需要在整体的构造器中创建部分的具体实例,完成对其的实例化。
下面通过具体的例子来更深刻的理解这三者的区别。先看第一个例子,其中Student类和BasketBall类之间是Association的关系:
public class Student{
private String name;
private int age;
BasketBall aBall;
public Student(String name, int age){
this.name=name;
this.age=age;
}
public void getBall(BasketBall aBall){
this.aBall=aBall;
}
public void play(){
System.out.println("I am playing basketball"+aBall);
}
}
class BasketBall{
private Color aColor;
private int size;
public BasketBall(Color aColor, int size){
this.aColor=aColor;
this.size=size;
}
}
class StudentAdmin{
public static void main(String aa[]){
Student aStudent=new Student("Leo", 25);
BasketBall aBasketBall=new BasketBall(Color.red, 32);
aStudent.getBall(aBasketBall);
aStudent.play();
}
}
再看一个Aggregation的例子,在这段代码中,Computer类和Monitor类之间是Aggregation的关系:
public class Computer{
private String cpu;
private float weight;
private Monitor aMonitor;
public Computer(String cpu, float weight, Monitor aMonitor){
this.cpu=cpu;
this.weight=weight;
this.aMonitor=aMonitor;
}
public void turnOn(){ System.out.println("I am on."); }
}
class Monitor{
private int inch;
private boolean isFlat;
//no information of computer
public Monitor(int inch, boolean isFlat){
this.inch=inch;
this.isFlat=isFlat;
}
}
class ComputerAdmin{
public static void main(String aa[]){
Monitor aMonitor=new Monitor(15, true);
System.out.println("I do something others here");
Computer aComputer=new Computer(586, 32.0, aMonitor);
System.out.println("Computer is :"+aComputer);
aComputer.turnOn();
}
}
最后是Composition的例子,与上一段代码不同的是,Computer类和Monitor类之间变成了Composition的关系:
public class Computer{
private String cpu;
private float weight;
private Monitor aMonitor;
public Computer(String cpu, float weight, int inch, boolean isFlat){
this.cpu=cpu;
this.weight=weight;
this.aMonitor=new Monitor(inch, isFlat);
}
public void turnOn(){ System.out.println("I am on."); }
}
class Monitor{
private int inch;
private boolean isFlat;
//no information of computer
public Monitor(int inch, boolean isFlat){
this.inch=inch;
this.isFlat=isFlat;
}
}
class ComputerAdmin{
public static void main(String aa[]){
//Monitor aMonitor=new Monitor(15, true);
Computer aComputer=new Computer(586, 32.0, 15, true);
System.out.println("Computer is :"+aComputer);
aComputer.turnOn();
}
}
通过以上的说明,相信大家对于Association、Aggregation和Composition三者之间的区别,会产生一个比较清晰的认识。如果你是刚刚接触面向对象编程的初学者,你会发现,这无论对于你工程前期程序类图的设计还是后期具体的代码实现,都是相当有帮助的。
转载自:http://it.cqnews.net/html/2013-12/25/content_29170755.htm