The relationship and representation between classes in UML class diagram (Class Diagram)

There are five types of relationships between classes:

 

1. Dependency

One-way , which means that a class depends on the definition of another class, and changes in one class will affect the other class, which is a " use a " relationship

If A depends on B, then B behaves as A's local variables, method parameters, static method calls, etc.

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1. publicclass Person {   
2.     publicvoid doSomething(){   
3.         Card card =  new  Card(); //local variable  
4.         ....  
5.     }  
6. }  

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1. publicclass Person {   
2.     public void  doSomething(Card card){ //Method parameters   
3.         ....  
4.     }  
5. }  

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1. publicclass Person {   
2.     publicvoid doSomething(){   
3.         int  id = Card.getId(); // static method call  
4.         ...  
5.     }  
6. }  

 

2. Association

One-way or two-way (usually we need to avoid using two-way association relationship), is a " has a " relationship, if A is unidirectionally related to B, then it can be said that A has a B, usually expressed as a global variable

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1. publicclass Person {   
2.     public Phone phone;  
3.       
4.     public void setPhone(Phone phone){        
5.         this.phone = phone;  
6.     }  
7.       
8.     public Phone getPhone(){          
9.         return phone;  
10.     }  
11. }  

 

3，聚合关系(Aggregation)

单向，关联关系的一种，与关联关系之间的区别是语义上的，关联的两个对象通常是平等的，聚合则一般不平等，有一种整体和局部的感觉，实现上区别不大

Class由Student组成，其生命周期不同，整体不存在了，部分依然存在，当前Team解散了，人还在，还可以加入别的组

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1. public class Team {  
2.     public Person person;  
3.       
4.     public Team(Person person){  
5.         this.person = person;  
6.     }  
7. }  

 

4，组合关系(Composition)

单向，是一种强依赖的特殊聚合关系

Head，Body，Arm和Leg组合成People，其生命周期相同，如果整体不存在了，部分也将消亡

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1. public class Person {  
2.     public Head head;  
3.     public Body body;  
4.     public Arm arm;  
5.     public Leg leg;  
6.       
7.     public Person(){  
8.         head = new Head();  
9.         body = new Body();  
10.         arm = new Arm();  
11.         leg = new Leg();  
12.     }  
13. }  

 

5. Inheritance

A class implements an interface, a class inherits an abstract class, and a class inherits from a parent class belongs to this relationship

It can be divided into finer points:

Realization: A class implementing an interface belongs to this relationship

Generalization: that is, the " is a " relationship, the class inherits the abstract class, and the class inherits the parent class belongs to this relationship