学步园

为什么要使用访问者设计模式?

先看一下下面这段代码

class Person{
	private String action ;
	private String name;
	public Person(String name){
		this.name = name;
	}
	public String getAction() {
		return action;
	}

	public void setAction(String action) {
		this.action = action;
	}

	public void getConclusion(){
		if(action == "Happy"){
			System.out.println(name+" feel "+"Happy");
		}
		else if(action == "Exciting"){
			System.out.println(name+" feel "+"Exciting");
		}
		else if(action == "Grieved"){
			System.out.println(name+" feel "+"Grieved");
		}
	}
}
public class Client {
	public static void main(String [] args){
		Person person = new Person("jeason");
		person.setAction("Happy");
		person.getConclusion();
	}
}

如果需要添加一种feeling那么需要去改动Person类,这里违背了开放封闭原则,getConclusion()需要对每种feeling都分别做操作,这里违背了单一职责原则.现在我们利用访问者模

式就可以就可以避免违反的两种原则.

什么是访问者设计模式?

表示一个作用于某对象结构中的各元素的操作。它使你可以在不改变各元素类的前提下定义作用于这些元素的新操作。

怎么使用访问者设计模式?

我们根据之前的需求和访问者模式的概念设计出UML：

具体代码：

State.class

public abstract class State {
	public abstract void eflectPerson(Person person);
}

Happy.class

public class Happy extends State{

	@Override
	public void eflectPerson(Person person) {
		// TODO Auto-generated method stub
		System.out.println(person.getName()+" feel happy");
	}

}

Grieved.class

public class Grieved extends State{

	@Override
	public void eflectPerson(Person person) {
		// TODO Auto-generated method stub
		System.out.println(person.getName()+" feel grieved");
	}

}

Exciting.class

public class Exciting extends State{

	@Override
	public void eflectPerson(Person person) {
		// TODO Auto-generated method stub
		System.out.println(person.getName()+" feel exciting");
	}

}

Person.class

public class Person {
	private String name;
	public Person(String name){
		this.name = name;
	}
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
	//双分派
	public void receiveState(State state){
		state.eflectPerson(this);
	}
}

这里解释一下什么叫双分派:假设有A,B两个对象,对象A中有一个方法需要用到对象B的引用,而在这个方法中去调用对象B的方法会用到对象A的引用。它使得执行结果不仅取决

于B类而且还取决于A类.

PersonStructure.class

public class PersonStructure {
	private ArrayList<Person> arrayList = new ArrayList<Person>();
	public void addPerson(Person person){
		arrayList.add(person);
	}
	public void remove(Person person){
		arrayList.remove(person);
	}
	public void displayPersonName(){
		for(Person p : arrayList){
			System.out.println("person name "+p.getName());
		}
	}
	public void receiveState(State state){
		for(Person p : arrayList){
			p.receiveState(state);
		}
	}
}

Client.class

public class Client {
	public static void main(String [] args){
		Person p1 = new Person("jeason");
		Person p2 = new Person("neo");
		Person p3 = new Person("kuku");
		PersonStructure personStructure = new PersonStructure();
		personStructure.addPerson(p1);
		personStructure.addPerson(p2);
		personStructure.addPerson(p3);
		personStructure.displayPersonName();
		personStructure.receiveState(new Happy());
	}
}

最后输出：

person name jeason
person name neo
person name kuku
jeason feel happy
neo feel happy
kuku feel happy