訪問者模式

訪問者模式UML.png

例子以公司招聘查看簡歷為背景

interface Visitor{
    void visitClerical(Clerical clerical);//查看文員簡歷
    void visitCoder(Coder coder);//查看程序員簡歷
}    

訪問者接口，分別對ElementA與ElementB進行訪問

class HRVisitor{
    public void visitClerical(Clerical clerical){
        //HR關注文員簡歷XXXXX
    }
    
    public void visitCoder(Coder coder){
        //HR看碼農簡歷，看經驗看學校
    }
}

HR訪問者

class CTOVisitor{
    public void visitClerical(Clerical clerical){
        //CTO不關心文員簡歷，可是是空實現
    }
    
    public void visitCoder(Coder coder){
        //CTO看碼農簡歷，看項目 看技能
    }
}

CTO訪問者

abstract class JobSeeker{
    public String name;
    public String school;
    public String skill;
    
    public JobSeeker(String name ,String school ,String skill){
        this.name = name;
        this.school = school;
        this.skill = skill;
    }
    
    public abstract void accept(Visitor visitor);
}

抽象求職者類

class Clerical extends JobSeeker{
    public Clerical(String name ,String school ,String skill){
        super(name , school , skill);
    }
    public void accept(Visitor visitor){
        visitor.visitClerical(this);
    }
}

文員求職者

class Coder extends JobSeeker{
    public Coder(String name ,String school ,String skill){
        super(name , school , skill);
    }
    public void accept(Visitor visitor){
        visitor.visitCoder(this);
    }
}

碼農求職者

public class JobStructure{
    List<JobSeeker> list = new ArrayList<>();
    
    public void seeResume(Visitor visitor){
        for(JobSeeker jobSeeker : list){
            jobSeeker.accept(visitor);
        }
    }
    
    public void addJobSeeker(JobSeeker jobSeeker){
        list.add(jobSeeker);
    }
}   

查閱簡歷類

class CLient{
    public static void main(String[] args){
        JobStructure jobStructure = new JobStructure();
        jobStructure.addJobSeeker(new Coder("小強","清華","Android"));
        jobStructure.addJobSeeker(new Coder("小民","北大","Word"));
        jobStructure.seeResume(new CTOVisitor());
        jobStructure.seeResume(new HRVisitor());
    }
}   

客戶端類，收集簡歷，CTO查看簡歷，HR查看簡歷