设计模式之访问者模式（Visitor）

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设计模式之访问者模式（Visitor）

本篇为 https://github.com/iluwatar/java-design-patterns/tree/master/visitor 阅读笔记
意图

主要将数据结构与数据操作分离。

关键代码

在数据基础类里面有一个方法接受访问者，将自身引用传入访问者。

优点

1、符合单一职责原则。
2、优秀的扩展性。
3、灵活性。

缺点

1、具体元素对访问者公布细节，违反了迪米特原则。
2、具体元素变更比较困难。
3、违反了依赖倒置原则，依赖了具体类，没有依赖抽象。

场景

指挥官，军官，士兵组成的军队了，每一等级可以接受其下属等级的问候

抽象军人

public abstract class Unit {

    private Unit[] children;

    public Unit(Unit... children) {
        this.children = children;
    }

    public void accept(UnitVisitor visitor){
     for (Unit child : children){
         child.accept(visitor);
     }
    }
}
```指挥官

```java
public class Commander extends Unit {
    public Commander(Unit... children) {
        super(children);
    }

    @Override
    public void accept(UnitVisitor visitor) {
        visitor.visitCommander(this);
        super.accept(visitor);
    }

    @Override
    public String toString() {
        return "Commander";
    }
}

军官

public class Sergeant extends Unit {

    public Sergeant(Unit... children) {
        super(children);
    }

    @Override
    public void accept(UnitVisitor visitor) {
        visitor.visitSergeant(this);
        super.accept(visitor);
    }

    @Override
    public String toString() {
        return "Sergeant";
    }
}

士兵

public class Soldier extends Unit {
    public Soldier(Unit... children) {
        super(children);
    }

    @Override
    public void accept(UnitVisitor visitor) {
        visitor.visitSolder(this);
        super.accept(visitor);
    }

    @Override
    public String toString(){
        return "soldier";
    }
}

访问者接口

public interface UnitVisitor {

    void visitSolder(Soldier soldier);
    void visitSergeant(Sergeant sergeant);
    void visitCommander(Commander commander);

}

指挥官访问者

public class CommanderVisitor implements UnitVisitor {
    @Override
    public void visitSolder(Soldier soldier) {

    }

    @Override
    public void visitSergeant(Sergeant sergeant) {

    }

    @Override
    public void visitCommander(Commander commander) {
        System.out.println(MessageFormat.format("Greeting {0}", commander));
    }
}

军官访问者

public class SergeantVisitor implements UnitVisitor {
    @Override
    public void visitSolder(Soldier soldier) {

    }

    @Override
    public void visitSergeant(Sergeant sergeant) {
        System.out.println(MessageFormat.format("Greeting {0}", sergeant));
    }

    @Override
    public void visitCommander(Commander commander) {

    }
}

士兵访问者

public class SoldierVisitor implements UnitVisitor {
    @Override
    public void visitSolder(Soldier soldier) {
        System.out.println(MessageFormat.format("Greeting {0}", soldier));
    }

    @Override
    public void visitSergeant(Sergeant sergeant) {

    }

    @Override
    public void visitCommander(Commander commander) {

    }
}

测试

@Test
    public void visitorTest() {
        Commander commander = new Commander(
                new Sergeant(
                        new Soldier(),
                        new Soldier(),
                        new Soldier()),
                new Sergeant(
                        new Soldier(),
                        new Soldier(),
                        new Soldier())
        );
        commander.accept(new SoldierVisitor());
        commander.accept(new SergeantVisitor());
        commander.accept(new CommanderVisitor());
    }

关键部分代码为

  @Override
    public void accept(UnitVisitor visitor) {
	    //visitor.visitSolder(this);
	    //visitor.visitSergeant(this);
        visitor.visitCommander(this);
        super.accept(visitor);
    }

父类的accept方法为

 public void accept(UnitVisitor visitor){
     for (Unit child : children){
         child.accept(visitor);
     }
    }

在数据基础类里面有一个方法接受访问者，将自身引用传入访问者。