解决的问题

Visitor Pattern是为了在不改变类本身的代码的前提下，给类添加新的特性。 例如，HTML有各种类型的节点（HtmlNode），例如HeadingNode、 AnchorNode等，需要不改变类的代码给类添加新的操作，例如HighlightOperation、PlainTextOperation等。

代码

Operation:

package com.cong.designpattern.visitor;

public interface Operation {
    public void apply(HeadingNode node);
    public void apply(AnchorNode node);
}

HighLightOperation:

package com.cong.designpattern.visitor;

public class HighlightOperation implements Operation{
    @Override
    public void apply(HeadingNode node) {
        System.out.println("Heading Highlight");
    }
    @Override
    public void apply(AnchorNode node) {
        System.out.println("Anchor Highlight");
    }
}

PlainTextOperation:

package com.cong.designpattern.visitor;

public class PlainTextOperation implements Operation{
    @Override
    public void apply(HeadingNode node) {
        System.out.println("Heading plain text");
    }
    @Override
    public void apply(AnchorNode node) {
        System.out.println("Anchor plain text");
    }
}

HtmlNode:

package com.cong.designpattern.visitor;

public abstract class HtmlNode {
    abstract void execute(Operation operation);
}

HeadingNode:

package com.cong.designpattern.visitor;

public class HeadingNode extends HtmlNode{
    @Override
    void execute(Operation operation) {
        operation.apply(this);
    }
}

AnchorNode:

package com.cong.designpattern.visitor;

public class AnchorNode extends HtmlNode{
    @Override
    void execute(Operation operation) {
        operation.apply(this);
    }
}

HtmlDocument:

package com.cong.designpattern.visitor;

import java.util.ArrayList;
import java.util.List;

public class HtmlDocument {
    private List<HtmlNode> nodes = new ArrayList<>();
    public void addNode(HtmlNode node) {
        this.nodes.add(node);
    }
    public void execute(Operation operation) {
        for (HtmlNode node : this.nodes) {
            node.execute(operation);
        }
    }
}

Test code:

HtmlDocument document = new HtmlDocument();
document.addNode(new HeadingNode());
document.addNode(new AnchorNode());

document.execute(new HighlightOperation());
document.execute(new PlainTextOperation());

UML

Visitor Pattern UML