visitor访问者模式

定义资源接口：

public interface Resource {
    void accept(Visitor visitor);
}

定义visitor，支持2种类型，reader类资源和writer资源，访问writer资源需要额外一个line参数：

public interface Visitor {
    void visit(Reader reader);

    void visit(Writer writer, String line);
}

public interface Reader {
    String read();
}

public class ReaderImpl implements Reader, Resource {
    @Override
    public String read() {
        return "Doc#1 factory riverwood staff manual.";
    }

    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

public class WriterImpl implements Writer, Resource {

    private String line;
    public WriterImpl(String line) {
        this.line = line;
    }

    @Override
    public int write(String line) {
        return null == line ? 0 : line.length();
    }

    @Override
    public void accept(Visitor visitor) {
        visitor.visit(this, line);
    }
}

定义实际访问Resource的两个visitor角色：

public class VipVisitorImpl implements Visitor {

    @Override
    public void visit(Reader reader) {
        String read = reader.read();
        System.out.println(read);
    }

    @Override
    public void visit(Writer writer, String line) {
        int write = writer.write(line);
        if (write > 0) {
            System.out.println("write success!");
        }
    }
}

public class OrdinaryVisitorImpl implements Visitor {

    @Override
    public void visit(Reader reader) {
        String read = reader.read();
        System.out.println(read);
    }

    @Override
    public void visit(Writer writer, String line) {
        System.out.println("no access to writer!");
    }

}

调用：

String line = "save this line";
        Resource reader = new ReaderImpl();
        Resource writer = new WriterImpl(line);

        Visitor ordinaryVisitor = new OrdinaryVisitorImpl();
        Visitor vipVisitor = new VipVisitorImpl();

        reader.accept(vipVisitor);
        reader.accept(ordinaryVisitor);

        writer.accept(vipVisitor);
        writer.accept(ordinaryVisitor);

执行输出：

Doc#1 factory riverwood staff manual.

Doc#1 factory riverwood staff manual.

write success!

no access to writer!