行为型 - 访问者(Visitor)

访问者模式(visitor pattern): 当你想要为一个对象的组合增加新的能力, 且封装并不重要时, 就使用访问者模式。@pdai

• 行为型 - 访问者(Visitor)

• 意图

• 类图

• 实现

• JDK

意图

为一个对象结构(比如组合结构)增加新能力。

类图

• Visitor: 访问者，为每一个 ConcreteElement 声明一个 visit 操作
• ConcreteVisitor: 具体访问者，存储遍历过程中的累计结果
• ObjectStructure: 对象结构，可以是组合结构，或者是一个集合。

ec923dc7-864c-47b0-a411-1f2c48d084de.png

实现

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

    class CustomerGroup {

        private List<Customer> customers = new ArrayList<>();

        void accept(Visitor visitor) {
            for (Customer customer : customers) {
                customer.accept(visitor);
            }
        }

        void addCustomer(Customer customer) {
            customers.add(customer);
        }
    }

    public class Customer implements Element {

        private String name;
        private List<Order> orders = new ArrayList<>();

        Customer(String name) {
            this.name = name;
        }

        String getName() {
            return name;
        }

        void addOrder(Order order) {
            orders.add(order);
        }

        public void accept(Visitor visitor) {
            visitor.visit(this);
            for (Order order : orders) {
                order.accept(visitor);
            }
        }
    }

    public class Order implements Element {

        private String name;
        private List<Item> items = new ArrayList();

        Order(String name) {
            this.name = name;
        }

        Order(String name, String itemName) {
            this.name = name;
            this.addItem(new Item(itemName));
        }

        String getName() {
            return name;
        }

        void addItem(Item item) {
            items.add(item);
        }

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

            for (Item item : items) {
                item.accept(visitor);
            }
        }
    }

    public class Item implements Element {

        private String name;

        Item(String name) {
            this.name = name;
        }

        String getName() {
            return name;
        }

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

    public interface Visitor {
        void visit(Customer customer);

        void visit(Order order);

        void visit(Item item);
    }

    public class GeneralReport implements Visitor {

        private int customersNo;
        private int ordersNo;
        private int itemsNo;

        public void visit(Customer customer) {
            System.out.println(customer.getName());
            customersNo++;
        }

        public void visit(Order order) {
            System.out.println(order.getName());
            ordersNo++;
        }

        public void visit(Item item) {
            System.out.println(item.getName());
            itemsNo++;
        }

        public void displayResults() {
            System.out.println("Number of customers: " + customersNo);
            System.out.println("Number of orders: " + ordersNo);
            System.out.println("Number of items: " + itemsNo);
        }
    }

    public class Client {
        public static void main(String[] args) {
            Customer customer1 = new Customer("customer1");
            customer1.addOrder(new Order("order1", "item1"));
            customer1.addOrder(new Order("order2", "item1"));
            customer1.addOrder(new Order("order3", "item1"));

            Order order = new Order("order_a");
            order.addItem(new Item("item_a1"));
            order.addItem(new Item("item_a2"));
            order.addItem(new Item("item_a3"));
            Customer customer2 = new Customer("customer2");
            customer2.addOrder(order);

            CustomerGroup customers = new CustomerGroup();
            customers.addCustomer(customer1);
            customers.addCustomer(customer2);

            GeneralReport visitor = new GeneralReport();
            customers.accept(visitor);
            visitor.displayResults();
        }
    }

    customer1
    order1
    item1
    order2
    item1
    order3
    item1
    customer2
    order_a
    item_a1
    item_a2
    item_a3
    Number of customers: 2
    Number of orders:    4
    Number of items:     6

JDK

• javax.lang.model.element.Element and javax.lang.model.element.ElementVisitor
• javax.lang.model.type.TypeMirror and javax.lang.model.type.TypeVisitor