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一、概述

对象结构很少改变，但需要经常新增对象上的操作。

被访问的的类里面加入一个接待访问者的接口。

二、示例

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();
    }
}

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


public interface Visitor {
    void visit(Customer customer);

    void visit(Order order);

    void visit(Item item);
}

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 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 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 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 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);
    }
}