访问者模式（Visitor）

目的

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。

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();
    }
}
运行结果
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中的体现

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