Visitor

把處理物件資料的方法獨立出來，可用相同的介面來處理不同的物件。

from: Visitor Pattern Tutorial with Java Examples

Example

Element

public interface ItemElement {

    public int accept(ShoppingCartVisitor visitor);
}

public class Book implements ItemElement {

    private int price;
    private String isbnNumber;

    public Book(int cost, String isbn){
        this.price = cost;
        this.isbnNumber = isbn;
    }

    public int getPrice() {
        return price;
    }

    public String getIsbnNumber() {
        return isbnNumber;
    }

    @Override
    public int accept(ShoppingCartVisitor visitor) {
        return visitor.visit(this);
    }

}

public class Fruit implements ItemElement {

    private int pricePerKg;
    private int weight;
    private String name;

    public Fruit(int priceKg, int wt, String nm){
        this.pricePerKg = priceKg;
        this.weight = wt;
        this.name = nm;
    }

    public int getPricePerKg() {
        return pricePerKg;
    }


    public int getWeight() {
        return weight;
    }

    public String getName(){
        return this.name;
    }

    @Override
    public int accept(ShoppingCartVisitor visitor) {
        return visitor.visit(this);
    }

}

Visitor

public interface ShoppingCartVisitor {

    int visit(Book book);
    int visit(Fruit fruit);
}

public class ShoppingCartVisitorImpl implements ShoppingCartVisitor {

    @Override
    public int visit(Book book) {
        int cost = 0;
        //apply 5$ discount if book price is greater than 50
        if(book.getPrice() > 50) {
            cost = book.getPrice() - 5;
        } else {
            cost = book.getPrice();
        }
        System.out.println("Book ISBN::"+book.getIsbnNumber() + " cost ="+cost);
        return cost;
    }

    @Override
    public int visit(Fruit fruit) {
        int cost = fruit.getPricePerKg() * fruit.getWeight();
        System.out.println(fruit.getName() + " cost = "+cost);
        return cost;
    }

}

Usage

public class ShoppingCartClient {

    public static void main(String[] args) {
        ItemElement[] items = new ItemElement[]{new Book(20, "1234"), new Book(100, "5678"),
                new Fruit(10, 2, "Banana"), new Fruit(5, 5, "Apple")};

        int total = calculatePrice(items);
        System.out.println("Total Cost = "+total);
    }

    private static int calculatePrice(ItemElement[] items) {
        ShoppingCartVisitor visitor = new ShoppingCartVisitorImpl();
        int sum = 0;
        for(ItemElement item : items){
            sum = sum + item.accept(visitor);
        }
        return sum;
    }

}

from: Visitor Design Pattern in Java