Introduction to Design Patterns (Iterator)

This article summarizes Iterator. According to wikipedia, "By making the means of enumerating the elements of a container object independent, we provide an iterator that does not depend on the internal specifications of the container." Reference: Iterator pattern

A programming language has a mechanism for grouping data such as arrays, lists, and hash tables into a single object, each of which has a suitable usage, and this pattern iterates over the elements of the grouped object.

** Main characters **

** Implement the pattern ** I would like to implement multiple restaurant menus according to the design pattern, imagining the food code of a commercial complex. Here, as an example, we define the menus of two stores, "hamburger" and "okonomiyaki". We also assume that each store holds data in different forms (arrays and lists).

** Iterator **

Iterator.java

public interface Iterator {
    boolean hasNext();
    Object next();
}

** Iterator concrete class **

SandwichMenuIterator.java

import java.util.ArrayList;
public class SandwichMenuIterator implements Iterator {
    ArrayList<MenuItem> items = new ArrayList<>();
    int position = 0;


    public SandwichMenuIterator(ArrayList<MenuItem> items) {
        this.items = items;
    }
    public Object next() {
        MenuItem mi = items.get(this.position);
        this.position++;
        return mi;
    }
    public boolean hasNext() {
        if (this.position >= this.items.size()) return false;
        if (this.items.get(this.position) == null) return false;
        return true;
    }
}

OkonomiyakiMenuIterator.java

public class OkonomiyakiMenuIterator implements Iterator {
    MenuItem[] items;
    int position = 0;

    public OkonomiyakiMenuIterator(MenuItem[] items) {
        this.items = items;
    }
    public Object next() {
        MenuItem mi = this.items[this.position];
        this.position++;
        return mi;
    }
    public boolean hasNext() {
        if (this.position >= this.items.length) return false;
        if (this.items[this.position] == null) return false;
        return true;
    }
}

** Aggregate **

Menu.java

public interface Menu {
    public Iterator createIterator();
}

** Concrete class of aggregate **

SandwichMenu.java

import java.util.ArrayList;
public class SandwichMenu implements Menu {
    ArrayList<MenuItem> menuItems = new ArrayList<>();

    public SandwichMenu() {
        addItem("Hamburger", "Ordinary sandwich", true, 120);
        addItem("cheeseburger", "Sandwich with cheese", true, 240);
        addItem("Fish burger", "Fried fish sandwich", true, 320);
    }
    public void addItem(String name, String desc, Boolean coffee, Integer price) {
        MenuItem mi = new MenuItem(name, desc, coffee, price);
        menuItems.add(mi);
    }
    public Iterator createIterator() {
        return new SandwichMenuIterator(menuItems);
    }
    public ArrayList<MenuItem> getMenuItems() {
        return menuItems;
    }
}

OkonomiyakiMenu.java

public class OkonomiyakiMenu implements Menu {
    static final int MAX_ITEMS = 2;
    MenuItem[] menuItems;
    Integer cnt = 0;

    public OkonomiyakiMenu() {
        menuItems = new MenuItem[MAX_ITEMS];
        addItem("Pork ball", "Grilled food using local pork", false, 600);
        addItem("Modern grilled", "Grilled dishes using local vegetables and noodles", false, 750);
    }
    public void addItem(String name, String desc, Boolean coffee, Integer price) {
        MenuItem mi = new MenuItem(name, desc, coffee, price);
        menuItems[cnt] = mi;
        cnt++;
    }
    public Iterator createIterator() {
        return new OkonomiyakiMenuIterator(menuItems);
    }
    public MenuItem[] getMenuItems() {
        return menuItems;
    }
}

** Class that holds menu data **

MenuItem.java

public class MenuItem {
    String name;
    String desc;
    Boolean coffee;
    Integer price;

    public MenuItem(String name, String desc, Boolean coffee, Integer price) {
        this.name = name;
        this.desc = desc;
        this.coffee = coffee;
        this.price = price;
    }

    public String getName() { return name; }
    public String getDesc() { return desc; }
    public Boolean getCoffee() { return coffee; }
    public Integer getPrice() { return price; }
}

** Main class **

Main.java

public class Main {
    public static void main(String args[]) {
        SandwichMenu sm = new SandwichMenu();
        OkonomiyakiMenu om = new OkonomiyakiMenu();
        Iterator sandwichIterator = sm.createIterator();
        Iterator okonomiyakiIterator = om.createIterator();
        printMenu(sandwichIterator);
        printMenu(okonomiyakiIterator);
    }
    public static void printMenu(Iterator iterator) {
        while (iterator.hasNext()) {
            MenuItem mi = (MenuItem)iterator.next();
            System.out.print(mi.getName());
            if(mi.getCoffee()) System.out.print("(With coffee)");
            System.out.println("," + mi.getPrice());
            System.out.println("--" + mi.getDesc());
        }
    }
}

result

$java Main
Hamburger(With coffee),120
--Ordinary sandwich
cheeseburger(With coffee),240
--Sandwich with cheese
Fish burger(With coffee),320
--Fried fish sandwich
Pork ball,600
--Grilled food using local pork
Modern grilled,750
--Grilled dishes using local vegetables and noodles

The Iterator pattern is to implement a function that sequentially accesses an object that collects data. This time, for two objects of different data types, we created a class (○○ MenuIterator.java) that accesses each object in sequence and output it on the screen. The Java language "java.util.Collection" interface implements the Iterator method, so it's easy to imagine.

Reference: [Head First Design Patterns-Design Patterns to Remember with Your Head and Body] (https://www.amazon.co.jp/Head-First%E3%83%87%E3%82%B6%E3%82%A4%E3%83%B3%E3%83%91%E3%82%BF%E3%83%BC%E3%83%B3-%E2%80%95%E9%A0%AD%E3%81%A8%E3%81%8B%E3%82%89%E3%81%A0%E3%81%A7%E8%A6%9A%E3%81%88%E3%82%8B%E3%83%87%E3%82%B6%E3%82%A4%E3%83%B3%E3%83%91%E3%82%BF%E3%83%BC%E3%83%B3%E3%81%AE%E5%9F%BA%E6%9C%AC-Eric-Freeman/dp/4873112494)