GoF Behavioral - Iterator Pattern

• Intent

To provide a way to access the elements of an aggregate object sequentially without exposing its underlying implementation.

java.util.Enumeration and java.util.Iterator are examples of the Iterator pattern.

• Benefits

1. It supports variations in the traversal of a collection(對於多種不同的物件集合提供同樣的走訪方法)


2. It simplifies the interface to the collection

• 適用情境

1. 當存取一個集合物件時，我不需要知道其內部是如何地進行運作，而很簡單的直接將我要的結果傳回


2.  多個物件走訪方式需要透過集合來進行


3. 以一個單一的介面，提供多種不同物件集合結構的存取

• 常見API - java.util.Iterator , java.util.Enumeration (ps : jdk 5.0以後有 Iterable interface)


• Class Diagram

[caption id="attachment_242" align="alignnone" width="458" caption="Iterator pattern"][/caption]

• Sample Code

import java.util.Iterator;

class IterableString implements Iterable<Character> {
    private String original;

    public IterableString(String original) {
        this.original = original;
    }

    public Iterator<Character> iterator() {
        return new InnerIterator();
    }

    private class InnerIterator implements Iterator<Character> {
        private int index;
        public boolean hasNext() {
            return index < original.length();
        }

        public Character next() {
            Character c = original.charAt(index);
            index++;
            return c;
        }

        public void remove() {}
    }
}

public class Main {
    public static void main(String[] args) {
        foreach(new IterableString("Justin"));
    }
    public static void foreach(Iterable<Character> iterable) {
        Iterator<Character> iterator = iterable.iterator();
        while(iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}