擴展型模式之Iterator（迭代器）模式

1.如果你期望提供順序地訪問集合元素的方法，可使用Iterator（迭代器）模式
2.舉例

//迭代器接口
interface Iterator {
    Object next();
    boolean hasNext();
}

//容器接口：裝載集合元素
public interface Collection {
    void add(Object obj);
    int size();
    Iterator iterator();
}

//具體集合類
public class ArrayList implements Collection{

    Object[] objs = new Object[10];
    int index = 0;
    @Override
    public void add(Object obj) {
        // TODO Auto-generated method stub
        if(index==objs.length){
            Object[] newObjects = new Object[objs.length * 2];
            System.arraycopy(objs, 0, newObjects, 0, objs.length);
            objs = newObjects;
        }
        objs[index] = obj;
        index++;
    }

    @Override
    public int size() {
        // TODO Auto-generated method stub
        return index;
    }

    @Override
    public Iterator iterator() {
        // TODO Auto-generated method stub
         return new ArrayListIterator(objs,index);
    }

}

//針對具體容器的迭代器
public class ArrayListIterator implements Iterator {
    private int currentIndex = 0;
    private Object[] objs;
    private int index;
    public ArrayListIterator(Object[] objs, int index) {
        // TODO Auto-generated constructor stub
        this.objs = objs;
        this.index = index;
    }

    @Override
    public Object next() {
        // TODO Auto-generated method stub
        Object obj = objs[currentIndex];
        currentIndex ++;
        return obj;
    }

    @Override
    public boolean hasNext() {
        // TODO Auto-generated method stub
        if(currentIndex>=index)
            return false;
        else
            return true;
    }

}

//測試類
public class Test {
    public static void main(String[] args) {
        ArrayList list = new ArrayList();
        for (int i = 0; i < 15; i++) {
            list.add("string " + i);
        }
        Iterator iterator = list.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());            
        }
        System.out.println(list.size());    
    }
}

//結果
string 0
string 1
string 2
string 3
string 4
string 5
string 6
string 7
string 8
string 9
string 10
string 11
string 12
string 13
string 14
15

3.總結：Iterator（迭代器）模式的意圖在於使得用戶可以順序訪問集合內的元素。在多線程的應用中，爲保證多線程程序中迭代的安全性，可以通過鎖住互斥對象，實現對集合的同步訪問。
4.參考：http://www.jb51.net/article/39790.htm