设计模式_16:迭代器模式

迭代器模式提供一种方法顺序访问一个聚合对象中的各个元素，而又不暴露该对象的内部表示。

当你需要对聚集进行多种方式遍历时，可以考虑用迭代器模式;

import java.util.ArrayList;import java.util.List;public class Main {    public static void main(String[] args) {        Aggregate<String> concreteAggregate = new ConcreteAggregate<>();        concreteAggregate.add("同学1号");        concreteAggregate.add("同学2号");        concreteAggregate.add("同学3号");        concreteAggregate.add("同学4号");        Iterator iterator = concreteAggregate.createIterator();        while (iterator.hasNext()){            System.out.println(iterator.next());        }        System.out.println("-----------------------------");        Iterator<String> iteratorDesc = new ConcreteIteratorDesc<>(concreteAggregate);        while (iteratorDesc.hasNext()){            System.out.println(iteratorDesc.next());        }    }}//聚集抽象类interface Aggregate<T>  {    Iterator createIterator();    void add(T t);    T getByIndex(int index);    int getCount();}//具体聚集类class ConcreteAggregate<T> implements Aggregate<T>{    private List<T> list = new ArrayList<>();    @Override    public Iterator createIterator() {        return new ConcreteIterator<>(this);    }    @Override    public void add(T t){        this.list.add(t);    }    @Override    public T getByIndex(int index){        return this.list.get(index);    }    @Override    public int getCount(){        return this.list.size();    }}//迭代器抽象类interface Iterator<T> {    T next();    boolean hasNext();    T currentItem();}//具体迭代器类(顺序)class ConcreteIterator<T> implements Iterator<T>{    private ConcreteAggregate<T> aggregate;    private int currentIndex = 0;           //当前集合的下标    public ConcreteIterator(ConcreteAggregate<T> aggregate) {        this.aggregate = aggregate;    }    @Override    public T next() {        T t = this.currentItem();        currentIndex++;        return t;    }    @Override    public boolean hasNext() {        return currentIndex < this.aggregate.getCount();    }    @Override    public T currentItem() {        return this.aggregate.getByIndex(currentIndex);    }}//具体迭代器类(逆序)class ConcreteIteratorDesc<T> implements Iterator<T>{    private Aggregate<T> aggregate;    private int currentIndex = 0;           //当前集合的下标    public ConcreteIteratorDesc(Aggregate<T> aggregate) {        this.aggregate = aggregate;        this.currentIndex = aggregate.getCount() - 1;    }    @Override    public T next() {        T t = this.currentItem();        currentIndex--;        return t;    }    @Override    public boolean hasNext() {        return currentIndex >= 0;    }    @Override    public T currentItem() {        return this.aggregate.getByIndex(currentIndex);    }}

运行结果:

同学1号同学2号同学3号同学4号-----------------------------同学4号同学3号同学2号同学1号

其实JAVA/C#的foreach in就是隐式调用了迭代器模式