迭代器(iterator)模式

模式名称：迭代器模式 Iterator

描述：一个一个遍历

对代码的作用：当数据容器内存储结构发生改变，不必修改客户端遍历的代码

容器接口：

public interface Container<T> {

    ContainerIterator<T> iterator();

    void add(T element);
}

迭代器接口：

public interface ContainerIterator<T> {

    T next();

    boolean hasNext();

    void setSize(int size);

    T prev();
}

容器实现：

public class ContainerImpl<T> implements Container<T> {

    private Object[] elements;

    private int index;

    private int size;

    private ContainerIterator iterator;

    public ContainerImpl() {
        elements = new Object[10];
        iterator = new ContainerIteratorImpl(elements, size);
    }

    @Override
    public ContainerIterator iterator() {
        return iterator;
    }

    @Override
    public void add(T element) {
        if(index > 9) {
            throw new ArrayIndexOutOfBoundsException();
        }
        elements[index++] = element;
        ++size;
        iterator.setSize(size);
    }
}

迭代器实现：

public class ContainerIteratorImpl<T> implements ContainerIterator<T> {

    private int index = 0;

    private Object[] elements;

    private int size;

    public ContainerIteratorImpl(Object[] elements, int size) {
        this.elements = elements;
        this.size = size;
    }

    @Override
    public T next() {
        return (T)elements[index++];
    }

    @Override
    public boolean hasNext() {
        return index < size;
    }

    @Override
    public void setSize(int size) {
        this.size = size;
    }

    @Override
    public T prev() {
        if(index < 1) {
            throw new ArrayIndexOutOfBoundsException();
        }
        return (T) elements[--index];
    }
}

容器元素：

public class User {

    private String name;

    private int age;

    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

客户端：

public class Main {

    public static void main(String[] args) {
        Container<User> userList = new ContainerImpl<>();
        userList.add(new User("小明", 20));
        userList.add(new User("小红", 23));
        userList.add(new User("小刚", 12));
        userList.add(new User("小离", 24));

        ContainerIterator<User> iterator = userList.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}

UML图：

总结：当ContainerImpl内部存储结构改变 我们只需要修改迭代器实现类的实现，而无需修改客户端代码，如果只有一两处修改，修改客户端没什么，当使用量特别大的时候，就比较棘手了，只需要修改迭代器实现，一劳永逸