迭代器模式

集合类中包含很多数据结构：数组、散列表、ArrayList等，这些集合类的迭代方法都不一样。为了隐藏迭代的具体实现 ，抽象集合类中引用抽象迭代器，具体的集合类引用具体的迭代类，在具体迭代类实现便利具体集合类的方法

package iterator;

import java.util.Iterator;

/**
 * 抽象集合类，引用抽象迭代类
 */
public interface Aggregate {
    public abstract Iterator createIterator();
}

package iterator;

import java.util.Iterator;

/**
 * 具体集合类
 */
public class ArrayShelf implements Aggregate {
    Book[] books;
    int index = 0;

    public ArrayShelf(int maxium) {
        books = new Book[maxium];
    }

    public Book getBookAt(int index) {
        return books[index];
    }

    public void appendBook(Book newBook) {
        books[index++] = newBook;
    }

    public int getLength() {
        return index;
    }

    @Override
    public Iterator createIterator() {
        return new ArrayIterator(this);
    }
}

package iterator;

import java.util.Iterator;

/**
 * 具体迭代类，Iterator是抽象迭代类
 */
public class ArrayIterator implements Iterator {
    private ArrayShelf bookShelf;
    private int index = 0;

    public ArrayIterator(ArrayShelf bookShelf) {
        this.bookShelf = bookShelf;
        index = 0;
    }

    @Override
    public boolean hasNext() {
        if (index < bookShelf.getLength()) {
            return true;
        } else {
            return false;
        }
    }

    @Override
    public Object next() {
        Book book = bookShelf.getBookAt(index++);
        return book;
    }
}

package iterator;

import java.util.ArrayList;
import java.util.Iterator;

/**
 * Created by marcopan on 17/4/7.
 */
public class ListIterator implements Iterator {
    ListShelf arrays = new ListShelf();
    int index = 0;

    public ListIterator(ListShelf arrays) {
        this.arrays = arrays;
        index = 0;
    }

    @Override
    public boolean hasNext() {
        return (index < arrays.getLength());
    }

    @Override
    public Object next() {
        return arrays.getBook(index++);
    }
}

package iterator;

import java.util.Iterator;
import java.util.List;
import java.util.ArrayList;

public class ListShelf implements Aggregate {
    List<Book> shelfList = new ArrayList<Book>();

    public ListShelf() {
        shelfList = new ArrayList();
    }

    public void append(Book book) {
        shelfList.add(book);
    }

    public Book getBook(int index) {
        return shelfList.get(index);
    }

    public int getLength() {
        return shelfList.size();
    }

    @Override
    public Iterator createIterator() {
        return new ListIterator(this);
    }
}

package iterator;

import java.util.Iterator;

/**
 * Created by marcopan on 17/4/7.
 */
public class IteratorTest {
    public static void main(String[] args) {
        ArrayShelf shelf = new ArrayShelf(4);
        shelf.appendBook(new Book("a"));
        shelf.appendBook(new Book("b"));
        shelf.appendBook(new Book("c"));
        shelf.appendBook(new Book("d"));

        /*ListShelf list = new ListShelf();
        list.append(new Book("a"));
        list.append(new Book("b"));
        list.append(new Book("c"));
        list.append(new Book("d"));*/

        Iterator it = shelf.createIterator();
        while (it.hasNext()) {
            Book book = (Book)it.next();
            System.out.println(book.getName());
        }
    }
}