图解设计模式（一）迭代器模式

迭代器模式

import java.util.ArrayList;

/**
 * 迭代器模式：
 * 适用于需要对聚合类实现遍历的场景。每个新增一个聚合类，需新增对应的迭代器。
 * @author zhaozx
 * @create 2018-11-22 下午7:57
 */
public class IteratorPattern {
    public static void main(String[] args) {
        BookShelf<Book> bookShelf = new BookShelf<>(5);
        bookShelf.appendBook(new Book("Java"));
        bookShelf.appendBook(new Book("C"));
        bookShelf.appendBook(new Book("Python"));
        bookShelf.appendBook(new Book("PHP"));
        

        Iterator<Book> iterator = bookShelf.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}

/**
 * 聚合类接口
 */
interface Aggregate<T> {
    Iterator<T> iterator();
}

/**
 * 迭代器接口
 * @param <T>
 */
interface Iterator<T> {
    boolean hasNext();
    T next();
}

class Book {
    private String name;
    public Book(String name) {this.name = name;}
    public String getName() {
        return name;
    }
    public String toString() {
        return new String("Book: " + name);
    }
}

/**
 * 集合类，包含对个对象，如Book，例子使用数组作为集合例子
 */
class BookShelf<T> implements Aggregate<T> {
    private ArrayList<T> book;
    private int last;
    public BookShelf (int maxSize) {
        book = new ArrayList<T>(maxSize);
    }
    public T getBookAt(int index) {
        return (T) book.get(index);
    }
    public void appendBook (T book) {
        this.book.add(last++, book);
    }
    public int getLength() {
        return last;
    }
    public Iterator<T> iterator() {
        return new BookShelfIterator<T>(this);
    }
}

/**
 * 迭代器实现，与聚合类BookShelf相对应。
 * 迭代器模式下，每一个聚合类应当有自己的迭代器实现类.
 * 迭代器类可以实现多种方式遍历聚合类，如顺序、倒序遍历
 */
class BookShelfIterator<T> implements Iterator {
    BookShelf<T> bookShelf;
    int cursor;
    public BookShelfIterator (BookShelf<T> bookShelf) {
        this.bookShelf = bookShelf;
    }

    @Override
    public boolean hasNext() {
        return cursor < bookShelf.getLength();
    }

    @Override
    public T next() {
        return (T)bookShelf.getBookAt(cursor++);
    }
}