①图解设计模式之 【Iterator】

首先附上 示例程序类图:

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实例源代码:
Aggregate.java

package IteratorPattern;

import java.util.Iterator;

public interface Aggregate {
    public abstract Iterator iterator();
}

Book.java

package IteratorPattern;

public class Book {
    private String name;

    public Book(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

BookShelf.java

package IteratorPattern;

import java.util.Iterator;

public class BookShelf implements Aggregate {
    private Book[] books;
    private int last = 0;

    public BookShelf(int maxsize) {
        this.books = new Book[maxsize];
    }

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

    public void appendBook(Book book) {
        this.books[last] = book;
        last++;
    }

    public int getLength() {
        return last;
    }

    @Override
    public Iterator iterator() {

        return new BookShelfIterator(this);
    }

}

BookShelfIterator.java

package IteratorPattern;

import java.util.Iterator;

public class BookShelfIterator implements Iterator {
    private BookShelf bookShelf;
    private int index;

    public BookShelfIterator(BookShelf bookShelf) {
        this.bookShelf = bookShelf;
        this.index = 0;
    }

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

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

}

Main.java

package IteratorPattern;

import java.util.Iterator;

public class Main {

    public static void main(String[] args) {
        BookShelf bookShelf = new BookShelf(4);
        bookShelf.appendBook(new Book("Around the World in 80 Days"));
        bookShelf.appendBook(new Book("Bible"));
        bookShelf.appendBook(new Book("Cinderella"));
        bookShelf.appendBook(new Book("Daddy-Long-Legs"));
        Iterator it = bookShelf.iterator();
        while (it.hasNext()) {
            Book book = (Book) it.next();
            System.out.println(book.getName());
        }
    }

}

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总结:

因为Iterator 将遍历与实现分离开来.

例如:while (it.hasNext()) {
Book book = (Book) it.next();
System.out.println(book.getName());
}

这段代码中,只是使用了Iterator 的 hasNext() 和next() 方法,并没有调用BookShelf 方法.

设计模式的作用就是帮助我们编写可复用的类。“可复用”就是将类实现为“组件”，当一个组件发生改变时，不需要对其他的组件进行修改或只需很小的修改即可应对。