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一、场景

• 以简化版“图书馆书籍管理系统”介绍迭代器模式的好处。
• 假设图书馆中有多个书架，每个书架上存放着不同数量的书籍。我们需要实现一个功能，能够遍历所有书架上的书籍，并打印出每本书的信息。

二、不采用迭代器模式

1、代码

// 书类
public class Book {private String title;public Book(String title) {this.title = title;}public String getTitle() {return title;}
}// 书架类
public class BookShelf {private Book[] books;private int last = 0;public BookShelf(int maxSize) {this.books = new Book[maxSize];}public void appendBook(Book book) {if (last >= books.length) {throw new IllegalArgumentException("书架已满");}this.books[last] = book;last++;}public int getLength() {return last;}public Book getBookAt(int i) {if (i < last && i >= 0) {return books[i];}throw new IllegalArgumentException("书架索引越界");}
}// 图书馆类
@Getter
@AllArgsConstructor
public enum BookShelfEnum {CS_BOOK_SHELF("计算机书架", 0),LITERATURE_BOOK_SHELF("文学书架", 1),NOVEL_BOOK_SHELF("小说书架", 2);private final String name;private final int index;
}public class Library {private BookShelf[] bookShelves;public Library() {this.bookShelves = new BookShelf[3];}public void appendBook(BookShelfEnum shelfEnum , Book book) {if (shelfEnum == null) {throw new IllegalArgumentException("书架类型不能为null");}BookShelf bookShelf = bookShelves[shelfEnum.getIndex()];if (bookShelf == null) {bookShelves[shelfEnum.getIndex()] = new BookShelf(5);bookShelf = bookShelves[shelfEnum.getIndex()];}bookShelf.appendBook(book);}public int getNumShelves() {return bookShelves.length;}public BookShelf getBookShelfAt(int i) {if (i < 0 || i >= bookShelves.length) {throw new IllegalArgumentException("书架索引越界");}return bookShelves[i];}
}// 客户端
public class Main {public static void main(String[] args) {Library library = new Library();library.appendBook(BookShelfEnum.CS_BOOK_SHELF, new Book("Java编程思想"));library.appendBook(BookShelfEnum.CS_BOOK_SHELF, new Book("Effective Java"));library.appendBook(BookShelfEnum.LITERATURE_BOOK_SHELF, new Book("三国演义"));library.appendBook(BookShelfEnum.LITERATURE_BOOK_SHELF, new Book("红楼梦"));library.appendBook(BookShelfEnum.NOVEL_BOOK_SHELF, new Book("白夜行"));library.appendBook(BookShelfEnum.NOVEL_BOOK_SHELF, new Book("简爱"));// 遍历所有书籍for (int i = 0; i < library.getNumShelves(); i++) {BookShelf bookShelf = library.getBookShelfAt(i);if (bookShelf == null) {continue;}for (int j = 0; j < bookShelf.getLength(); j++) {Book book = bookShelf.getBookAt(j);System.out.println(book.getTitle());}}}
}

2、缺点

• 不采用迭代器模式时，当我们去遍历图书馆的书籍的时候，不得不知道图书馆是如何存储书籍的。

  • 首先，我们得知道图书馆有书架组成，我们需要遍历若干个书架。
  • 其次，对于每个书架，如果有书的话，我们要遍历每本书。

• 但我们并不希望客户端知道图书馆是如何存储书籍的。这时候怎么办？需要迭代器模式。

三、采用迭代器模式

• 迭代器模式是一种行为设计模式， 让我们能在不暴露集合底层存储结构 （列表、 栈和树等） 的情况下遍历集合中所有的元素。

1、代码

// 书架类
public class BookShelf implements Iterable<Book> {private Book[] books;private int last = 0;public BookShelf(int maxSize) {this.books = new Book[maxSize];}public void appendBook(Book book) {this.books[last] = book;last++;}private Book getBookAt(int index) {return books[index];}private int getLength() {return last;}@Overridepublic Iterator<Book> iterator() {return new BookIterator(this);}private class BookIterator implements Iterator<Book> {private BookShelf bookShelf;private int index;public BookIterator(BookShelf bookShelf) {this.bookShelf = bookShelf;this.index = 0;}@Overridepublic boolean hasNext() {return index < bookShelf.getLength();}@Overridepublic Book next() {return bookShelf.getBookAt(index++);}}
}// 图书馆类
public class Library implements Iterable<Book> {private BookShelf[] bookShelves;public Library() {this.bookShelves = new BookShelf[3];}public void appendBook(BookShelfEnum shelfEnum, Book book) {if (shelfEnum == null) {throw new IllegalArgumentException("书架类型不能为null");}BookShelf bookShelf = bookShelves[shelfEnum.getIndex()];if (bookShelf == null) {bookShelves[shelfEnum.getIndex()] = new BookShelf(5);bookShelf = bookShelves[shelfEnum.getIndex()];}bookShelf.appendBook(book);}@Overridepublic Iterator<Book> iterator() {return new LibraryIterator(this);}private class LibraryIterator implements Iterator<Book> {private Library library;private int bookShelfIndex = 0;private Iterator<Book> bookIterator;public LibraryIterator(Library library) {this.library = library;}@Overridepublic boolean hasNext() {if (bookIterator != null && bookIterator.hasNext()) {return true;}while (bookShelfIndex < library.bookShelves.length) {BookShelf bookShelf = library.bookShelves[bookShelfIndex++];if (bookShelf == null) {continue;}bookIterator = bookShelf.iterator();if (bookIterator.hasNext()) {return true;}}return false;}@Overridepublic Book next() {return bookIterator.next();}}
}// 客户端
public class Main {public static void main(String[] args) {Library library = new Library();library.appendBook(BookShelfEnum.CS_BOOK_SHELF, new Book("Java编程思想"));library.appendBook(BookShelfEnum.CS_BOOK_SHELF, new Book("Effective Java"));library.appendBook(BookShelfEnum.LITERATURE_BOOK_SHELF, new Book("三国演义"));library.appendBook(BookShelfEnum.LITERATURE_BOOK_SHELF, new Book("红楼梦"));library.appendBook(BookShelfEnum.NOVEL_BOOK_SHELF, new Book("白夜行"));library.appendBook(BookShelfEnum.NOVEL_BOOK_SHELF, new Book("简爱"));// 遍历所有书籍Iterator<Book> iterator = library.iterator();while (iterator.hasNext()) {Book book = iterator.next();System.out.println(book.getTitle());}}
}

2、优点

• 采用迭代器模式后，遍历图书馆的所有书籍，客户端完全不知道图书馆是如何存储书籍的。

四、总结

• 采用迭代器模式的思路：

  • （1）理解需求：如果自定义了一个较复杂结构，客户端又需要遍历这个复杂结构的元素，这时候要想到使用迭代器模式。

  • （2）复杂结构要实现Iterator结构：

    public class ComplexStructure implements Iterator<Element>  {@Overridepublic Iterator<Element> iterator() {return new ComplexStructureIterator(this);}private class ComplexStructureIterator implements Iterator<Element> {private ComplexStructure complexStructure;public ComplexStructureIterator(ComplexStructure complexStructure) {this.complexStructure = complexStructure;}@Overridepublic boolean hasNext() {// 待实现}@Overridepublic Element next() {// 待实现}}
    }
    

  • （3）客户端使用迭代器：

    Iterator<Element> iterator = library.iterator();
    while (iterator.hasNext()) {Element element = iterator.next();...
    }
    

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