设计模式之迭代器设计模式

• 迭代器模式的设计如下
  
• 核心：congtainer的实现和container的迭代（interator）是分开的

package design_model;

import java.util.Iterator;

/**
 * @author: gavin
 * @Time: 2024/10/10
 * @Desc:
 **/

class Book{
    private String name;
    Book(String name){
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

class BookSelf {
    private Book[]books;
    private int lastIndex=0;
    BookSelf(int maxSize){
        books = new Book[maxSize];
    }

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

    public void addBook(Book book){
        books[lastIndex]=book;
        lastIndex++;
    }

    public int getSize(){
        return lastIndex;
    }

    public Iterator iterator(){
        return new BookSelfIterator(this);
    }
}

//实现迭代器的书架迭代器
class BookSelfIterator implements Iterator{
    private BookSelf bookSelf;
    private  int index;
    BookSelfIterator(BookSelf bookSelf){
        this.bookSelf = bookSelf;
        index = 0;
    }
    @Override
    public boolean hasNext() {
        if(index<bookSelf.getSize()){
            return true;
        }else{
            return false;
        }
    }

    @Override
    public Object next() {
        Book book =  bookSelf.getBookByIndex(index);
        index+=1;
        return book;
    }
}

public class Iterator_model {
    public static void main(String[] args) {
        BookSelf bookSelf = new BookSelf(4);
        bookSelf.addBook(new Book("1 book"));
        bookSelf.addBook(new Book("2 book"));
        bookSelf.addBook(new Book("3 book"));
        bookSelf.addBook(new Book("4 book"));
        Iterator iterator = bookSelf.iterator();
        while(iterator.hasNext()){
            Book next = (Book)iterator.next();
            System.out.println(next.getName());
        }
    }
}

1. 迭代器设计模式可以让可迭代对象的具体实现（即BookSelf）和业务迭代的逻辑（while(iterator.hasNext)…）分开。（将存储数据和遍历数据的职责分离。）
2. 即如果我Bookself底层不用数组实现，而是用链表或者arraylist做，我只需调整（BookSeft找元素和遍历元素）的逻辑。
3. 而具体迭代器的实现逻辑不需要改变，即BookSelfIterator不需要改变，业务上使用iterator也不需要改变，即下面的迭代代码也不需要改：

		while(iterator.hasNext()){
            Book next = (Book)iterator.next();
            System.out.println(next.getName());
        }

下面我把bookself底层换成arraylist做一遍

package design_model;

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

/**
 * @author: gavin
 * @Time: 2024/10/10
 * @Desc:
 **/

class Book{
    private String name;
    Book(String name){
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

class BookSelf {
    private ArrayList<Book> books;
    private int lastIndex=0;
    BookSelf(int maxSize){
        books = new ArrayList<>(maxSize);
    }

    public Book getBookByIndex(int index){
        return books.get(index);
    }

    public void addBook(Book book){
       books.add(book);
        lastIndex++;
    }

    public int getSize(){
        return lastIndex;
    }

    public Iterator iterator(){
        return new BookSelfIterator(this);
    }
}

//实现迭代器的书架迭代器
class BookSelfIterator implements Iterator{
    private BookSelf bookSelf;
    private  int index;
    BookSelfIterator(BookSelf bookSelf){
        this.bookSelf = bookSelf;
        index = 0;
    }
    @Override
    public boolean hasNext() {
        if(index<bookSelf.getSize()){
            return true;
        }else{
            return false;
        }
    }

    @Override
    public Object next() {
        Book book =  bookSelf.getBookByIndex(index);
        index+=1;
        return book;
    }
}

public class Iterator_model {
    public static void main(String[] args) {
        BookSelf bookSelf = new BookSelf(4);
        bookSelf.addBook(new Book("1 book"));
        bookSelf.addBook(new Book("2 book"));
        bookSelf.addBook(new Book("3 book"));
        bookSelf.addBook(new Book("4 book"));
        Iterator iterator = bookSelf.iterator();
        while(iterator.hasNext()){
            Book next = (Book)iterator.next();
            System.out.println(next.getName());
        }
    }
}

1. 可以看到我只是修改了bookself的定位元素和迭代的逻辑，具体的迭代器逻辑BookSelfIterator没有动过。代码一样是正常的。

总结

1. 迭代器模式可以让我们封装一切可以遍历集合，提供统一的遍历方式。无论你底层是数组，链表，arraylist，vector等等，我下面的迭代代码始终一致：

		while(iterator.hasNext()){
            Book next = (Book)iterator.next();
            System.out.println(next.getName());
        }