23种设计模式第三部分关系模式（4）迭代子模式

绝地灬酷狼 2022-07-17 04:09 141阅读 0赞

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## 理解 ##

迭代器模式就是顺序访问聚集中的对象，一般来说，集合中非常常见，如果对集合类比较熟悉的话，理解本模式会十分轻松。这句话包含两层意思：一是需要遍历的对象，即聚集对象，二是迭代器对象，用于对聚集对象进行遍历访问。

![f7571a69-3c85-3fe1-b781-e460563a40a8.jpg][]

## 实现 ##

两个接口：

public interface Iterator {  
        //前移  
        public Object previous();  
          
        //后移  
        public Object next();  
        public boolean hasNext();  
          
        //取得第一个元素  
        public Object first();  
    }

public interface Collection {  
          
        public Iterator iterator();  
          
        /*取得集合元素*/  
        public Object get(int i);  
          
        /*取得集合大小*/  
        public int size();  
    }

两个实现：

public class MyCollection implements Collection {  
      
        public String string[] = {"A","B","C","D","E"};  
        @Override  
        public Iterator iterator() {  
            return new MyIterator(this);  
        }  
      
        @Override  
        public Object get(int i) {  
            return string[i];  
        }  
      
        @Override  
        public int size() {  
            return string.length;  
        }  
    }

public class MyIterator implements Iterator {  
      
        private Collection collection;  
        private int pos = -1;  
          
        public MyIterator(Collection collection){  
            this.collection = collection;  
        }  
          
        @Override  
        public Object previous() {  
            if(pos > 0){  
                pos--;  
            }  
            return collection.get(pos);  
        }  
      
        @Override  
        public Object next() {  
            if(pos<collection.size()-1){  
                pos++;  
            }  
            return collection.get(pos);  
        }  
      
        @Override  
        public boolean hasNext() {  
            if(pos<collection.size()-1){  
                return true;  
            }else{  
                return false;  
            }  
        }  
      
        @Override  
        public Object first() {  
            pos = 0;  
            return collection.get(pos);  
        }  
      
    }

测试:

public class Test {  
      
        public static void main(String[] args) {  
            Collection collection = new MyCollection();  
            Iterator it = collection.iterator();  
              
            while(it.hasNext()){  
                System.out.println(it.next());  
            }  
        }  
    }

输出

A B C D E

我们简单模拟了一个集合类的过程，感觉是不是很爽？其实JDK中各个类也都是这些基本的东西，加一些设计模式，再加一些优化放到一起的，只要我们把这些东西学会了，掌握好了，我们也可以写出自己的集合类，甚至框架！

## 其他实现： ##

![iterator_pattern_uml_diagram.jpg][]

## Step 1 ##

Create interfaces.

Iterator.java

public interface Iterator {
       
       public boolean hasNext();
       public Object next();
    }

Container.java

public interface Container {
       
       public Iterator getIterator();
    }

## Step 2 ##

Create concrete class implementing the Container interface. This class has inner class NameIterator implementing the Iterator interface.

NameRepository.java

public class NameRepository implements Container {
       
       public String names[] = {
       "Robert" , "John" ,"Julie" , "Lora"};
    
       @Override
       public Iterator getIterator() {
       
          return new NameIterator();
       }
    
       private class NameIterator implements Iterator {
       
    
          int index;
    
          @Override
          public boolean hasNext() {
       
          
             if(index < names.length){
       
                return true;
             }
             return false;
          }
    
          @Override
          public Object next() {
       
          
             if(this.hasNext()){
       
                return names[index++];
             }
             return null;
          }		
       }
    }

## Step 3 ##

Use the NameRepository to get iterator and print names.

IteratorPatternDemo.java

public class IteratorPatternDemo {
       
    	
       public static void main(String[] args) {
       
          NameRepository namesRepository = new NameRepository();
    
          for(Iterator iter = namesRepository.getIterator(); iter.hasNext();){
       
             String name = (String)iter.next();
             System.out.println("Name : " + name);
          } 	
       }
    }

## Step 4 ##

Verify the output.

Name : Robert
    Name : John
    Name : Julie
    Name : Lora

http://www.tutorialspoint.com/design\_pattern/iterator\_pattern.htm

[57a92d42-4d84-3aa9-a8b9-63a0b02c2c36.jpg]: /images/20220717/c9fc521ddba3422f8e9cfcdc68bf8801.png
[5e2feb58-4170-3c07-a370-ed99bdcab223.jpg]: /images/20220717/ae7fc18712d84727adae504bacfeab40.png
[f7571a69-3c85-3fe1-b781-e460563a40a8.jpg]: /images/20220717/719cf8a355214a38bb52267522e44a82.png
[iterator_pattern_uml_diagram.jpg]: /images/20220717/7d86bdd43eae400a9dd7affa442da208.png