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Wednesday, 4 July 2018

Design Patterns - Iterator Pattern

Design Patterns - Iterator Pattern


Iterator pattern is very commonly used design pattern in Java and .Net programming environment. This pattern is used to get a way to access the elements of a collection object in sequential manner without any need to know its underlying representation.
Iterator pattern falls under behavioral pattern category.

Implementation

We're going to create a Iterator interface which narrates navigation method and a Container interface which retruns the iterator . Concrete classes implementing the Container interface will be responsible to implement Iteratorinterface and use it
IteratorPatternDemo, our demo class will use NamesRepository, a concrete class implementation to print a Names stored as a collection in NamesRepository.
Iterator Pattern UML Diagram

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

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