Introduction to the Iterator pattern for Java design patterns

1. First define a container Collection interface.

package com.njupt.zhb.learn.iterator;
public interface Collection {
 void add(Object o);
 int size();
 Iterator iterator();
}

2. Define an interface to the Iterator Iterator

package com.njupt.zhb.learn.iterator;
public interface Iterator {
 Object next();
 boolean hasNext();
}

3. Define an ArrayList, implement the Collection interface, and write an inner class that implements the Iterator interface.

package com.njupt.zhb.learn.iterator;
import com.njupt.zhb.learn.iterator.Collection;
public class ArrayList implements Collection {
 Object[] objects = new Object[10];
 int index = 0;
 public void add(Object o) {
  if(index == objects.length) {
   Object[] newObjects = new Object[objects.length * 2];
   System.arraycopy(objects, 0, newObjects, 0, objects.length);
   objects = newObjects;
  }
  objects[index] = o;
  index ++;
 }

 public int size() {
  return index;
 }

 public Iterator iterator() {

  return new ArrayListIterator();
 }

 private class ArrayListIterator implements Iterator {
  private int currentIndex = 0;
  @Override
  public boolean hasNext() {
   if(currentIndex >= index) return false;
   else return true;
  }
  @Override
  public Object next() {
   Object o = objects[currentIndex];
   currentIndex ++;
   return o;
  }

 }
}

4. Write the test program TestMain

package com.njupt.zhb.learn.iterator;
import com.njupt.zhb.learn.iterator.ArrayList;
public class TestMain {
 public static void main(String[] args) {
  Collection c = new ArrayList();
  for(int i=0; i<15; i++) {
   c.add("string "+i);
  }
  System.out.println(c.size());
  Iterator it = c.iterator();
  while(it.hasNext()) {
   Object o = it.next();
   System.out.println(o.toString() + " ");
  }
 }
}

Operation results:

15
string 0 
string 1 
string 2 
string 3 
string 4 
string 5 
string 6 
string 7 
string 8 
string 9 
string 10 
string 11 
string 12 
string 13 
string 14 

As you can see from the above, the design pattern USES polymorphism in object orientation everywhere. The interface calls a function in a subclass. (link: http://xiazai.jb51.net/201307/yuanma/MyIterator_jb51net.rar)