Null Pointer Exception--need help tonight!

Hey there so I'm basically working on a bonus project for class and I'm having trouble with this null pointer exception. I'm supposed to create a class called SimpleSet that implements the dictionary interface but only uses keys, not values. The instructor gave us a jar file that contains all required classes to run SimpleSet. Anyway the method thats throwing the exception is the add method. Here's the code for the class:

Code Java:

import java.util.*; 
 
public class SimpleSet<K extends Comparable<? super K>> 
{ 
 
/* use SimpleSortedDictionary object to hold set items  
     Note: Value is not needed, just set it to Boolean       */ 
private DictionaryInterface<K, Boolean> items; 
 
/* default constructor */ 
public SimpleSet() 
{
	DictionaryInterface<K, Boolean> items = new SimpleSortedDictionary();
 
}
 
/* Adds a given object to the set. If the given object already exists in the set, 
    return false; otherwise, return true */ 
public boolean add(K item)  //NOTE: this calls the dictionary interface method: public V add(K key, V value); 
 
{
	boolean result;
	if(items.add(item, true) != null) 
	{
		result = true;
	}
	else result = false;
	return result;
}
 
/* Removes a given object from the set. */ 
public void remove(K item) 
{
	items.remove(item);
}
 
/* Sees whether the set contains a given object. */ 
public boolean contains(K item) 
{
	return items.contains(item);
}
 
/* Clears all objects from the set. */ 
public void clear()
{
	items.clear();
}
 
/* Gets the number of objects in the set. */ 
public int getSize() 
{
	return items.getSize();
}
 
/* Returns an iterator to the set. */ 
public Iterator<K> getIterator() 
{
	return items.getKeyIterator();
}
 
/* Returns a set that combines the items in two sets 
    Note: the union of current ("this") set and input otherset.  */ 
public SimpleSet<K> union(SimpleSet<K> otherSet) 
{
	return null;
}
 
/* Returns a set of those items that occur in both of two sets (the intersection). 
         Note: the intersection of current ("this") set and input otherset.  */ 
public SimpleSet<K> intersection(SimpleSet<K> otherSet) 
{
	return null;
}
 
/* Display objects */ 
public String toString() 
{
	return "Set print out: "+this;
}
 
/* may define tests here */ 
public static void main(String args[]) 
{
	SimpleSet setA = new SimpleSet();
	Integer five = new Integer(5);
	setA.add(five);
	setA.toString();
 
}
} // end SimpleSet

error message:

Exception in thread "main" java.lang.NullPointerException
at SimpleSet.add(SimpleSet.java:22)
at SimpleSet.main(SimpleSet.java:83)