Project - Manipulating ArrayList (Union, Intersect, etc)

I am working on a project for class and I'm a bit lost. I have filled out as much of the code as I could, but I think it's wrong.
I need to create and ArrayList and implement methods specified by the teacher using specified parameters.

Here is the code I have so far.

Code java:

---

package setapplication;
 
import java.util.*;
 
public class Set 
{
 
private ArrayList<String>elements;
 
/**
 * creates an empty set
 */
 
public Set()
{
    elements = null;
}
 
/**
 * creates a set using the elements of the ArrayList s.
 * @param s the ArrayList whose elements are used to create this set.
 * @throws IllegalArgumentException if s contains duplicity.
 */    
 
public Set(ArrayList<String> s)
{
    int i;
    elements = new ArrayList<String>();
 
    for(i=0;i<s.size();i++)
    {
        if(elements.contains(s.get(i)))
        {throw new IllegalArgumentException("Set(ArrayList<String>)duplicity not allowed in sets");}
 
        elements.add(s.get(i));
 
    }
}
 
/**
 * creates a set using the elements of the array s.
 * @param s the array whose elements are used to create this set.
 * @throws illegalArgumentException if s contains duplicity.
 */    
 
public Set(String[] s)
   {
       int i;
       elements = new ArrayList<String>();
       for(i=0; i<s.length; i++)
       {
          if (elements.contains(s[i]))
          {throw new IllegalArgumentException("Set(String[]):duplicity not allowed in sets");}
          elements.add(s[i]);
       }
   }
 
   /**
    * determines whether a set contains the specified element
    * @param elt an element
    * @return true if elt is an element of this set; otherwise, false
    */
 
   public boolean isElement(String elt)
   {
       return elements.contains(elt);
   }
 
   /**
    * determines the size of this set.
    * @return the size of this set.
    */
 
   public int cardinality()
   {
       return elements.size();
   }
 
   /**
    * computes the intersection of this set and the
    * specified set.
    * @param s a set
    * @return a set representing the intersection of this set
    *         and s.
    */
 
   public Set intersect(Set s)
   {
       int i;
       ArrayList<String> result = new ArrayList<String>();
       for (i=0;i<s.cardinality();i++)
       {
           if (this.isElement(s.elements.get(i)))
           {result.add(s.elements.get(i));}
       }  
       return new Set(result);           
   }
 
    /**
    * computes the union of this set and the specified set.
    * @param s a sets
    * @return a set representing the union of this set
    *         and s.
    */
 
  public Set union(Set s)
   {
       int i;
       ArrayList<String> result = new ArrayList<String>();
       for(i=0;i<s.cardinality();i++)
       {
           if (this.isElement(s.elements.get(i)))
           {result.remove(s.elements.get(i));}
       }
       return new Set(result);
   }
 
   /**
    * computes the difference between this set and the
    * specified set.
    * @param s a set
    * @return a set representing the difference between
    *         this set and s.
    */
 
   public Set diff(Set s)
   {
       int i;
       ArrayList<String> result = new ArrayList<String>();
       for(i=0;i<s.cardinality();i++)
       {
           if (this.isElement(s.elements.get(i)) && s.isElement(s.elements.get(i)))
           {result.remove(s.elements.get(i));}
       }
       return new Set(result);
   }
 
   /**
    * computes the symmetric difference between this set
    * and the specified set.
    * @param s a set
    * @return a set representing the symmetrix difference
    *         between this set and s.
    */
 
   public Set symDiff(Set s)
   {
       int i;
       ArrayList<String> result = new ArrayList<String>();
       for(i=0;i<s.cardinality();i++)
       {
           if (this.isElement(s.elements.get(i)) && !s.isElement(s.elements.get(i)))
           {result.add(s.elements.get(i));}
       }
       return new Set(result);
   }
 
   /**
    * computes the Cartesian product for this set
    * and the specified set.
    * @param s a set
    * @return a set representing the Cartesian product
    *         of this set and s.
    */
 
   public Set xProduct(Set s)
   {
       //implement this method
   }
 
 
   /**
    * determines whether a set is empty
    * @return true if this set is empty; otherwise, false
    */
 
   public boolean isEmpty()
   {
       return elements.isEmpty();
   }
 
   /**
    * determines whether this set is equal to the specified
    * set.
    * @param s a set
    * @return true if this set is equal to s; otherwise, false
    */
 
   public boolean equals(Set s)
   {
       return elements.equals(s.elements);
   }
 
   /**
    * determines whether this set is a subset of the specified set.
    * @param s a set
    * @return true if this set is a subset of s; otherwise, false
    */
 
   public boolean subset(Set s)
   {
        return elements.containsAll(s.elements);
   }
 
   /**
    * determines whether this set is a proper subset of the specified set.
    * @param s a set
    * @return true if this set is a proper subset of s; otherwise, false
    */
 
   public boolean properSubset(Set s)
   {
       if(elements.equals(s.elements) && elements.containsAll(s.elements))
       {return false;}
       else{
           return true;
       }
 
   }
 
   /**
    * returns a string {x1,x2,...,xn} representing this set,
    * where x1,x2,...,xn are elements of this set.
    * @return a string representation of this set formatted
    *         as specified.
    */
 
    @Override
   public String toString()
   {
       //implement this method            
   }

---

Any guidance would be appreciated

Mistakes i noted:
1. your union function is wrong. no items are ever added to result. an easy way to do it would be to have result contain the same contents of elements and then add items from s if they are not already in result.
2. same thing for diff. here you could iterate through each set and add any item in each set that is not found in the other.

all code above those 2 are correct. below, i'm not sure what you're trying to do.

Quote:

---

Originally Posted by cosecant

Mistakes i noted:
1. your union function is wrong. no items are ever added to result. an easy way to do it would be to have result contain the same contents of elements and then add items from s if they are not already in result.
2. same thing for diff. here you could iterate through each set and add any item in each set that is not found in the other.

all code above those 2 are correct. below, i'm not sure what you're trying to do.

---

How would I go about doing that?

Ok, if (this.isElement(s.elements.get(i))) is checking to see if this.isElement is an element in S, correct?

if it is, I want to add it to my result. If it is not, I do not want to add it to my result.

{result.addAll(s.elements.get(i));} ?