Question

public static <E> ArrayList<E> union
        (ArrayList<E>array1, ArrayList<E>array2)
{
  //arrayUnion will be the arrayList that will be returned
  ArrayList <E> arrayUnion  = new ArrayList <E>(array1);   
  arrayUnion.addAll(array2);
  E current;

  for(int i = 0; i < arrayUnion.size(); i++)
  {
      for(int j = 0; j < arrayUnion.size(); j++)
      {
          current = arrayUnion.get(i);
          if(current.equals(arrayUnion.get(j)))
          {
              arrayUnion.remove(j);
          }
      }
  }

      return arrayUnion;
}

For my test of this method, this was the output:

The first list is [ww, ee, rr, t, yy]

The second list is [ww, ss, ee, dd]

The union of both ArrayLists is: [ee, t, ww, dd]

What went wrong..? I’ve been stuck on this for far too long and I never want to hear the word Union again. Plz help

Answer 1

Your code has to do check the current item is checked with itself. If not you have to remove the item and decrease the j with one as you have to check again the item replaced at j. I have modified your code to work for your case. Just look at the condition check for item to be removed.

public static <E> ArrayList< E > union( ArrayList< E > array1, ArrayList< E > array2 ) {
    // arrayUnion will be the arrayList that will be returned
    ArrayList< E > arrayUnion = new ArrayList< E >( array1 );
    arrayUnion.addAll( array2 );
    E current;

    for ( int i = 0; i < arrayUnion.size( ); i++ ) {

        for ( int j = 0; j < arrayUnion.size( ); j++ ) {
            current = arrayUnion.get( i );

            if ( i != j && current.equals( arrayUnion.get( j ) ) ) {
                arrayUnion.remove( j );
                --j;// This is set to check the item which replace the removed item at previous statement
            }
        }
    }

    return arrayUnion;
}