java - Exception being thrown incorrectly -

i using pre-written classes textbook , implementing them program. duplicate error being thrown every time try add new element tree, , have no idea why, , haven't manipulated written code @ all.

basically, program prompts user enter information, , take info , add student binary search tree, have following textbook:

abstractbinarytree class:

public abstract class abstractbinarysearchtree<e extends comparable<? super e>> implements binarysearchtree<e>   public void add( e element ) { if ( element == null ) {   throw new searchtreeexception(); } setroot( add( null, this.root(), element ) ); size++; } ... 

linkedbst class:

public class linkedbst<e extends comparable<? super e>> extends abstractbinarysearchtree<e> {  public linkedbst( e element ) { if ( element == null ) {   throw new java.lang.illegalargumentexception( "null element illegal" ); } this.root = new bstnode<e> ( element ); this.size = 1; }  protected void setroot( bstnode<e> newroot ) { this.root = newroot; }  protected bstnode<e> add( bstnode<e> parent, bstnode<e> node, e element ) { if ( node == null ) { // base case   node = new bstnode<e> ( element );   node.parent = parent; } else { // recursive case   int compareresult = element.compareto( node.element );   if ( compareresult < 0 ) { // recursive case - left     node.leftchild = add( node, node.leftchild, element );   }   else if ( compareresult > 0 ) { // recursive case - right     node.rightchild = add( node, node.rightchild, element );   }   else {     throw new searchtreeexception( "duplicate element: " + element.tostring() );   } }  return node; }  ... 

as binarysearchtree interface, , i'm calling method seperate class:

... abstractbinarysearchtree<student> tree = new linkedbst<student>(); //this problem? ... tree.add(new student(studentnumber, firstname, lastname, major, gpa)); 

so, know partially works @ least. i'm able add @ least 1 record tree, when try add second 1 regardless of information, keeps throwing searchtreeexception a(also pre written) , won't add others. still new abstract classes binary search trees doing best @ implementing this, if can see going wrong awesome!

if @ comparable interface , compareto(..) method, states

returns negative integer, zero, or positive integer object less than, equal to, or greater specified object.

if you're seeing searchtreeexception, means piece of code

int compareresult = element.compareto( node.element ); if ( compareresult < 0 ) { // recursive case - left     node.leftchild = add( node, node.leftchild, element ); } else if ( compareresult > 0 ) { // recursive case - right     node.rightchild = add( node, node.rightchild, element ); } else {     throw new searchtreeexception( "duplicate element: " + element.tostring() ); } 

used implementation of compareto(..) returned 0.

from comments, stated using default (ide-)created implementation of method returns 0. need fix compareto method works documented in javadoc.