Which of the following 4 implementations could not be used for contains?

public boolean contains(T anEntry) {

  Node currentNode = firstNode;

  while (!anEntry.equals(currentNode.getData())) {
    currentNode = currentNode.getNext();
  } // end while

  return true;
} // end contains

• public boolean contains(T anEntry)
  {
    return getIndexOf(anEntry) > -1;
  }
  

• public boolean contains(T anEntry)
  {
    return getFrequencyOf(anEntry) > 0;
  }
  

• public boolean contains(T anEntry)
  {
    boolean found = false;
    Node currentNode = firstNode;
  
    while (!found && (currentNode != null)) {
      if (anEntry.equals(currentNode.getData()))
         found = true;
      else
         currentNode = currentNode.getNext();
    } // end while
  
    return found;
  } // end contains