Which of the following 5 implementations could not be used for contains?

public boolean contains(T anEntry)
{
   return !isEmpty();
}

• public boolean contains(T anEntry)
  {
    boolean found = false;
    int i = 0;
    while ( !found && (i < numberOfEntries))
    {
      if (anEntry.equals(contents[i]))
      {
        found = true;
      }
      i++;
    }
    return found;
  }

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

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

• public boolean contains(T anEntry)
  {
    int i = 0;
    while (i < numberOfEntries)
    {
      if (anEntry.equals(contents[i]))
      {
        return true;
      }
      i++;
    }
    return false;
  }