我的数据结构类正在以skiplist的形式实现自己的数据结构。我正在研究的方法是sublist。我已经处理了大多数错误，但是我得到一个错误，说在我的系统中抛出了一个异常 testSubList() 方法。我不确定这可能是什么，因为我相信我已经用输入变量覆盖了所有的错误。假设所有其他方法都起作用，是什么导致了这个错误？

public List<E> subList(int fromIndex, int toIndex)
{
  //subList method written by Ryan Schubert
//if a trivial case where the index is out of bounds occurs throw exception
  if (fromIndex < 0 || toIndex > this.size() || toIndex < fromIndex){
    throw new IndexOutOfBoundsException();
  }
//make new skiplist to add values to
    List<E> sub = new SkipList<E>();
//run through the skiplist between the indices and add the values to sub
    for(int i = fromIndex; i<toIndex; i++)
    {
        sub.add(this.get(i));
    }
    return sub;
}  
public static boolean testSubList()
    {
      //testSubList method written by Ryan Schubert
      //make a new list and add a bunch of values to it
        List<String> testList = new SkipList<String>();

        testList.add("test1");
        testList.add("test2");
        testList.add("test3");
        testList.add("test4");
        testList.add("test5");
        testList.add("test6");
        //make a new list and fill it with sub list values
        List<String> testSubList = new SkipList<String>();
        testSubList = testList.subList(1, 4);
        //compare the test list to the sublist
        if (testList.containsAll(testSubList))
        {
            System.out.println("true");
            return true;
        }
        else
        {
            System.out.println("false");
            return false;
        }
    }