How to Flip a Double Linked List in java and put it into a new List-CodePudding

I wanted to flip a Double Linked List and put it into a new List. But I don't know how to solve the Problem. My Code looks like this and I think it makes sense, but it doesn't work at all.

getPred = previous getSucc = next

    public DoublyLinkedList fliping()
    {
        DoublyLinkedList newList = new DoublyLinkedList();
        Element current = last;
        Element toInsert = newList.first;
        while(current != null)
        {
            current.disconnectPred();
            Element temp = current.getPred();
            Element temp2 = current.getPred().getPred();
            if(toInsert == newList.first)
            {
                temp.connectAsSucc(temp2);
                temp2.connectAsPred(temp);
                toInsert.connectAsSucc(temp);
                newList.size  ;
            }
            if(temp2 == null)
            {
                temp.connectAsSucc(temp2);
                toInsert.connectAsSucc(temp);
                newList.size  ;
            }
            temp.connectAsSucc(temp2);
            temp2.connectAsPred(temp);
            toInsert.connectAsSucc(temp);
            toInsert.connectAsPred(temp2);
            toInsert = toInsert.getSucc();
            current = current.getPred();
            newList.size  ;
            size--;
        }
        return newList;
    }

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "Element.getPred()" because the return value of "Element.getPred()" is null
    at DoublyLinkedList.fliping(DoublyLinkedList.java:282)
    at Testumgebung.testDoublyLinkedList(Testumgebung.java:35)
    at Testumgebung.main(Testumgebung.java:7)

I tried to disconnect the last element and the second last element. Than i tried to connect them to each other and put them togeter, as the first and second element into the new List

CodePudding user response：

If your DoublyLinkedList implements the interface java.util.List I would first ensure that you can copy it (e.g. with a copy-constructor that initializes it with any other Collection or List - don't implement the clone method, too much hassle with that!).

Then simply use the method java.util.Collections.reverse(List<?>) on a 1:1 copy of your original list.