while循环中的条件不起作用？

Question

private void Scan(DoublyLinkedList dList) { // T(n) = O(n)
    DNode p1 = dList.getFirst();

    while (p1 != null) {
        DNode p2 = p1.next;
        System.out.println(p1.getElement().toString()); // <--- Here it throws NullPointerException.

        if (p2.next != null) {
            DNode p3 = p2.next;

            if (p3.getElement() != null) {
                boolean b = Determinate.isPointRightSide(p1.getElement(), p2.getElement(),p3.getElement());

                if (b == true) {
                    p1 = p1.next;
                } else {
                    p1.next = p3;
                    p3.prev = p1;
                    dList.remove(p2);
                    p1 = p1.prev;
                }
            } else break;
        }else break;
    }
}

the exception:

run:
Exception in thread "main" java.lang.NullPointerException
X :8.0  Y: 9.0angle0.0lol
        at ConvexHull.GrahamVersion.Scan(GrahamVersion.java:102)
        at ConvexHull.GrahamVersion.grahamScan(GrahamVersion.java:83)
        at ConvexHull.GrahamVersion.<init>(GrahamVersion.java:25)

It throws NullPointerException on System.out.println(p1.getElement().toString()); . It means that it doesn't pay attention to the condition of while loop?

Answer 1

p1 is not null , but p1.getElement() returned null .

Remove the toString() call. You don't need it there in sysout. It will then just print null as "null".

Answer 2

The condition in the while loop does work. The NullPointerException is thrown on p1.getElement() , which is null and therefore cannot be dereferenced. If p1 were null , the line above it ( p2 = p1.next ) would fail.