Inserting an Object/Integer into a Singly Linked List and sorting it

Question

I've made a Singly Linked List but I keep getting a NullPointerException. The insert method is supposed to add an Object to Singly Linked List then I take all the elements of the SLL and put them into MyVector class where I then use my quick sort algorythm I made for MyVector class then put the Objects back into the SSL. I'm not totally sure why I keep getting the error.

Exception in thread "main" java.lang.NullPointerException at java.lang.Integer.compareTo(Integer.java:978) at java.lang.Integer.compareTo(Integer.java:37) at collection.SortedSLList.remove(SortedSLList.java:51) at collection.SortedSLList.insert(SortedSLList.java:39) at lab.Lab6.test(Lab6.java:15) at main.Main.main(Main.java:15) Java Result: 1

public void insert(Object element) {
    if(head == null) {
        head = tail = new SLListNode(element, null);
        ++size;
        return;
    }
    tail = tail.next = new SLListNode(element, null);
    ++size;
    MyVector temp = new MyVector();
    int i = size;
    Object t = head.data;
    while(temp.size() < i) {
        temp.append(t);
        remove(t); //this line
        t = head.next;
    }
    MySort.quickSort(temp);
    i = 0;
    while(size < temp.size()) {
        insert(temp.elementAt(0));
        ++i;
    }
}
public boolean remove(Object element) {
    if(head == null) return false;
    if(((Comparable)(head.data)).compareTo(element) == 0) { //this line
        if(head == tail) {
            head = tail = null;
            return true;
        }
        head = head.next;
        return true;
    }
    if(head == tail) return false;
    SLListNode ref = head;
    while(ref.next != tail) {
        if(((Comparable)(ref.next.data)).compareTo(element) == 0) {
            ref.next = ref.next.next;
            return true;
        }
        ref = ref.next;
    }
    if(((Comparable)(tail.data)).compareTo(element) == 0) {
        tail = ref;
        tail.next = null;
        return true;
    }
    return false;
}

can you post your stacktrace please, or let us know on what line the error is — PermGenError
– PermGenError, Commented Nov 4, 2012 at 19:42
Not sure what stacktrace is but I marked the lines in the code and added the error message that NetBeans gives me. — Schwarz
– Schwarz, Commented Nov 4, 2012 at 19:45

Kevin Day · Accepted Answer · 2012-11-04 20:13:45Z

1

The exception trace says that you are calling remove(null). For some reason, head.data or head.next contains null. I suggest that you add a printout here:

Object t = head.data;
while(temp.size() < i) {
    System.out.println("Looking at " + t); // <-- add here
    temp.append(t);
    remove(t); //this line
    t = head.next;
}

Then watch what those values are doing. You'll see that one of them comes up null.

answered Nov 4, 2012 at 20:13

Kevin Day

16.6k8 gold badges50 silver badges72 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

CrazyCasta · Accepted Answer · 2012-11-04 20:02:28Z

0

The problem is where you do:

while(temp.size() < i) {
    temp.append(t);
    remove(t); //this line
    t = head.next;
}

The problem is that you have removed the head (t), so you should be setting t equal to head.data, not head.next.

edited Nov 4, 2012 at 20:02

answered Nov 4, 2012 at 19:51

CrazyCasta

28.6k4 gold badges50 silver badges73 bronze badges

1 Comment

CrazyCasta Over a year ago

Actually I guess it should be head.data since you're deleting the data, not the node.

Collectives™ on Stack Overflow

Inserting an Object/Integer into a Singly Linked List and sorting it

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related