Building my own queue in Java with nodes

Question

I'm trying to make my add method work as you can see from my code below & it's giving me tremendous difficulty figuring out what's wrong with my code.

NodeFN class:

public class NodeFN {
    private String data; // Data for node.
    private NodeFN next; // Next node.

public NodeFN(String data) {
    this.data = data; // Take the data value passed in & store it in the data field.
    this.next = null; // Take the next node & store it in the next field.
}

// Mutator functions.
public String getData() {return data;}
public NodeFN getNext() {return next;}
public void setData(String d) {data = d;}
public void setNext(NodeFN n) {next = n;}
}

Queue class:

public class Queue {
    NodeFN head = null; // Head of node.
    public String n;

public Queue() {} // Default constructor.

public Queue(String n) { 
    head = new NodeFN(n); // head is now an object of NodeFN which holds a string.
}

public void add(String n) {
    NodeFN nn = new NodeFN(n); // nn is now an object of NodeFN which holds a string.
        if(nn == null) { // If the new node (nn) is empty.
            head = nn; // Then allow head to equal to the new node(nn).
        } else {
            // If new node (nn) alphabetically comes first compared to head 
            if(nn.getData().compareTo(head.getData()) < 0) { 
                nn.setNext(head); 
                head = nn;
            }       
        }
    }

public static void main(String[] args) {
    Queue q = new Queue();
    q.add("some string to test to see if it prints in console");

    System.out.println(q);
    }
}

Printing objects to debug them, is a bad practice, by the way. You should learn to use a debugger. — OneCricketeer, Nov 03 '16 at 21:16
Single step through the execution in a debugger and see what the code you wrote actually does. You will learn a lot by figuring out why it does what it does (rather than what you thought it would do). — Rob, Nov 03 '16 at 21:17

score 2 · Answer 1 · answered Nov 03 '16 at 21:15

Lets start here:

NodeFN nn = new NodeFN(n); 
if(nn == null) {

Those two lines dont make sense together. In Java, the first line is guaranteed to return something that is not null. The only thing that could happen is: the constructor could throw an exception, but then you would never reach the next line.

Thus: you are never going to take the "then" branch of your if.

Then:

if(nn.getData().compareTo(head.getData())

... will only work ... if head is != null.

But surprise: out of your two constructors for the queue class, only the second one makes sure head is != null.

Long story short: that is how a NullPointerException comes into existence! When you change

Queue q = new Queue();

to

Queue q = new Queue("whatever");

your example shouldn't fail any more. But of course, the code is still buggy then. The key thing to fix: make sure that head is initialized; and make sure that you compare against null ... in those occasions where null is possible!

user6904265 · Answer 2 · 2016-11-03T21:23:59.767

0

You should check if head is null:

 if(head == null) {
            head = nn;
        } else {
           ...    
        }

then you should also consider this else condition:

if(nn.getData().compareTo(head.getData()) < 0) { 
    nn.setNext(head); 
    head = nn;
} 
else {
    head.setNext(nn)
}

Finally, you should implement the toString() method in Queue class..

edited Nov 03 '16 at 21:23

answered Nov 03 '16 at 21:16

user6904265

1,938
1
16
21

score 0 · Answer 3 · answered Nov 03 '16 at 21:16

0

New node will never be null, you've jest created it.

answered Nov 03 '16 at 21:16

Konrad Szałkowski

166
1
4

Building my own queue in Java with nodes

3 Answers3