Java NullPointerException / Type Generics Question

Question

I have created a Linked List class from scratch in Java. The goal is for this Linked List class to be immutable. I am using generic types (suspected part of issue). The class has three attributes:

boolean isEmpty;
T data;
LinkedListClass<T> next;

There are also two constructors for creating this class, one is private and the other public:

private LinkedListClass(boolean isEmpty, T data, LinkedListClass<T> next){

    this.isEmpty = isEmpty;
    this.data = data;
    this.next = next;

}

...

public LinkedListClass(){

        LinkedListClass<T> newList = new LinkedListClass<T>(true, null, null);

    }

The reason for this is so the user does not have direct access to the variables isEmpty, data and next. So when the user creates a Linked List, the aforementioned variables are assigned to true, null and null respectively by default. N.b. Any method that appends or changes the list in such a way that it is not empty anymore, will hence change the isEmpty value to false.

I have made a copyList() function:

 public LinkedListClass<T> copyList() {

    LinkedListClass<T> clonedList = new LinkedListClass<T>();

    if(this.isEmpty) {

        return clonedList;
    }

    clonedList = new LinkedListClass<T>(false, this.data, this.next.copyList() ); //this is line 32


    return clonedList;


}

In the main method I am simply calling the copy function like so:

public class prog7 {

public static void main (String args[]) {

    LinkedListClass<Integer> list1 = new LinkedListClass();
    LinkedListClass<Integer> list2 = new LinkedListClass();
    list2 = list1.copyList();                                 //this is line 7

}

}

Upon compiling, the following error occurs:

Exception in thread "main" java.lang.NullPointerException
at LinkedListClass.copyList(LinkedListClass.java:32)
at prog7.main(prog7.java:7)

I think the NullPointerException is thrown because when creating a LinkedListClass<T>, two of its values are null. So when trying to change them or reference them, the exception is thrown. One workaround I have considered is replacing null in the public LinkedListClass<T> constructor with the int value 0. However my IDE complains that the expected type is T, but I am providing it with an int. So I think this may be a question regarding type generics.

What semantics has `isEmpty`? Does it mean that `data` is 'empty' (null?), or that the linked list does not have any more nodes? Why does _every_ element in the linked list have an `isEmpty` flag? — isnot2bad, Dec 20 '19 at 15:27
I know that this has probably nothing to do with your question. The problem is imho that your implementation is somehow akward, and I don't see much use in fixing just the specific null-pointer problem while all other things remain untouched. — isnot2bad, Dec 20 '19 at 15:29
The only possible cause of a NullPointerException in the code you have shown us is that `this.next` is null when `copyList` is called. — VGR, Dec 20 '19 at 17:19

score 2 · Accepted Answer · edited Dec 20 '19 at 15:22

2

You have a problem with your public constructor. It should look like that:

public LinkedListClass() {
     this(true, null, null);
}

In this line

LinkedListClass<T> newList = new LinkedListClass<T>(true, null, null);

you don't set parameters at all, you create a new variable which is not used afterwards. So isEmpty has a default value (false). Read more about constructor overloading.

edited Dec 20 '19 at 15:22

Naman

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answered Dec 20 '19 at 15:14

Daria Pydorenko

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Java NullPointerException / Type Generics Question

1 Answers1