How do I implement generics <E> in a linked List?

Question

I have been trying to create a linked list that uses generics to return a data type of the user's choosing. The problem is that my method public E get(int sub) is not recognizing my return cursor.contents as a type E generic.

public E get(int sub)
{
    Node cursor = head; //start at the beginning of linked list. 

    for (int c = 1; c <= sub; c++)
    {
        cursor = cursor.next; //move forward by one. 
    }

    return cursor.contents;//return the element that the cursor landed on. 
}


 public class Node <E>
{
        public E contents; 
    @SuppressWarnings("rawtypes")
    public Node next = null; //points to the next node
    //a method has no return type and has the same name as the class
    public Node(E element)
    {
        this.contents = element; 
    }
}

as I have shown above the contents parameter is declared as type E in the Node, but the get method will not recognize cursor.contents as a proper return type.

The system recomments that I either change the return type to Object, which is not an option. Or I change contents to a type E which has already been done, but it still gives me a compilation error.

I added the generics to the node cursor to make it node<E> cursor, but now when I test it with the code below it gives me a new error and I cant figure out why my code wont return the proper output. — Jon Doe
– Jon Doe, Commented May 10, 2015 at 2:13
public class Demo1 { public static void main(String[] args) { MyLinkedList<String> t = new MyLinkedList<String>(); t.add("Thousand Oaks"); for (int x = 0; x < t.size(); x++) { System.out.println(t.get(x)); } t.add("Hollywood"); for (int x = 0; x < t.size(); x++) { System.out.println(t.get(x)); } for (int x = 0; x < 500; x++) { t.add("Junk data " + x); } System.out.println(t.get(64)); System.out.println(t.get(499)); System.out.println(t.get(319)); System.out.println("--------"); System.out.println("Cool! I didn't crash!"); } } — Jon Doe
– Jon Doe, Commented May 10, 2015 at 2:14

EpicPandaForce · Accepted Answer · 2015-05-08 05:24:39Z

1

That's because you need to change it to:

public E get(int sub)
{
    Node<E> cursor = head; //you forgot the generics here

    for (int c = 1; c <= sub; c++)
    {
        cursor = cursor.next; 
    }

    return cursor.contents;
}


 public class Node <E>
{
    public E contents; 
    public Node<E> next = null; //you also even suppressed the raw type here

    public Node(E element)
    {
        this.contents = element; 
    }
}

answered May 8, 2015 at 5:24

EpicPandaForce

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2 Comments

Ron Over a year ago

You should explain where the OP went wrong instead of just providing a copy-pasteable answer

Ron Over a year ago

There could be more ;) I think it's interesting to note how and why the original return type disagreed with the declared return type.

1337joe · Accepted Answer · 2015-05-08 05:04:47Z

1

You're not setting the generic type on the declaration of your Node cursor variable. What happens when you change that to Node<E> cursor.

Also, you're not providing context of the linked list class itself - that's where the generic <E> should be declared.

answered May 8, 2015 at 5:04

1337joe

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1 Comment

gengkev Over a year ago

True, seems like either solution would work! If the first solution is used, however, Node should be a static class. Otherwise the <E> from the LinkedList class can be used instead.

EpicPandaForce · Accepted Answer · 2015-05-08 08:33:52Z

1

In your method

public E get(int sub)

You initialize a cursor as a Node instead of Node<E>.

Node cursor = head; //start at the beginning of linked list.

This will cause the element type to be Object which is what you get when you write

return cursor.contents;

To Solve:

Either use a Node<E> or explicitly cast the return to E

edited May 8, 2015 at 8:33

EpicPandaForce

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answered May 8, 2015 at 5:28

Ron

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Comments

gengkev · Accepted Answer · 2015-05-08 05:04:17Z

0

Are these part of a class with a typed parameter, such as MyLinkedList<E>? The problem may be that you added a <E> type parameter to the Node class as well, which may refer to a different class E that is not necessarily the same E referenced by the outer class. Try changing Node <E> to Node.and see if it works.

answered May 8, 2015 at 5:04

gengkev

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