JAVA Why do I get a type error when assigning these variables (T extends Comparable<T>)

Question

So, let's say we have these two classes.

public class Main<T extends Comparable<T>> {

   public List<T> toList(T t){

      Node n = new Node();

      n.value = t; //This assignment1 gives no issues.



      List<T> list = new List<T>();

      list.add(n.value); //This assignment2 gives a type error.

      return list;

     }

}



class Node <T extends Comparable<T>> {

   T value = null;

}

So, why does assignment1 go through without a type error, while assignment2 gives me the following error:

add (T) in List cannot be applied to (java.lang.Comparable)

It disappears when I do:

list.add((T)n.value);

But I would like to understand the reason first to figure out if above solution is the correct one.

EDIT: For now, I should specify that the code that uses these classes are Integer objects. So, I as the developer can assume that the T types are the same. But I am not sure if the code has some underlying routine that can change the values during casting.

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Answer 1

Node n = new Node();

should be

Node<T> n = new Node<T>();

If you ever refer to Node without <something> after it, you are telling the compiler to ignore all type checking on Node. This will give you warnings, as it should.

JAVA Why do I get a type error when assigning these variables (T extends Comparable<T>)

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