Stream operation returns an Object instead of a List - java-8

Question

I have the following code that executes as I intend:
import java.util.*;
import java.util.stream.Collectors;
public class HelloWorld{
public static void main(String []args){
HelloWorld.TreeNode rootNode = new HelloWorld().new TreeNode<Integer>(4);
List<Integer> traversal = rootNode.inorderTraversal();
// Prints 4
System.out.println(
String.join(",",
traversal
.stream()
.map(Object::toString)
.collect(Collectors.toList())
)
);
}
class TreeNode<K extends Comparable<K>> {
TreeNode<K> left;
TreeNode<K> right;
K val;
TreeNode(K val, TreeNode<K> left, TreeNode<K> right) {
this.val = val;
this.left = left;
this.right = right;
}
TreeNode(K val) {
this(val, null, null);
}
List<K> inorderTraversal() {
List<K> list = new ArrayList<>();
list.add(this.val);
return list;
}
}
}
However, if I replace the commented line with
System.out.println(
String.join(",",
rootNode.inorderTraversal()
.stream()
.map(Object::toString)
.collect(Collectors.toList())
)
);
I get the following error:
HelloWorld.java:14: error: no suitable method found for join(String,Object)
String.join(",",
^
method String.join(CharSequence,CharSequence...) is not applicable
(varargs mismatch; Object cannot be converted to CharSequence)
method String.join(CharSequence,Iterable<? extends CharSequence>) is not
applicable
(argument mismatch; Object cannot be converted to Iterable<? extends
CharSequence>)
Note: HelloWorld.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error
I saw this very similar issue (Why does this java 8 stream operation evaluate to Object instead of List<Object> or just List?), but I don't see how my solution doesn't circumvent the problem that user had because rootNode.inorderTraversal() return a List<Integer> instead of a List.
Thanks in advance for any assistance!

Answer 1

This is because you are using raw types. Parameterize it with the generic types like so.
HelloWorld.TreeNode<Integer> rootNode = new HelloWorld().new TreeNode<>(4);
This will fix the issue. If you don't supply a generic type parameter on the left-hand side, the List is declared as a raw type.