difference between < extends Comparable > and < extends Comparable < T > >?

Question

I tried to cast String to T type, but it says "Inconvertible types; cannot cast 'java.lang.String' to 'T'. However, when I remove "< T >" from < T extends Comparable< T > >, and fix that to < T extends Comparable >, it is okay. What is the difference? Below is the code:

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

public void createGraph(Scanner in) {
    String line;
    String[] elements;
    while (in.hasNextLine()) {
        line = in.nextLine();
        elements = line.split("\\s+");
        insertVertex((T)elements[0]); // << This is the part
    }

Answer 1

Your graph is of type T. That means I can have a graph of Persons, for example.

Then, in createGraph, you try to insert a String vertex in the graph. But String cannot be cast to a Person.

If you want a createGraph method that creates a Graph<String>, then you need it to make it static, like this:

public Graph<String> createGraph(Scanner in) {
    String line;
    String[] elements;
    Graph<String> graph = new Graph<>();
    while (in.hasNextLine()) {
        line = in.nextLine();
        elements = line.split("\\s+");
        graph.insertVertex(elements[0]);
    }
    // ...
    // Anything else, like adding edges
    // ...
    return graph;
}

EDIT: The reason your code compiles with no problem when you remove the <T> in Comparable<T> is that it becomes a graph of Comparables (instead of a graph of persons or a graph of strings), and then there's no problem in adding both a String and a Person to it.