How to create Java List of Type E extends Comparable<? super E>

Question

I am trying to implement the Quicksort method written in java on Rosettacode.org.

http://rosettacode.org/wiki/Sorting_algorithms/Quicksort#Java

However, I do not know how to add elements to a LinkedList of type E in order to use the method.

public static <E extends Comparable<? super E>> void main(String[] args) {

        List<E> list = new LinkedList<E>();

        list.add(1);
}

I get the following error when I try to compile:

QuickSort.java:12: error: no suitable method found for add(int)         list.add(1);
            ^
    method List.add(int,E) is not applicable
      (actual and formal argument lists differ in length)
    method List.add(E) is not applicable
      (actual argument int cannot be converted to E by method invocation conversion)
    method Collection.add(E) is not applicable
      (actual argument int cannot be converted to E by method invocation conversion)   where E is a type-variable:
    E extends Comparable<? super E> declared in method <E>main(String[]) 1 error make: *** [c] Error 1

Answer 1

There are tons of issues here.

First, why are you declaring your main method as static<E extends Comparable<? super E>> static<E extends Comparable<? super E>> .

Secondly, you have bounded the list to a generic type E but have not specified what E is. Thus, to add an int into a list that has no specific type will cause a conversion issue by the compiler. Also, an int is a primitive type, it doesn't follow, even when it's autoboxed to java.lang.Integer , it doesn't satisfy the constraint since E is not specific/specified.

I hope this helps.

---

Update:

Based on the link you provided, this is how you will to use the quickSort() function.

List<Integer> intList = new ArrayList<>(); //If using JDK 7 and higher.

OR

List<Integer> intList = new ArrayList<Integer>(); //If using JDK 6 and JDK 5.

Now...

//Add all your items in the list.
intList.add(1);
intList.add(50);
intList.add(10);
intList.add(8);
intList.add(-24);
//...etc.

//Sort,
intList = quickSort(intList);

Since E is bounded to an object that is Comparable , it will accept any list that conforms to that bounding.

Answer 2

In order to add Integer ,You should change

 List<E> list = new LinkedList<E>();

to

List<Integer> list = new LinkedList<Integer>();

The reason why you are getting error is due to the fact that when you declared List<E> then all you can add into list is Element of type E and Integer is not E