This is a review question for my upcoming intro to programming exam and I need some help fixing my code. The question explicitly states to write a method that takes a List of integers and returns a new ArrayList in reverse order. It also says that we can assume we have methods prepend and append which add a new element to the beginning/end.

Here is my current code which gives me a NullPointerException

public static ArrayList<Integer> reverse(List<Integer> list){
	ArrayList<Integer> list2 = new ArrayList<Integer>();

	list.add(5);
	list.add(10);
	list.add(15);
	list.add(20);
	
	System.out.println(list);

	for(int i = 0, j = list.size()-1; i < j; i++){
		list2.add(i, list.remove(j));
	}
	
	return list2;
}
public static void main(String[] args){
	reverse(null);
}

Any help would be greatly appreciated. Thank you.

Edit: I am getting closer, 2 of the 4 objects are being reversed and displayed. More information is in the comments below.

Edit2: Thank you so much for all the help today guys, I understand this concept a lot better after reading through everyone's comments!

//code inside a reverse method that has the list object 
//passed as a parameter from main method
int size = list.size();
//create temporary list object to hold contents of original list in reverse
MyListReferenceBased temp = new MyListReferenceBased(); 
for (int i = 0; i < size; i++) 
{ 
//list add method requires which index your adding to, and object
//get method just gets the object at the specified index in list
temp.add(i, list.get((size-i)-1)); 
//remove method just removes object at specified index
list.remove((size-i)-1);                       
} 
//now that original list is empty, put the contents back in 
for (int i = 0; i<size; i++)
{ 
list.add(i, temp.get(i));            
} 
temp.removeAll(); 
//end result: original list is now reversed and temporary array is all empty 
System.out.println("List has been Reversed."); 
//is there a way to get rid of the 2nd loop?? Not possible to just assign?

So this code is in a reverse method inside the main class for reversing a linked list. Well reversing a list in general, since the user of the main class isn't suppose to interact with the linked list directly but just has access to the linked list methods (Didn't do reverse inside linked list class on purpose). Rather in the main/driver class, I'm reversing the list by first creating a second/temporary linked list, adding over each of the elements from the original list (in reverse) into the new list. And then since I need the contents to be in the original list and only need the temporary list during the duration of this method, I copy over the elements back into the old list.

The list object is instantiated as local in the main method of this main class and then calls the reverse method, passing the list object as a parameter. Instead of having the second loop, isn't there a way to just assign the temporary list object to the original list? I was able to do this when I the underlying implementation of the list was using arrays, but not sure how to it with linked lists.

Or any other efficient work around? Remember this is specifically for reversing a linked list without directly being inside the linked list class.