How does line 26 work?

Question about the task

Under discussion

I'm confused as to how that line works. Wouldn't the list.remove delete the first string and then make it impossible to move that string to the end? Shouldn't it be " list.remove(list.add(0)); " instead?

package com.codegym.task.task07.task0720; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.ArrayList; /* Shuffled just in time */ public class Solution { public static void main(String[] args) throws IOException { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); int N = Integer.parseInt(reader.readLine()); int M = Integer.parseInt(reader.readLine()); ArrayList<String> list = new ArrayList<String>(); for (int i = 0; i < N; i++) { String text = reader.readLine(); list.add(text); } for (int i = 0; i < M; i++) { list.add(list.remove(0)); } for (String s : list) { System.out.println(s); } } }

Comments (4)

Popular
New
Old

You must be signed in to leave a comment

Nouser

Level 36 , Germany

10 December 2020, 06:59

from the java docs: Removes the element at the specified position in this list (optional operation). Shifts any subsequent elements to the left (subtracts one from their indices). Returns the element that was removed from the list. So remove returns the element it deleted. For easier understanding you could write it that way

String removedElement = list.remove(0);
list.add(removedElement);

The Chief

Level 18 , United States

13 December 2020, 05:14

So from my understanding, the remove operation deletes the element but you can still access the removed element afterwards? Also, why won't "list.remove(list.add(0));" work? Is it because the add operation can't be used with only an index number?

Nouser

Level 36 , Germany

13 December 2020, 07:52

No, the remove method removes the element from the list but it has a return value. And this return value is of the type the list elements are and in that return value you'll find the deleted element. If you just call

list.remove(0);

the return value is saved nowhere and therefore lost.

String removedElement = list.remove(0);

Here you save the removed element in a String named removedElement. However you have deleted the element from the list. You may have noticed, that we accessed the list element using an index. The remove method has an overloaded version taking an Object (of the type you declared the list). So if you have a String list and an element "test", remove("test"); will remove that element. Here the return value is not the removed element (you already know it) but a boolean value indicating if removing the element succeded. The add method takes an element (of a type you set the list to) and adds it to the list. Or, the overloaded method takes an index (int) and the element. So when you try to add(0); that will fail, cause 0 is an int and not of type String. What you can try is to

boolean result = list.remove(list.add("test"));

but result will be false. Why is that so? add() also hase a return value, a boolean if adding "test" succeeded. So the result here would be a boolean true. In the end your call would look similar to:

boolean result = list.remove(true);

and true is of the wrong type and of courese not in the list. It is always a good idea to know what the methods you use do. Oracle has a good documentation. You will need to get used to it first. More about List

The Chief

Level 18 , United States

13 December 2020, 21:05

Okay, I understand it better now.