Explain pls this "list.add(list.remove(0));"

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Java Syntax, Level 7, Lesson 12

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I just have a question about this part of code

list.add(list.remove(0));

Can you pls explain the order of actions here? if remove(0) is performed first, how then add method knows what was in in the 0? It's basically self-explanatory, however, I didn't know we can add to the list and element which we are removing. Please provide some details. Thanks.

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Gellert Varga

Level 23 , Szekesfehervar, Hungary

1 April 2021, 20:53

I solved this task quite differently before. So the code and the question you shared was unusual for me too, I didn’t understand either. But Nouser's response helped me. And i looked at the ArrayList class and its methods in the Oracle API. The point: The list.remove(0) method doesn't just cause to remove the item. (But so far, I have thought that.) This remove() method also has a return value! It returns to you the removed element! If you try the following code, this will explain everything:

ArrayList<String> list = new ArrayList<>();
list.add("0.abc");
list.add("1.def");
list.add("2.ghi");
list.add("3.jkl");
String str = list.remove(0);
System.out.println(str);
System.out.println(list);

Output: 0.abc [1.def, 2.ghi, 3.jkl] So if you write this:

list.add(list.remove(0));

then it means the sum of these two codes: 1.) list.add(list.get(0)); 2.) list.remove(0);

Guadalupe Gagnon

Level 37 , Tampa, United States

1 April 2021, 21:04

IntelliJ will tell you the return values as well. Pay attention to the intelli-sense box that pops up when you are typing. I figured out a lot of things such as the return type of the .remove() method from IntelliJ:

Guadalupe Gagnon

Level 37 , Tampa, United States

1 April 2021, 21:09

It gives you a lot of information. For example:

In this pic I typed "Set" and it brought up a whole list of things. The icons on the left show what type the item is: Class Interface Abstract Class and it will show Methods Enumerations Exceptions Errors etc

Gellert Varga

Level 23 , Szekesfehervar, Hungary

1 April 2021, 21:12

Well that's very good!:) Now I'm starting to feel the drawback of not using IntelliJ...

Nouser

Level 36 , Germany

31 March 2021, 07:18

1. You may confuse how a list is working. It doesn't save objects but reference variables. So if you 'delete' an element, then you do not delete an object but you remove the reference to that object from the list. 2. remove returns the reference to the object. So after the remove method has been executed the add method got passed the return value of the remove method. And that's the reference to the object. The add method is able to add the reference again to the end of the list.