Array not populating: Incorrect use of add method?

Question

In my for-loop below I am trying to add all car IDS associated to a persons with a certain status.

My code compiles and runs, however the size of arrayList: listOfCarIds is remaining 0. I.e. nothing seems to be added

Is there a mistake in how I am implementing this?

for(int i=0 ; i < personsWithStatus.size() ; i++)
{
  idOfCarRelatingToPerson = personsWithStatus.get(i).getCarId();

  List<String> listOfCarIds = new ArrayList();

  listOfCarIds.add(idOfCarRelatingToPerson);
}

You are creating a new listOfCarIds in each iteration and discarding it. Perhaps you intended to initalise the field not the local variable before the loop. — Peter Lawrey
– Peter Lawrey, Commented Feb 26, 2016 at 14:13

Pragnani Kinnera · Accepted Answer · 2016-02-26 14:21:22Z

6

Declare your ArrayList outside as the ArrayList you have declared inside the for loop will initialize every time and has the scope within the for loop only.

  List<String> listOfCarIds = new ArrayList();
for(int i=0 ; i < personsWithStatus.size() ; i++) {
       idOfCarRelatingToPerson = personsWithStatus.get(i).getCarId();
       listOfCarIds.add(idOfCarRelatingToPerson);
            }

edited Feb 26, 2016 at 14:21

answered Feb 26, 2016 at 14:06

Pragnani Kinnera

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3 Comments

java123999 Over a year ago

Why is this the case?

Michael Over a year ago

@java123999 Every time you are looping through the list in your code, you are declaring a new list and adding that item to it. After the loop completes, the list has gone out of scope and in inaccessible.

Pragnani Kinnera Over a year ago

@java123999 I have added a more description, to make you understand

David Glickman · Accepted Answer · 2016-02-26 14:08:26Z

1

You are declaring your ArrayList in the loop. Each time it goes through the loop it resets listOfCarIDs to a blank ArrayList.

Declare the ArrayList outside the loop, and only use the loop to add to it.

answered Feb 26, 2016 at 14:08

David Glickman

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