Adding to an Integer already in an ArrayList

Question

I have an already existing ArrayList<Integer> and I'd like to add 1 to an Integer at a specific index. However, it gives me the error that "The left-hand side of an assignment must be a variable." It's something like this:

arrayListOfIntegers.get(i) += 1;

StriplingWarrior · Accepted Answer · 2014-09-10T20:58:59.333

10

The += operation is supposed to act on a variable--a local variable, a field, etc. And Integers are immutable, so you can't really change their value directly--5 will always be 5, and if you add 1 to it, you end up with a new number (6).

So you need to first "get" the value that is at the given index, and then "set" the value at that index to the new number that comes from adding one to the original value:

arrayListOfIntegers.set(i, arrayListOfIntegers.get(i) + 1);

edited Sep 10 '14 at 20:58

answered Sep 10 '14 at 20:53

StriplingWarrior

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Please add an explanation of why this change is necessary. – Bobulous Sep 10 '14 at 20:55
@Arkanon: I was working on it. Does this new explanation make sense? – StriplingWarrior Sep 10 '14 at 20:57
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@dfriend21 because `Integer` is immutable and, on top of that, [**Java is pass by value, never by reference**](http://stackoverflow.com/q/40480/1065197). – Luiggi Mendoza Sep 10 '14 at 21:01

score 0 · Answer 2 · edited Sep 10 '14 at 20:58

0

Your not assigning the value to anything you need to do

arrayListOfInteger.set(i, (arrayListOfIntegers.get(i) + 1));

edited Sep 10 '14 at 20:58

Luiggi Mendoza

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answered Sep 10 '14 at 20:56

brso05

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Adding to an Integer already in an ArrayList

2 Answers2