Arraylist - using addition to existing index - add both new and old values

Question

I've got an ArrayList. The first index has a value of 10.

I want to add 10 to this index so that now it contains 20.

I tried:

ArrayListName.add(0, 10);

Which obviously overrides the field. How can I add in the 10 instead?

If this is a duplicate please refer me to the appropriate article

Many thanks.

Don't use ArrayListName as a variable name, java convention says a variable should start with lower case letter, not upper case (unless that's a final const, and then it should be all caps) — amit
– amit, Commented Feb 16, 2015 at 21:53
Amit I obviously would follow normal naming convention when using variables. I used the capitals for clarity but I see now it probably inhibits anyone else looking at this in the future. — Joseph Erickson
– Joseph Erickson, Commented Feb 16, 2015 at 22:00

Connorelsea · Accepted Answer · 2015-02-16 21:56:20Z

1

What you are doing adds a new element to the array, at index zero, containing value 10. The "add()" method adds an element to the array, and does not perform mathematic addition.

To add a number to an existing element you'd do

ArrayListName.set(0, ArrayListName.get(0) + 10);

This replaces the element at index 0, with a new value that has the value of the original content of index 0, plus 10.

edited Feb 16, 2015 at 21:56

answered Feb 16, 2015 at 21:51

Connorelsea

2,4987 gold badges30 silver badges52 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Oliver Charlesworth Over a year ago

add inserts an element; it doesn't replace it.

Connorelsea Over a year ago

Oops. I mistyped in confusion. I changed the code. It works now. If you could remove the downvote now that it works I'd be grateful. Thanks for pointing out my mistake!

Connorelsea Over a year ago

Oh. Well then! Hopefully whoever did sees that I remedied my mistake. Thanks for pointing it out.

Connorelsea Over a year ago

@JosephErickson Thank you. But you do know if it's the correct answer you're supposed to mark the check mark, right? If it's not correct, then thanks anyways!

Joseph Erickson Over a year ago

I did know. It gives a cool down from the moment its asked for like 10 minutes ish.

Collectives™ on Stack Overflow

Arraylist - using addition to existing index - add both new and old values

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related