Java ArrayList OutofBounds when adding an item

Question

I have this array:

ArrayList<Problem> problems = new ArrayList <Problem>( 100 );

then I try to make an object to put in it:

Problem p = new Problem ();
p.setProblemName( "Some text" );

Then I try to add the object to the array:

problems.set(1, p);

But at this point the system throws a runtime exception:

03-12 18:58:04.573: E/AndroidRuntime(813): Caused by: java.lang.IndexOutOfBoundsException: Invalid index 1, size is 0

But if I increase the initial size of the array to 100. Why does this error happen? It seems this is super straight forward.

Thanks!

twain249 · Accepted Answer · 2012-03-12 19:18:25Z

2

You don't use set to add to an ArrayList you use it to overwrite an existing element.

problems.set(1, p); //Overwrite the element at position 1

You use add

problems.add(p);

will add it at the end

problems.add(1, p);

will add it at index 1, this will throw an IndexOutOfBoundsException for
index < 0 or index > ArrayList.size(). Which will be the case on the first attempt for an add.

Also just for your knowledge

problems.add(ArrayList.size(), p); //Works the same as problems.add(p);

edited Mar 12, 2012 at 19:18

answered Mar 12, 2012 at 19:03

twain249

5,6861 gold badge23 silver badges26 bronze badges

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

Comments

Adrian · Accepted Answer · 2012-03-12 22:34:07Z

2

ArrayList#set()

Throws: IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size())

size() returns number of elements in the array list, not the capacity.

edited Mar 12, 2012 at 22:34

Adrian

5,6808 gold badges57 silver badges90 bronze badges

answered Mar 12, 2012 at 19:02

vladimir e.

7333 silver badges7 bronze badges

Comments

assylias · Accepted Answer · 2012-03-12 19:09:11Z

1

When you write ArrayList<Problem> problems = new ArrayList <Problem>( 100 );, you only tell Java that you think you are going to use that kind of capacity (which optimises the size of the underlying array) but the list still has a size of 0.

You need to use add():

problems.add(p);

will add p in the first position.

List<Problem> problems = new ArrayList <Problem>();
Problem p = new Problem ();
p.setProblemName( "Some text" );

problems.add(p);

Problem p2 = problems.get(0); //p2 == p

edited Mar 12, 2012 at 19:09

answered Mar 12, 2012 at 19:03

assylias

330k84 gold badges680 silver badges806 bronze badges

Comments

Kayhan Asghari · Accepted Answer · 2012-03-12 19:21:06Z

0

you should write this instead:

problems.add(0, p);

You haven't any 0th member that you want to insert p into 1st place!

edited Mar 12, 2012 at 19:21

answered Mar 12, 2012 at 19:03

Kayhan Asghari

2,8641 gold badge30 silver badges47 bronze badges

1 Comment

twain249 Over a year ago

Set won't work. The ArrayList has no elements yet so Size = 0. Set throws and IndexOfOutBoundsException if the index is >= size.

Joe R. · Accepted Answer · 2012-03-12 19:13:08Z

-1

try

problems.set(0, p);

the first position in an array is always 0.

edit you should be using the .add() method to add an object to the array though

edited Mar 12, 2012 at 19:13

answered Mar 12, 2012 at 19:04

Joe R.

12311 bronze badges

2 Comments

GeekedOut Over a year ago

when I set it to 0, I get this error: java.lang.IndexOutOfBoundsException: Invalid index 0, size is 0

twain249 Over a year ago

Set will not work if there isn't an element in the list already. He has to use add. Set fails in index < 0 || index >= size. With a size=0 set always fails.

Collectives™ on Stack Overflow

Java ArrayList OutofBounds when adding an item

5 Answers 5

Comments

Comments

Comments

1 Comment

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

Comments

Comments

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related