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!

Answer 1

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);