What is going on? new ArrayList( 10 ) throws Exception when add( 2, "something" )

[Dan Bizman]

I created the below ArrayList (java 1.4.2_03) and it's throwing an out of bounds exception!

ArrayList a = new ArrayList( 34 );
 
//EVEN WHEN I USE THIS LINE, IT STILL THROWS THE EXCEPTION
//a.ensureCapacity( 34 );
a.add( 10, "something" );

What could I be doing wrong? Is there a bug in 1.4.2_03?

[Nathaniel Stoddard]

Capacity and size are two different things. When you supply and index for a set(int, Object), the int has to be within the range of 0 to the size. Despite setting the capacity to 34, the size is still 0, and therefore you get an exception thrown.

[Ilja Preuss]

To be more specific, the size is the number of elements currently in the list, the capacity is the number of elements the list can hold before the internally used array needs to be resized. Setting the capacity is a pure performance optimization.