Adding Elements to An ArrayList

// adding an element to the middle of a list
arrayList.add( i, object );

// adding an element to the end of a list
arrayList.add( object );

ArrayIndexOutOfBoundsException

• Using a negative index.
• Indexing past the current end of the ArrayList with get or set. It will only automatically grow with add, and it will grow only one element at a time. It won’t let you even use set one element past the end. setCapacity is of no help. It only increases the size of the internal array, not the logical array.
• Doing a lastIndexOf starting out past the end.

GrowingArrayList

With an ordinary ArrayList you can’t set elements until you have grown the ArrayList sufficiently first with add. If you try it, you will get an ArrayIndexOutOfBoundsException or IndexOutOfBoundsException.

If you want an ArrayList that grows automatically when you use a set index too big, here is the complete source code for it.

Removing Elements From An ArrayList

Trimming ArrayLists

When I am absolutely sure there are no new elements coming for the ArrayList, I typically use toArray and use the much faster raw array from then on. I think of ArrayLists mainly as tools to build a Collections, and I think of arrays as tools to process them. However, most other programmers are not nearly so keen as I am on naked arrays. They prefer to do everything with ArrayLists. when you use generics and for:each it is relatively easy to flip back and forth between each style.

ArrayList.add or ArrayList. addAll will not discard duplicates. To do that you would need to check with ArrayList.contains before adding, or create a HashSet which naturally dedups, then use ArrayList.addAll to effectively convert it to an ArrayList.

ArrayList ⇔ array

Thread Safety of ArrayLists

// making an ArrayList threadsafe
ArrayList a = new ArrayList( 100 );
Collection threadSafeList = Collections.synchronizedCollection( a );

Generics with ArrayLists

Learning More