When I test my add method for the ArrayStringList with one element, it shows $2.$ For example, when I add$(0,"$a$"),$ the list looks like $[$a$,$ a$].$ What could be the bug? The javadoc API says: Inserts an item into this string list at the specified index position. If an item was already at that position, then that item and subsequent items are shifted to the right $($i$.$e$.,$ one is added to their indices$).$ It throws a NullPointerException $-$ if item is null, an IllegalArgumentException $-$ if item is empty, and an IndexOutOfBoundsException $-$ if index is out of range $($index $<mtext\; id="EditorElement\_320371"></mtext><mn\; id="EditorElement\_320372">0</mn><mtext\; id="EditorElement\_320373"></mtext><mi\; id="EditorElement\_320374">|</mi><mi\; id="EditorElement\_320375">|</mi>$ index $>$ size$())$