question: public void add ( OffsettedItem item ) { throwIfPrepared ( ) ; try { if ( item . getAlignment ( ) > getAlignment ( ) ) { throw new IllegalArgumentException ( "incompatible item alignment" ) ; } } catch ( NullPointerException ex ) { throw new NullPointerException ( "item == null" ) ; } items . add ( item ) ; }
target:  Adds an item to this instance. This will in turn tell the given item that it has been added to this instance. It is invalid to add the same item to more than one instance, nor to add the same items multiple times to a single instance.