question: public BType addBuilder ( int index , MType message ) { ensureMutableMessageList ( ) ; ensureBuilders ( ) ; SingleFieldBuilder < MType , BType , IType > builder = new SingleFieldBuilder < MType , BType , IType > ( message , this , isClean ) ; messages . add ( index , null ) ; builders . add ( index , builder ) ; onChanged ( ) ; incrementModCounts ( ) ; return builder . getBuilder ( ) ; }
target:  Inserts a new builder at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).