question: public BType addBuilder ( MType message ) { ensureMutableMessageList ( ) ; ensureBuilders ( ) ; SingleFieldBuilder < MType , BType , IType > builder = new SingleFieldBuilder < MType , BType , IType > ( message , this , isClean ) ; messages . add ( null ) ; builders . add ( builder ) ; onChanged ( ) ; incrementModCounts ( ) ; return builder . getBuilder ( ) ; }
target:  Appends a new builder to the end of this list and returns the builder.