question: private void ensureBuilders ( ) { if ( this . builders == null ) { this . builders = new ArrayList < SingleFieldBuilder < MType , BType , IType > > ( messages . size ( ) ) ; for ( int i = 0 ; i < messages . size ( ) ; i ++ ) { builders . add ( null ) ; } } }
target:  Ensures that the list of builders is not null. If it's null, the list is created and initialized to be the same size as the messages list with null entries.