list.stream() 
.collect(Collectors.collectingAndThen(
    Collectors.groupingBy(A::getId, Collector.of( 
        () -> new A<>(null, 0d, null), (acc, a) -> { 
            acc.setId(a.getId()); 
            acc.setQuant(Double.sum(acc.getQuant(), a.getQuant())); 
            acc.setType(a.getType()); 
        }, (a1, a2) -> { 
            a1.setQuant(Double.sum(a1.getQuant(), a2.getQuant())); 
            return a1; 
        }, a -> new B<>(a.getId(), new NewClass(a.getQuant(), a.getType()) )
    )), Map::values 
  )) 
        
        // Set .forEach(System.out::println);