stream流之去重求和

根据商品id去重，并对同一商品id的商品数量求和，取集合

List<ProductDO> collect = productList.stream()
        .collect(Collectors.toMap(ProductDO::getProductId, a -> a, (o1, o2) -> {
                    o1.setNum(o1.getNum() + o2.getNum());
                    return o1;
                })
        ).values().stream().collect(Collectors.toList());