关于java：lambda表达式中的错误返回类型：BigDecimal无法转换为long

Bad return type in lambda expression: BigDecimal cannot be converted to long

我试图通过速度在Java流中编写查询。
当我在选择中尝试sum (l_extendedprice * (1 - l_discount))时，出现此错误：

Bad return type in lambda expression: BigDecimal cannot be converted to long. Operator '-' cannot be applied to 'int', 'java.math.BigDecimal'.

我的代码是这样的：

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JoinComponent joinComponent = app.getOrThrow(JoinComponent.class);
Join<Tuple6<Customer, Orders, Lineitem, Supplier, Nation, Region>> join = joinComponent
.from(CustomerManager.IDENTIFIER)
.innerJoinOn(Orders.O_CUSTKEY).equal(Customer.C_CUSTKEY)
.where(Orders.O_ORDERDATE.greaterOrEqual(sqlDate))
.where(Orders.O_ORDERDATE.lessThan(sqlDate2))
.innerJoinOn(Lineitem.L_ORDERKEY).equal(Orders.O_ORDERDATE)
.innerJoinOn(Supplier.S_SUPPKEY ).equal(Customer.C_NATIONKEY)
.innerJoinOn(Nation.N_NATIONKEY).equal(Supplier.S_NATIONKEY)
.innerJoinOn(Region.R_REGIONKEY).equal(Nation.N_REGIONKEY)
.where(Region.R_NAME.equal("ASIA"))
.build(Tuples::of);

Comparator<Tuple1<String>> comparator = Comparator
.comparing((Function<Tuple1<String>, String>) Tuple1::get0)
.thenComparing(Tuple1::get0);

Map<Tuple1<String>, LongSummaryStatistics> grouped = join.stream()
.collect(groupingBy(t -> Tuples.of(t.get4().getNName()),
() -> new TreeMap<>(comparator),
summarizingLong(t->t.get2().getLDiscount()*(1-t.get2().getLDiscount()))
));

我该如何解决？