如何使用lamda表达式对list进行求和

//根据id查询库存
List<ProductStock> list = productStockMapper.selectList(Wrappers.<ProductStock>lambdaQuery().in(Product::id, idList))
//查询所有库存总和
Integer qtySum = list.stream().mapToInt(ProductStock::getStockQty).sum();

//查询当前所有商品库存
List<Integer> stockList = productStockMapper.selectStock();
//查询所有库存总和
Integer stockSum = stockList .stream().mapToInt(Integer::intValue).sum();

//根据id查询库存
List<ProductStock> list = productStockMapper.selectList(Wrappers.<ProductStock>lambdaQuery().in(Product::id, idList))
//查询所有库存金额总和
BigDecimal stockAmt = list .stream().map(ProductStock::getStockAmt).reduce(BigDecimal::add).orElse(Bigdecimal.ZERO);

//查询当前所有商品库存
List<Bigdecimal> stockList = productStockMapper.selectStock();
//查询所有库存金额总和
Bigdecimal stockAmt = stockList.stream().reduce(BigDecimal::add).orElse(Bigdecimal.ZERO);

list.RemoveAll(e=>e==400);

list.RemoveAll(e=>e%100==0);

int sum=0;
int result = list.ForEach(val=>sum+=val);

bool result = list.Exists(e=>e==400)

bool result = list.TrueForAll(e=>e==400)

var result = list.Find(e=>e%100==0);

var result = list.FindLast(e=>e%100==0);

var result = list.FindAll(e=>e%100==0);

int result = list.FindIndex(e=>e%100==0);

int result = list.FindLastIndex(e=>e%100==0);

list.Sort();//二分查找前先必须先升序排序
int result = list.BinarySearch(e=>e%100==0);

internal class Book : IComparable<Book>
{
    public int ID { get; set; }
    public string Name { get; set; }
    public double Price { get; set; }
    public int CompareTo([AllowNull] Book other)
    {
        if (other == null) return 1;
        return this.ID - other.ID;//返回正数，this>other；返回0，this=other；返回负数，this<other；
    }
    public override string ToString()
    {
        return JsonSerializer.Serialize(this);
    }
    public override bool Equals(object obj)
    {
        if (obj == null) return false;
        Book other = obj as Book;
        if (other.ID == this.ID && other.Name == this.Name && other.Price == this.Price) return true;
        return false;
    }
}