如何将具有相同键的值组合到hashMap中的一个值中？

I am tasked with the following problem: given an array of IngredientPortions (an object i made), if two of the elements in the IngredientPortions are the same Ingredient (Ingredient is another object that is a component of IngredientPortion objects), I am supposed to combine the IngredientPortion elements. I am tasked with the following problem: given a