如何使用Java Stream API从Map中提取特定实例变量的值？

I have a HashMap, for each key the value is a User Defined Object. This user defined object has variable of type ArrayList. How can I extract this ArrayList into one List using Java stream API. So, the size of my HashMap is 3. Size of the array list inside each user defined object is 1. Final out should be an arraylist of size 3I have a HashMap, for each key the value is a U