在java8中对一组数据进行并行计算?

我有一组这样的数据:

Set<CustomObject> testSet = [{id: a1, qty: 3}, 
                             {id: a2, qty: 9},
                             {id: a3, qty: 5},
                             {id: a4, qty: 8},
                             {id: a5, qty: 12},
                             ...
                             {id: a200, qty: 7}];

ID分为3组,可使用以下方法找到:

//The getGroup method is implemented in the class CustomObject.
//I am using hazelcast map to store few id's that are inclusive, and
//one of the id that is in the request of the api is the current id.
public String getGroup(String id){
     HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
     if(id.equals(this.id)){
       return "currentId";
     }else if(id.equals(hazelcastInstance.getMap("idMap").get(id))){
       return "inclusive";
     } else {
       return "exclusive";
     }
}

上面的testSet包含大量数据,我想使用Java根据上面的分组方法执行Set中每个对象的数量总和。

我尝试使用流,但这不允许我使用Java8Streams的groupingBy方法中的getGroup方法。