如何在Java集合上执行Groupby，Sum，Avg，Min，Max？

Question

I have been trying to build a functionality which will enable me filter data in Java collections. The end goal is to have somethings as follow.

SomeBuilder(Collections)
.groupby("Key")
.filter("Key","Value")
.sum()
.getResult() ;

The functionality mostly resembles to sql world but not fully. My problem is how to filter collections like List<HashMap<String,Object> ?

How can I apply groupby logic on such collections ?

A snippet of code showing filtering of such collection will help a lot. If Guava can do this please give an example.

Edit:- I have added an answer for this question but please suggest better ways of doing it. Hope this help others. For doing Sum ,Avg ,Count will update as I go ahead with my project.

Answer 1

Ok This is to answer my own question and which may help others. I will be using the builder pattern to get the effect of method chaining. For filtering of

List<HashMap<String,Object>>

, I used Guava, here is an example of what I tried to do. Not the best way but a starter.

  private static Collection<HashMap<String, Object>>  filter(
        List<HashMap<String, Object>> resMap, final String key,
        final Object value) {
    Predicate<Map<String, Object>> keyValueMatch = new Predicate<Map<String, Object>>() {
        public boolean apply(Map<String, Object> stringStringMap) {
            return value.equals( stringStringMap.get(key));
        }
    };

    Collection<HashMap<String, Object>>  transformedList = Collections2.filter(resMap, keyValueMatch);
    return transformedList;
}

The usage will be as follow.

 Collection<HashMap<String, Object>>  transformedList =  filter(input,"Dep","GlobalWarming");

Hope this help. Any better suggestions are welcome.