Java 8 : how to extract a HashMap from a ArrayList of HashMap in Java 8 using streams?

I have the following sample result when I query a database :

dataList  = [{ name : name1, rollno: rollno1 }, { name : name2, rollno: rollno2 } ]

I want to convert this list of hashmaps into a single hashmap using Java 8 streams.
I tried using Collectors.toMap() but i am not sure how to refer to rollNo as key and the hashmap as the value inside the toMap method.

Exected output :

{ rollno1 : { name : name1, rollno: rollno1 } , rollno2 : { name : name2, rollno: rollno2 } }

I tried doing it using a for each loop on the list and then adding the rollno as key to a hashmap and the hashmap as value itself of that rollno.

    HashMap<String,HashMap<String,String>> newMap = new HashMap();

    for(HashMap<String,String> record : dataList){
        String key = record.get("rollno").toString();
        newMap.put(key,record);
    }

Is there a way to refactor this code using functional streams in Java 8?
Will using streams collect method give any performance advantage over the foreach for doing this?

Will appreciate any leads. Thanks

As I don't know the type of Object you are using, so I am performing this on String. But it is valid for any type of object.

 HashMap result = (HashMap<String, HashMap<String, String>>) listOfHashMaps.stream()
               .collect(Collectors.toMap(e-> e.get("roll"),e->e));

As type casting will help you to achieve this.

Here's complete example how to do it

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.stream.Collectors;

public class Main {

    public static void main(String[] args) {
        List<HashMap<String, String>> input = new ArrayList<>();
        HashMap<String, String> subinput1 = new HashMap<>();
        subinput1.put("name", "name1");
        subinput1.put("rollno", "rollno1");
        input.add(subinput1);
        HashMap<String, String> subinput2 = new HashMap<>();
        subinput2.put("name", "name2");
        subinput2.put("rollno", "rollno2");
        input.add(subinput2);
        HashMap<String, HashMap<String, String>> result = (HashMap<String, HashMap<String, String>>) input.stream()
                .collect(Collectors.toMap(v -> (String) v.get("rollno"), e -> e));
        System.out.println(result);
    }
}

It iterates over a collection of HashMaps, takes the key in which it should be stored in the result HashMap, then it creates a Map of Maps where the key is the "rollno" from the input map, and value is the input map itself.

You can use Collectors.toMap with Function.identity() as below,

list.stream()
        .collect(toMap(e->e.get("rollno"), Function.identity()));

public static void main(String[] args) {
    List<HashMap<String, String>> input = new ArrayList<>();
      
    HashMap<String, String> subinput1 = new HashMap<>();
    subinput1.put("name", "name1");
    subinput1.put("rollno", "rollno1");
    input.add(subinput1);

    HashMap<String, String> subinput2 = new HashMap<>();
    subinput2.put("name", "name2");
    subinput2.put("rollno", "rollno2");
    input.add(subinput2);
      
    //Test key conflict
    HashMap<String, String> subinput3 = new HashMap<>();
    subinput2.put("name", "name3");
    subinput2.put("rollno", "rollno2");
    input.add(subinput2);

    System.out.println("input:"+ JSONObject.toJSONString(input));

    HashMap<String, HashMap<String, String>> result = (HashMap<String, HashMap<String, String>>) 
       input.stream()
            .collect(Collectors.toMap(
                v -> (String) v.get("rollno"), 
                Function.identity(),(oldValue, newValue) -> newValue
            ));
    //fastjson  hashmap-toString use =
    System.out.println(JSONObject.toJSONString(result));
}

Probably something like this:

Map<RollNo, List<Roll>> rollsPerType = rolls.stream()
  .collect(groupingBy(Roll::getRollNo));

where Roll is the main object and RollNo is the property inside (since you didn't provide the actual definitions).