Combine similar elements in a Java Stream

How to combine consecutive similar objects that are in a Java Stream?

Till now I could not find a good solution, so I hope you can help.

Suppose we have a class like the following:

class Someclass {
     String type;
     int count;
}

When having a Stream { “a”, 1 }, { “a”, 2}, { “b”, 4}, { “b”, “5” } and that needs to be processed as { “a”, 3 }, { “b”, 9 }.

Combining means of course, adding the counts for the objects with the same type.

How to do?

Answer

You can use Collectors.toMap to collect the stream into the desired Map.

Demo:

import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

class MyType {
    String type;
    int count;

    public MyType(String type, int count) {
        this.type = type;
        this.count = count;
    }

    public String getType() {
        return type;
    }

    public int getCount() {
        return count;
    }
}

public class Main {
    public static void main(String[] args) {
        List<MyType> list = List.of(new MyType("a", 1), 
                                    new MyType("a", 2), 
                                    new MyType("b", 4), 
                                    new MyType("b", 5));

        Map<String, Integer> map = list.stream()
                .collect(Collectors.toMap(MyType::getType, MyType::getCount, Integer::sum));

        System.out.println(map);
    }
}

Output:

{a=3, b=9}