How do I group elements in a list

In this Q&A, we'll go over functions provided by Java stream classes that allow a collection of elements to be grouped

Collectors class provide groupingBy methods that allow a list of elements to be grouped by a key.  It provides options to group elements into List or Set values.  Using groupingBy method we can also get sum, average or count for a key.

See demo code below

package com.javahowdoi.stream;

import java.util.*;

import java.util.stream.Collectors;

import static junit.framework.TestCase.assertEquals;

public class Group

{

    private static class Inner {

        public int i;

        public String s;

        public static Integer getI(Inner in) {return in.i;}

        @Override

        public boolean equals(Object other) {

            if(!(other instanceof Inner))

                return false;

            Inner i2 = (Inner) other;

            return this.i == i2.i ;

        }

        @Override

        public int hashCode() {

            return this.i;

        }

    }

    public static void main(String[] args ) {

        List<Integer> l = Arrays.asList(1, 2, 3, 3, 4, 5,5, 6);

        List<Inner> li = new ArrayList<>();

        for(int i=0; i < 5;  ++i) {

            Inner in = new Inner();

            in.i = i; in.s = String.format("%d", i);

            li.add(in); li.add(in);

        }

        assertEquals("map size match", 10, li.size() );

        Map<Integer, List<Inner>> m = li.stream().collect(Collectors.groupingBy(Inner::getI));

        assertEquals("map size match", 5, m.size() );

        // assert each sub list is of size 2

        for(int i=0; i < 5;  ++i) {

            List<Inner> lg = m.get(i);

            assertEquals("Set size match", 2, lg.size() );

        }

       Map<Integer, Set<Inner>> m1 = li.stream().collect(Collectors.groupingBy(Inner::getI, Collectors.toSet()));

        assertEquals("map size match", 5, m1.size() );

        // assert each sub set is of size 1

        for(int i=0; i < 5;  ++i) {

            Set<Inner> s = m1.get(i);

            assertEquals("Set size match", 1, s.size() );

        }

        Map<Integer, Integer> m2 = li.stream().collect(Collectors.groupingBy(Inner::getI, Collectors.summingInt(Inner::getI)));
        for(int i=0; i < 5;  ++i) {
            int sum = m2.get(i);
            // assert sum for a given key
            assertEquals("Set size match", i * 2, sum );

        }
    }

}