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Example of Java 8 Streams groupingBy feature

Statement: Let’s say you have a list of integers which you want to group into even and odd numbers.

Create a list of integers with four values 1,2,3 and 4:

List<Integer> numbers = new ArrayList<>();

Now group the list into odd and even numbers:

Map<String, List<Integer>> numberGroups= 
numbers.stream().collect(Collectors.groupingBy(i -> i%2 != 0 ? "ODD" : "EVEN"));

This returns a map of (“ODD/EVEN” -> numbers).

Printing the segregated list along with its offset (ODD/EVEN):

for (String offset : numberGroups.keySet()) {
  for (Integer i : numberGroups.get(offset)) {
    System.out.println(offset +":"+i);



Refer Github for complete program.

Why Java 8 ?

In simple words java 8 allows us to write code more precisely and concisely, which is better than writing verbose code in the java versions prior to java 8.

Example: Let’s sort a collection of cars based on their speed.

Java versions prior to java 8 :

Collections.sort(fleet, new Comparator() {
  public int compare (Car c1, Car c2) { 
  return c1.getSpeed().compareTo(c2.getSpeed());

Instead of writing a verbose code like above, using java 8 we can write the same code as:

Java 8 :


The above code is more concise and could be read as “sort fleet comparing Car’s speed”.

So why write a boilerplate code which is not related to the problem statement. Instead you can write concise code which is related to the problem statement and has SQL like readability.