Most Frequently asked Java Streams coding and answers

Input - 1

Assume you are given a list of numbers like {1,7,8,9,5,2,36,4,78,222,24,9}

Answers

1) Given a list of numbers, return the sum of all numbers.

Approach: We can use reduce() method whenever we want to produce a single resultant value as output, for example, maximum, minimum, sum, product, etc.

You can refer Stream.reduce for more details.

Optional<Integer> sum = list.stream().reduce((a, b) -> a+b);
System.out.println("sum is: "+sum.get());        

output

2) Given a list of numbers, return the average of all numbers

Approach: We can use mapToInt() followed by average() whenever we want to perform an average on the list of integers.

output

3) Given a list of numbers, square them and filter the numbers which are greater 100 and then find the average of them

Approach: Here we need to do 3 things

• We need to square each number ( We can use map())
• Filter whose value is greater than 100 (we can use filter())
• Find average of those (we can use mapToInt() and average() together)

If we look at the numbers, only 36, 78, 222, 24 squares are greater than 100.

• (1296 + 6084 + 49284 + 576)/4 = 14310.0

output

4) Given a list of numbers, return the even and odd numbers separately

Approach: We can use filter() and Collectors.toList() to get both even and odd numbers as two separate lists.

Even Numbers:

output

Odd Numbers

output

5) Given a list of numbers, find out all the numbers starting with 2

Approach: Here we need to do 4 things

• Convert Integer to String to perform startsWith operation on it
• Filter the strings that starts with 2
• Convert String to Integer on filtered data
• Collect the final Integers as List and store

output

6) Given a list of numbers, print the duplicate numbers

Approach 1: Using frequency() method of Collections class. It counts the frequency of the specified element in the given list. If count > 1 then that element is duplicate one

output

duplicates: [9]        

Approach 2: Using Set to collect only duplicates.

output

duplicates: [9]        

7) Given a list of numbers, print the maximum and minimum values

Approach: Using max() and min() we can get maximum and minimum values from a list along with Comparator.comparing().

Maximum Value:

output

Maximum Value: 222        

Minimum Value:

output

Minimum Value: 1        

8) Given a list of numbers, sort them in ASC and DESC order and print

Approach: Using sorted(), We can sort a list in ASC or DESC order.

ASC Order

output

ASC Order: [1, 2, 4, 5, 7, 8, 9, 9, 24, 36, 78, 222]        

DESC Order

output

DESC Order: [222, 78, 36, 24, 9, 9, 8, 7, 5, 4, 2, 1]        

9) Given a list of numbers, return the first 5 elements and their sum

Approach: We can use limit() followed by reduce().

First 5 Elements

output

First 5 Elements Sum

output

first5elementsSum: 30        

10) Given a list of numbers, skip the first 5 numbers and return the sum of the remaining numbers

Approach: We can use skip() to skip the first n numbers in a list.

output

Sum after first 5 elements skip: 375        

11) Given a list of numbers, return the cube of each number

Approach: We can use map() here.

List<Integer> cubes = list.stream().map(num -> num*num*num).collect(Collectors.toList());
System.out.println("Cubes: "+cubes);        

output

Cubes: [1, 343, 512, 729, 125, 8, 46656, 64, 474552, 10941048, 13824, 729]        

Thanks To: shivaprasadgurram.hashnode.dev/