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Welcome back to my blog.

In my last article I shared list of questions and in this article I'm going to share answers for Input - 1 questions. In subsequent articles I will share answers for Input - 2 questions.

Questions link : Most Frequently asked Java Streams coding questions - Part 1

Input - 1

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

List<Integer> list = Arrays.asList(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

sum is: 405

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 list of integers.

double avg = list.stream().mapToInt(a->a).average().orElse(0);
System.out.println("average is: "+avg);

output

average is: 33.75

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)

double avg1 = list.stream().map(a->a*a).filter(a -> a>100).mapToInt(a->a).average().orElse(0);
System.out.println("average is: "+avg1);

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

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

output

average is: 14310.0

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:

List<Integer> evens = list.stream().filter(num->num%2==0).collect(Collectors.toList());
System.out.println("evens: "+ evens);

output

evens: [8, 2, 36, 4, 78, 222, 24]

Odd Numbers

List<Integer> odds = list.stream().filter(num -> num%2!=0).collect(Collectors.toList());
System.out.println("odds: "+ odds);

output

odds: [1, 7, 9, 5, 9]

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

List<Integer> startsWith2 = list.stream().map(num -> String.valueOf(num)).filter(n -> n.startsWith("2")).map(Integer::valueOf).collect(Collectors.toList());
System.out.println("startsWith2: "+startsWith2);

output

startsWith2: [2, 222, 24]

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.

Set<Integer> duplicates = list.stream().filter(num -> Collections.frequency(list,num) > 1).collect(Collectors.toSet());
System.out.println("duplicates: "+duplicates);

output

duplicates: [9]

Approach 2: Using Set to collect only duplicates.

Set<Integer> duplicates1 = new HashSet<>();
Set<Integer> dup = list.stream().filter(num -> !duplicates1.add(num)).collect(Collectors.toSet());
System.out.println("duplicates: "+dup);

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:

int max = list.stream().max(Comparator.comparing(Integer::valueOf)).get();
System.out.println("Maximum Value: "+max);

output

Maximum Value: 222

Minimum Value:

int min = list.stream().min(Comparator.comparing(Integer::valueOf)).get();
System.out.println("Minimum Value: "+min);

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

List<Integer> asc_order = list.stream().sorted().collect(Collectors.toList());
System.out.println("ASC Order: "+asc_order);

output

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

DESC Order

List<Integer> desc_order = list.stream().sorted(Collections.reverseOrder()).collect(Collectors.toList());
System.out.println("DESC Order: "+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

List<Integer> first5elements = list.stream().limit(5).collect(Collectors.toList());
System.out.println("first5elements: "+first5elements);

output

first5elements: [1, 7, 8, 9, 5]

First 5 Elements Sum

int first5sum = list.stream().limit(5).reduce((a,b) -> a+b).get();
System.out.println("first5elementsSum: "+first5sum);

output

first5elementsSum: 30

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

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

int sum1 = list.stream().skip(5).reduce((a,b) -> a+b).get();
System.out.println("Sum after first 5 elements skip: "+sum1);

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]

Note: Keep in mind about Integer overflow.

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Thanks for reading :) Happy coding :)