Top Java 8 Stream Coding Interview Questions for Experienced

Top 8 Java 8 Stream API Coding Interview Questions & Solutions

Java 8 Stream API is one of the most frequently asked topics during interviews for experienced Java developers. In this article, we'll solve some of the most commonly asked Java 8 coding interview questions with clean and optimized solutions.

Question 1

1. Increase Employee Salary if Salary is Less than ₹20,000

Problem Statement

Given a list of employees, increase the salary by ₹10,000 for employees whose salary is less than ₹20,000 and print the updated employee list using Java 8 Streams.

Java Solution

Java 8

import java.util.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        List<Employee> employees = Arrays.asList(
            new Employee("John", 15000),
            new Employee("David", 28000),
            new Employee("Steve", 18000),
            new Employee("Alex", 35000)
        );

        employees.stream()
            .map(emp -> {
                if (emp.getSalary() < 20000) {
                    emp.setSalary(emp.getSalary() + 10000);
                }
                return emp;
            })
            .forEach(System.out::println);
    }
}

Output

John -> 25000.0
David -> 28000.0
Steve -> 28000.0
Alex -> 35000.0

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Question 2

2. Remove Special Characters and Count Frequency of Each Word

Problem Statement

Given the following string, remove all special characters and print the frequency of each word using Java 8 Streams.

Input

String data = "java@ #is &an ++Object";

Java Solution

Java 8

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        String data = "java@ #is &an ++Object";

        String cleaned = data.replaceAll("[^a-zA-Z ]", "");

        Map<String, Long> frequency =
            Arrays.stream(cleaned.split("\\s+"))
                .filter(word -> !word.isEmpty())
                .collect(Collectors.groupingBy(
                    Function.identity(),
                    Collectors.counting()
                ));

        frequency.forEach((k, v) ->
            System.out.println(k + " -> " + v));
    }
}

Output

java -> 1
is -> 1
an -> 1
Object -> 1

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Question 3

3. Print Employees Whose Salary is Greater Than Their Manager

Problem Statement

Given a list of employees and their respective managers, print the employees whose salary is greater than their manager using Java 8 Streams.

Java Solution

Java 8

import java.util.*;

class Employee {
    String name;
    double salary;
    Employee manager;

    Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    void setManager(Employee manager) {
        this.manager = manager;
    }
}

public class Main {
    public static void main(String[] args) {
        Employee manager = new Employee("John", 50000);

        Employee emp1 = new Employee("David", 60000);
        Employee emp2 = new Employee("Steve", 45000);

        emp1.setManager(manager);
        emp2.setManager(manager);

        List<Employee> employees = Arrays.asList(emp1, emp2);

        employees.stream()
            .filter(e -> e.salary > e.manager.salary)
            .forEach(e -> System.out.println(e.name));
    }
}

Output

David

Explanation

The filter() operation compares each employee's salary with their manager's salary and prints only those employees whose salary is higher.

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Question 4

4. Group Anagrams from the List

Problem Statement

Group all the anagrams together using Java 8 Streams.

Input

eat, tea, tan, ate, nat, bat

Java Solution

Java 8

import java.util.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        List<String> words = Arrays.asList("eat", "tea", "tan", "ate", "nat", "bat");

        Map<String, List<String>> result =
            words.stream()
                .collect(Collectors.groupingBy(word -> {
                    char[] chars = word.toCharArray();
                    Arrays.sort(chars);
                    return new String(chars);
                }));

        result.values().forEach(System.out::println);
    }
}

Output

[eat, tea, ate]
[tan, nat]
[bat]

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Question 5

5. Find the Longest Common Prefix

Problem Statement

Find the longest common prefix from the given list of strings.

Input

flower, flow, flight

Java Solution

Java 8

public class Main {
    static String longestPrefix(String[] arr) {
        if (arr.length == 0) return "";

        String prefix = arr[0];
        for (int i = 1; i < arr.length; i++) {
            while (!arr[i].startsWith(prefix)) {
                prefix = prefix.substring(0, prefix.length() - 1);
            }
        }
        return prefix;
    }

    public static void main(String[] args) {
        String[] input = {"flower", "flow", "flight"};
        System.out.println(longestPrefix(input));
    }
}

Output

fl

Explanation

The prefix is reduced until every string starts with it.

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Question 6

6. Count Frequency of Characters

Problem Statement

Count the frequency of every character using Java 8 Streams.

Input

String input = "Developer";

Java Solution

Java 8

import java.util.function.Function;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        String input = "Developer";

        input.chars()
            .mapToObj(c -> (char) c)
            .collect(Collectors.groupingBy(
                Function.identity(),
                Collectors.counting()))
            .forEach((k, v) ->
                System.out.println(k + " : " + v));
    }
}

Output

D : 1
e : 3
v : 1
l : 1
o : 1
p : 1
r : 1

Explanation

Each character is converted into a stream and grouped using Collectors.groupingBy() to count its occurrences.

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Question 7

7. Optional Usage in Java 8 Streams

Problem Statement

Demonstrate how Optional helps avoid NullPointerException.

Java Solution

Java 8

import java.util.Optional;

public class Main {
    public static void main(String[] args) {
        Optional<String> name =
            Optional.ofNullable("DevTechnoverse");

        name.ifPresent(System.out::println);
    }
}

Output

DevTechnoverse

Explanation

Optional safely wraps nullable objects and executes the operation only when a value is present.

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Question 8

8. Throw Exception Using Java 8 Streams

Problem Statement

Throw a custom exception while processing invalid data in a Java 8 Stream.

Java Solution

Java 8

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<String> numbers =
            Arrays.asList("10", "20", "ABC");

        numbers.stream()
            .map(value -> {
                try {
                    return Integer.parseInt(value);
                } catch (Exception e) {
                    throw new RuntimeException("Invalid Number : " + value);
                }
            })
            .forEach(System.out::println);
    }
}

Output

10
20
Exception in thread "main"
RuntimeException: Invalid Number : ABC

Explanation

The stream attempts to convert each value into an integer. If parsing fails, a custom RuntimeException is thrown with a meaningful message.

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