Java Interview Question: Find Highest Salary by Department Using Streams (groupingBy Explained)

 

🔹 Introduction

This is a very common Java Streams interview question, especially for backend roles.

👉 It tests your understanding of:

• groupingBy()
• maxBy()
• Comparator

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🔹 Problem Statement

Given a list of employees, find the highest salary in each department.

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🔹 Example

Input:
IT → 50000, 70000  
HR → 40000, 60000  

Output:
IT → 70000  
HR → 60000  

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🔹 Approach (Using Java Streams)

💡 Explanation

1. Group employees by department
2. Find max salary in each group
3. Use Collectors.maxBy()

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🔹 Employee Class

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

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

    public String getDepartment() { return department; }
    public double getSalary() { return salary; }
}

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🔹 Java Code (Streams Solution)

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

public class HighestSalary {
    public static void main(String[] args) {

        List<Employee> employees = Arrays.asList(
            new Employee("A", "IT", 50000),
            new Employee("B", "IT", 70000),
            new Employee("C", "HR", 40000),
            new Employee("D", "HR", 60000)
        );

        Map<String, Optional<Employee>> result =
            employees.stream()
                     .collect(Collectors.groupingBy(
                         Employee::getDepartment,
                         Collectors.maxBy(Comparator.comparing(Employee::getSalary))
                     ));

        result.forEach((dept, emp) ->
            System.out.println(dept + " -> " + emp.get().getSalary()));
    }
}

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🔹 Output

IT -> 70000  
HR -> 60000  

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🔍 Internal Working (Important for Interviews)

• groupingBy() → groups by department
• maxBy() → finds highest salary
• Comparator.comparing() → compares salary

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🔹 Time Complexity

• O(n)

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🔹 Key Takeaways

✔ groupingBy() is very important
✔ maxBy() helps find max values
✔ Real-world interview question

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🔹 Conclusion

This is a must-know Java Streams problem for backend developers.
Mastering this will help you crack mid-level interviews.

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🔗 Also Read

👉 First Non-Repeating Character in Java
👉 Check Palindrome String in Java
👉 Reverse a String
👉 Count Character Frequency in Java
👉 Separate Even and Odd Numbers Using Streams
👉 Find Duplicate Characters in a String