Java Stream Scenario Based Interview Question

You have a list of employees with fields like name, age, and department. Write a code snippet to group employees by department using Java Streams.

Solution

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

class Employee {
    private String name;
    private int age;
    private String department;

    // Constructor
    public Employee(String name, int age, String department) {
        this.name = name;
        this.age = age;
        this.department = department;
    }

    // Getters
    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public String getDepartment() {
        return department;
    }

    @Override
    public String toString() {
        return name + " (" + age + ")";
    }
}

public class StreamExample {
    public static void main(String[] args) {
        // Create a list of employees
        List<Employee> employees = Arrays.asList(
                new Employee("John", 25, "HR"),
                new Employee("Jane", 28, "Finance"),
                new Employee("Jake", 22, "HR"),
                new Employee("Jill", 35, "Finance"),
                new Employee("Joe", 29, "IT")
        );

        // Group employees by department
        Map<String, List<Employee>> employeesByDepartment = employees.stream()
                .collect(Collectors.groupingBy(Employee::getDepartment));

        // Print the grouped employees
        employeesByDepartment.forEach((department, empList) -> {
            System.out.println(department + ": " + empList);
        });
    }
}

Explanation
stream(): Creates a stream from the list of employees.

Collectors.groupingBy(Employee::getDepartment):
Groups employees by the value of their department field.
Returns a Map>, where the key is the department, and the value is a list of employees in that department.

forEach():
Iterates over the Map and prints the department name and the list of employees.

Output
For the given list:

[
    new Employee("John", 25, "HR"),
    new Employee("Jane", 28, "Finance"),
    new Employee("Jake", 22, "HR"),
    new Employee("Jill", 35, "Finance"),
    new Employee("Joe", 29, "IT")
]

The output will be:

HR: [John (25), Jake (22)]
Finance: [Jane (28), Jill (35)]
IT: [Joe (29)]

Edge Cases
Empty List: If the list is empty, the resulting map will also be empty.

All Employees in the Same Department: The map will have a single key with all employees grouped under it.

[
    new Employee("John", 25, "HR"),
    new Employee("Jane", 28, "HR"),
    new Employee("Jake", 22, "HR")
]

Output:

HR: [John (25), Jane (28), Jake (22)]

Key Takeaways
Collectors.groupingBy() is a powerful tool for grouping data in streams.

Use toString() to customize how objects appear in the output.