Java Interview Question: Separate Even and Odd Numbers Using Streams (With Examples)

 

🔹 Introduction

Separating even and odd numbers is a common Java interview question.
It helps test your understanding of Java Streams, filtering, and partitioning.

👉 In this article, we will solve this using:

• Traditional approach
• Java Streams (modern approach)

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🔹 Problem Statement

Given a list of integers, separate even and odd numbers.

Example:

Input: [1, 2, 3, 4, 5, 6]  
Output: Even: [2, 4, 6], Odd: [1, 3, 5]  

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🔹 Approach 1: Using Loop (Basic)

💡 Explanation

• Traverse list
• Check number % 2
• Store in separate lists

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👨‍💻 Java Code

import java.util.*;

public class EvenOdd {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4,5,6);

        List<Integer> even = new ArrayList<>();
        List<Integer> odd = new ArrayList<>();

        for (int num : list) {
            if (num % 2 == 0) {
                even.add(num);
            } else {
                odd.add(num);
            }
        }

        System.out.println("Even: " + even);
        System.out.println("Odd: " + odd);
    }
}

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🔹 Approach 2: Using Java Streams (Recommended)

💡 Explanation

• Use partitioningBy()
• It splits data into two groups (true/false)

👉 true → even
👉 false → odd

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👨‍💻 Java Code (Streams)

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

public class EvenOdd {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4,5,6);

        Map<Boolean, List<Integer>> result =
                list.stream()
                    .collect(Collectors.partitioningBy(n -> n % 2 == 0));

        System.out.println("Even: " + result.get(true));
        System.out.println("Odd: " + result.get(false));
    }
}

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🔹 Output

Even: [2, 4, 6]  
Odd: [1, 3, 5]  

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🔹 Time Complexity

• O(n)

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🔹 Key Takeaways

✔ partitioningBy() is very important
✔ Streams make code concise
✔ Frequently asked in interviews

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🔹 Conclusion

This is a must-know Java Streams problem.
Understanding partitioningBy() helps in solving many real-world problems.

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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

👉 Find Duplicate Characters in a String