30 Java Coding Interview Questions That Keep Showing Up in Product Based Companies

Master the Java coding patterns most frequently asked in product company interviews with clean, beginner-friendly solutions.

Sanajit Jana

7 min read

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May 19, 2026

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If you are preparing for Java interviews in product companies, coding rounds are something you simply cannot avoid.

Whether you’re targeting:

• Amazon
• Adobe
• Walmart
• Oracle
• PayPal
• Atlassian
• Flipkart
• Razorpay
• Or any strong backend engineering role

You will almost always face coding problems focused on problem-solving fundamentals.

Here’s the interesting part most developers eventually realize:

Top companies rarely ask completely random questions.

Most interview problems revolve around a small group of patterns:

• Arrays
• Strings
• HashMaps
• Sliding Window
• Stack & Queue
• Two Pointers
• Basic Data Structures

The difference between average candidates and strong candidates is not intelligence.

It’s pattern recognition.

In this article, we’ll go through 30 Java coding interview questions that repeatedly appear in product company interviews, along with clean and beginner-friendly solutions.

If you truly understand these problems, you will already be ahead of most candidates.

1. Reverse a String Without Using Built-in Functions

One of the oldest and most common Java interview questions.

Problem

Reverse a string manually without using StringBuilder.reverse().

Solution

public class ReverseString {

    public static void main(String[] args) {

        String str = "Java";
        char[] arr = str.toCharArray();

        int left = 0;
        int right = arr.length - 1;

        while (left < right) {

            char temp = arr[left];
            arr[left] = arr[right];
            arr[right] = temp;

            left++;
            right--;
        }

        System.out.println(new String(arr));
    }
}

What Interviewers Actually Test

This question looks simple, but interviewers are checking:

• Two-pointer understanding
• In-place swapping logic
• String manipulation basics
• Clean coding style

2. Find the First Non-Repeated Character

A classic HashMap problem.

Problem

Find the first character that appears only once in a string.

Solution

import java.util.*;

public class FirstUniqueChar {

    public static char firstUnique(String str) {

        Map<Character, Integer> map = new LinkedHashMap<>();

        for (char c : str.toCharArray()) {
            map.put(c, map.getOrDefault(c, 0) + 1);
        }

        for (Map.Entry<Character, Integer> entry : map.entrySet()) {

            if (entry.getValue() == 1) {
                return entry.getKey();
            }
        }

        return '_';
    }

    public static void main(String[] args) {
        System.out.println(firstUnique("swiss"));
    }
}

Why This Question Matters

This problem tests whether you understand:

• Frequency counting
• HashMap usage
• Order preservation
• Iteration efficiency

3. Check if a String is Palindrome

This appears everywhere from beginner interviews to backend screening rounds.

Problem

Check whether a string reads the same forward and backward.

Solution

public class Palindrome {

    public static boolean isPalindrome(String str) {

        int left = 0;
        int right = str.length() - 1;

        while (left < right) {

            if (str.charAt(left) != str.charAt(right)) {
                return false;
            }

            left++;
            right--;
        }

        return true;
    }

    public static void main(String[] args) {
        System.out.println(isPalindrome("madam"));
    }
}

Concepts Tested

• Two-pointer technique
• Edge-case handling
• String traversal

4. Find Duplicate Elements in an Array

One of the most frequently repeated array questions.

Solution

import java.util.*;

public class DuplicateElements {

    public static void main(String[] args) {

        int[] arr = {1, 2, 3, 4, 2, 5, 1};

        Set<Integer> set = new HashSet<>();

        for (int num : arr) {

            if (!set.add(num)) {
                System.out.println("Duplicate: " + num);
            }
        }
    }
}

Why Interviewers Love This

It quickly reveals whether candidates understand:

• HashSet behavior
• Time complexity
• Efficient lookup operations

5. Find the Largest Element in an Array

Solution

public class LargestElement {

    public static int findMax(int[] arr) {

        int max = arr[0];

        for (int num : arr) {

            if (num > max) {
                max = num;
            }
        }

        return max;
    }
}

Hidden Interview Goal

Interviewers often use simple problems to evaluate:

• Confidence
• Code clarity
• Handling of corner cases

6. Find the Second Largest Number

Solution

public static int secondLargest(int[] arr) {

    int first = Integer.MIN_VALUE;
    int second = Integer.MIN_VALUE;

    for (int num : arr) {

        if (num > first) {

            second = first;
            first = num;

        } else if (num > second && num != first) {

            second = num;
        }
    }

    return second;
}

What Makes This Important

Strong candidates solve this in:

• One traversal
• O(n) time
• Without sorting

That’s exactly what interviewers want.

7. Count Frequency of Characters in a String

Solution

import java.util.*;

public class CharFrequency {

    public static void main(String[] args) {

        String str = "programming";

        Map<Character, Integer> map = new HashMap<>();

        for (char c : str.toCharArray()) {
            map.put(c, map.getOrDefault(c, 0) + 1);
        }

        System.out.println(map);
    }
}

Concepts Being Tested

• Frequency mapping
• HashMap operations
• Iterative logic

8. Check if Two Strings are Anagrams

Solution

import java.util.Arrays;

public class Anagram {

    public static boolean isAnagram(String s1, String s2) {

        char[] a = s1.toCharArray();
        char[] b = s2.toCharArray();

        Arrays.sort(a);
        Arrays.sort(b);

        return Arrays.equals(a, b);
    }
}

Why This Appears Often

Because it tests:

• Sorting
• Arrays
• String processing
• Clean decomposition

9. Fibonacci Series

Solution

public class Fibonacci {

    public static void main(String[] args) {

        int n = 10;

        int a = 0;
        int b = 1;

        for (int i = 1; i <= n; i++) {

            System.out.print(a + " ");

            int c = a + b;
            a = b;
            b = c;
        }
    }
}

What Companies Observe

Even small problems reveal:

• Loop understanding
• Variable transitions
• Logical thinking

10. Check Prime Number

Solution

public static boolean isPrime(int num) {

    if (num <= 1) {
        return false;
    }

    for (int i = 2; i <= Math.sqrt(num); i++) {

        if (num % i == 0) {
            return false;
        }
    }

    return true;
}

Optimization Insight

Using Math.sqrt(num) avoids unnecessary iterations.

Interviewers notice these optimizations immediately.

The Real Secret Behind Product Company Interviews

Most developers prepare incorrectly.

They jump from one random problem to another without understanding the deeper patterns.

But interviews are usually pattern repetition in disguise.

If you deeply master:

• Arrays
• Strings
• HashMaps
• Sliding Window
• Stack
• Queue
• Recursion
• Binary Search

You can solve a massive percentage of interview problems.

This is exactly how strong candidates become fast problem solvers.

Not by memorizing solutions.

By recognizing patterns quickly.

What Interviewers Actually Care About

A lot of developers think interviews are about writing genius-level algorithms.

Most of the time, they are not.

Interviewers usually evaluate:

• Problem-solving approach
• Code readability
• Communication
• Optimization awareness
• Edge-case handling
• Confidence under pressure

A clean simple solution often beats a complicated clever solution.

Every single time.

Takeaways

You do not need to solve 5,000 problems to crack Java interviews.

You need:

• Strong fundamentals
• Pattern recognition
• Consistent practice
• Clean coding habits

That’s it.

Start with these 30 questions.

Practice them properly.

Understand why each solution works.

And over time, coding interviews will stop feeling random.

They’ll start feeling predictable.

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