🧩 Sharpen Your Logic with These 10 Java Mini Challenges

In this post, I’ll walk you through 10 simple but powerful Java mini programs. Each one targets a specific concept, and together they’ll help you think like a programmer, not just a code writer.

Let’s dive in! 🚀
1️⃣ Reverse a String (Manual Way):

public class ReverseString {
    public static void main(String[] args) {
        String str = "Java";
        String reversed = "";
        for (int i = str.length() - 1; i >= 0; i--) {
            reversed += str.charAt(i);
        }
        System.out.println(reversed); // Output: avaJ
    }
}

✅ Explanation: Loops from the last character to the first, building a new reversed string.
✅ Enforces: Loops, string indexing, and concatenation.

2️⃣ Check if a Number is Prime

public class PrimeCheck {
    public static void main(String[] args) {
        int num = 29, count = 0;
        for (int i = 1; i <= num; i++) {
            if (num % i == 0) count++;
        }
        System.out.println(count == 2 ? "Prime" : "Not Prime");
    }


}

✅ Explanation: A prime number has exactly 2 divisors: 1 and itself.
✅ Enforces: Loops and conditional logic.

3️⃣ Fibonacci Series

public class Fibonacci {
    public static void main(String[] args) {
        int n = 10, a = 0, b = 1;
        for (int i = 0; i < n; i++) {
            System.out.print(a + " ");
            int sum = a + b;
            a = b;
            b = sum;
        }
    }
}

✅ Explanation: Generates the first n numbers in the Fibonacci sequence.
✅ Enforces: Iteration and updating variables.

4️⃣ Factorial of a Number

public class Factorial {
    public static void main(String[] args) {
        int n = 5, fact = 1;
        for (int i = 1; i <= n; i++) {
            fact *= i;
        }
        System.out.println("Factorial: " + fact); // Output: 120
    }
}

✅ Explanation: Multiplies numbers from 1 to n.
✅ Enforces: Looping and accumulator logic.

5️⃣ Palindrome Check

public class Palindrome {
    public static void main(String[] args) {
        String str = "level";
        String reversed = new StringBuilder(str).reverse().toString();
        System.out.println(str.equals(reversed) ? "Palindrome" : "Not Palindrome");
    }
}

✅ Explanation: A palindrome reads the same forwards and backwards.
✅ Enforces: String comparison and reversal.

6️⃣ Sum of Digits

public class DigitSum {
    public static void main(String[] args) {
        int num = 1234, sum = 0;
        while (num > 0) {
            sum += num % 10;
            num /= 10;
        }
        System.out.println("Sum of digits: " + sum); // Output: 10
    }
}

✅ Explanation: Extracts digits using % and /.
✅ Enforces: Loops and modular arithmetic.

7️⃣ Find Largest Element in an Array

public class LargestInArray {
    public static void main(String[] args) {
        int[] arr = {2, 9, 5, 1, 7};
        int max = arr[0];
        for (int n : arr) {
            if (n > max) max = n;
        }
        System.out.println("Largest: " + max);
    }
}

✅ Explanation: Iterates through array to find maximum value.
✅ Enforces: Arrays and enhanced for-loops.

8️⃣ Count Vowels in a String

public class VowelCount {
    public static void main(String[] args) {
        String str = "Hello Java";
        int count = 0;
        for (char c : str.toLowerCase().toCharArray()) {
            if ("aeiou".indexOf(c) != -1) count++;
        }
        System.out.println("Vowels: " + count); // Output: 4
    }
}

✅ Explanation: Checks if characters belong to the vowel set.
✅ Enforces: Character iteration and string operations.

9️⃣ Simple Number Guessing Game

import java.util.*;

public class GuessGame {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int number = new Random().nextInt(10) + 1;
        System.out.print("Guess a number (1-10): ");
        int guess = sc.nextInt();

        if (guess == number) {
            System.out.println("Correct!");
        } else {
            System.out.println("Wrong! Number was " + number);
        }
    }
}

✅ Explanation: Random number generation with user input.
✅ Enforces: Scanner, randomness, and conditionals.

🔟 Matrix Addition

public class MatrixAddition {
    public static void main(String[] args) {
        int[][] a = {{1,2},{3,4}};
        int[][] b = {{5,6},{7,8}};
        int[][] sum = new int[2][2];

        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 2; j++) {
                sum[i][j] = a[i][j] + b[i][j];
            }
        }

        for (int[] row : sum) {
            for (int val : row) System.out.print(val + " ");
            System.out.println();
        }
    }
}

✅ Explanation: Adds two 2×2 matrices element by element.
✅ Enforces: Nested loops and multidimensional arrays.

🎯 Wrapping Up

These mini-programs might look simple, but together they cover:

Loops & conditionals

Arrays & strings

Methods & modular arithmetic

Randomness & user input

💬 Your turn:
Which one of these programs have you already mastered?
Which one do you want me to expand into a full breakdown?

Drop your thoughts in the comments! 🚀

📚 Resources to Practice More:

W3Resourse - Java Exercises

GeeksforGeeks - Java Programs