Important Java Programs

Collection of most important Java programs frequently asked in exams and interviews - patterns, number programs, string programs, and array programs.

Number Programs

1. Check Armstrong Number

java example

public boolean isArmstrong(int num) {
    int original = num, sum = 0;
    int digits = String.valueOf(num).length();
    while (num > 0) {
        int d = num % 10;
        sum += Math.pow(d, digits);
        num /= 10;
    }
    return sum == original;
}
// 153 = 1³ + 5³ + 3³ = 153 ✅

2. Reverse a Number

java example

public int reverse(int num) {
    int rev = 0;
    while (num != 0) {
        rev = rev * 10 + num % 10;
        num /= 10;
    }
    return rev;
}
// 12345 → 54321

3. Check Palindrome Number

java example

public boolean isPalindrome(int num) {
    return num == reverse(num);
}
// 121 → true, 123 → false

4. Swap Two Numbers Without Temp

java example

a = a + b;  // a=15
b = a - b;  // b=5 (original a)
a = a - b;  // a=10 (original b)
// OR using XOR
a = a ^ b; b = a ^ b; a = a ^ b;

5. GCD and LCM

java example

int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
int lcm(int a, int b) { return (a * b) / gcd(a, b); }

6. Sum of Digits

java example

int sumOfDigits(int n) {
    int sum = 0;
    while (n > 0) { sum += n % 10; n /= 10; }
    return sum;
}

String Programs

12. Count Vowels and Consonants

Example

String s = "Hello World"; int vowels = 0, consonants = 0; for (char c : s.toLowerCase().toCharArray()) { if ("aeiou".indexOf(c) != -1) vowels++; else if (c >= 'a' && c <= 'z') consonants++; }

13. Check Anagram

java example

boolean isAnagram(String a, String b) {
    char[] x = a.toLowerCase().toCharArray();
    char[] y = b.toLowerCase().toCharArray();
    Arrays.sort(x); Arrays.sort(y);
    return Arrays.equals(x, y);
}

14. Remove Whitespace

java example

String result = str.replaceAll("\\s+", "");

15. Count Words in String

java example

int words = str.trim().split("\\s+").length;

Array Programs

16. Find Max and Min

Example

int max = arr[0], min = arr[0]; for (int num : arr) { if (num > max) max = num; if (num < min) min = num; }

17. Bubble Sort

Example

for (int i = 0; i < n-1; i++) { for (int j = 0; j < n-i-1; j++) { if (arr[j] > arr[j+1]) { int temp = arr[j]; arr[j] = arr[j+1]; arr[j+1] = temp; } } }

18. Linear Search

Example

int search(int[] arr, int key) { for (int i = 0; i < arr.length; i++) { if (arr[i] == key) return i; } return -1; }

19. Matrix Addition

Example

int[][] result = new int[rows][cols]; for (int i = 0; i < rows; i++) for (int j = 0; j < cols; j++) result[i][j] = a[i][j] + b[i][j];

20. Matrix Transpose

Example

int[][] transpose = new int[cols][rows]; for (int i = 0; i < rows; i++) for (int j = 0; j < cols; j++) transpose[j][i] = matrix[i][j];

Recursion Programs

21. Tower of Hanoi

java example

void hanoi(int n, char from, char to, char aux) {
    if (n == 1) { System.out.println(from + " → " + to); return; }
    hanoi(n-1, from, aux, to);
    System.out.println(from + " → " + to);
    hanoi(n-1, aux, to, from);
}

22. Power Function

java example

double power(double base, int exp) {
    if (exp == 0) return 1;
    if (exp < 0) return 1.0 / power(base, -exp);
    return base * power(base, exp - 1);
}

23. Sum of Array (Recursive)

Example

int sum(int[] arr, int n) { if (n == 0) return 0; return arr[n-1] + sum(arr, n-1); }

OOP Programs

24. Singleton Pattern

java example

class Singleton {
    private static Singleton instance;
    private Singleton() {}
    public static synchronized Singleton getInstance() {
        if (instance == null) instance = new Singleton();
        return instance;
    }
}

25. Custom Exception

java example

class InsufficientBalanceException extends Exception {
    public InsufficientBalanceException(double amount) {
        super("Insufficient balance. Tried to withdraw: " + amount);
    }
}

Number Programs

1. Check Armstrong Number

java example

public boolean isArmstrong(int num) {
    int original = num, sum = 0;
    int digits = String.valueOf(num).length();
    while (num > 0) {
        int d = num % 10;
        sum += Math.pow(d, digits);
        num /= 10;
    }
    return sum == original;
}
// 153 = 1³ + 5³ + 3³ = 153 ✅

2. Reverse a Number

java example

public int reverse(int num) {
    int rev = 0;
    while (num != 0) {
        rev = rev * 10 + num % 10;
        num /= 10;
    }
    return rev;
}
// 12345 → 54321

3. Check Palindrome Number

java example

public boolean isPalindrome(int num) {
    return num == reverse(num);
}
// 121 → true, 123 → false

4. Swap Two Numbers Without Temp

java example

a = a + b;  // a=15
b = a - b;  // b=5 (original a)
a = a - b;  // a=10 (original b)
// OR using XOR
a = a ^ b; b = a ^ b; a = a ^ b;

5. GCD and LCM

java example

int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
int lcm(int a, int b) { return (a * b) / gcd(a, b); }

6. Sum of Digits

java example

int sumOfDigits(int n) {
    int sum = 0;
    while (n > 0) { sum += n % 10; n /= 10; }
    return sum;
}

String Programs

12. Count Vowels and Consonants

Example

String s = "Hello World"; int vowels = 0, consonants = 0; for (char c : s.toLowerCase().toCharArray()) { if ("aeiou".indexOf(c) != -1) vowels++; else if (c >= 'a' && c <= 'z') consonants++; }

13. Check Anagram

java example

boolean isAnagram(String a, String b) {
    char[] x = a.toLowerCase().toCharArray();
    char[] y = b.toLowerCase().toCharArray();
    Arrays.sort(x); Arrays.sort(y);
    return Arrays.equals(x, y);
}

14. Remove Whitespace

java example

String result = str.replaceAll("\\s+", "");

15. Count Words in String

java example

int words = str.trim().split("\\s+").length;

Array Programs

16. Find Max and Min

Example

int max = arr[0], min = arr[0]; for (int num : arr) { if (num > max) max = num; if (num < min) min = num; }

17. Bubble Sort

Example

for (int i = 0; i < n-1; i++) { for (int j = 0; j < n-i-1; j++) { if (arr[j] > arr[j+1]) { int temp = arr[j]; arr[j] = arr[j+1]; arr[j+1] = temp; } } }

18. Linear Search

Example

int search(int[] arr, int key) { for (int i = 0; i < arr.length; i++) { if (arr[i] == key) return i; } return -1; }

19. Matrix Addition

Example

int[][] result = new int[rows][cols]; for (int i = 0; i < rows; i++) for (int j = 0; j < cols; j++) result[i][j] = a[i][j] + b[i][j];

20. Matrix Transpose

Example

int[][] transpose = new int[cols][rows]; for (int i = 0; i < rows; i++) for (int j = 0; j < cols; j++) transpose[j][i] = matrix[i][j];

Recursion Programs

21. Tower of Hanoi

java example

void hanoi(int n, char from, char to, char aux) {
    if (n == 1) { System.out.println(from + " → " + to); return; }
    hanoi(n-1, from, aux, to);
    System.out.println(from + " → " + to);
    hanoi(n-1, aux, to, from);
}

22. Power Function

java example

double power(double base, int exp) {
    if (exp == 0) return 1;
    if (exp < 0) return 1.0 / power(base, -exp);
    return base * power(base, exp - 1);
}

23. Sum of Array (Recursive)

Example

int sum(int[] arr, int n) { if (n == 0) return 0; return arr[n-1] + sum(arr, n-1); }

OOP Programs

24. Singleton Pattern

java example

class Singleton {
    private static Singleton instance;
    private Singleton() {}
    public static synchronized Singleton getInstance() {
        if (instance == null) instance = new Singleton();
        return instance;
    }
}

25. Custom Exception

java example

class InsufficientBalanceException extends Exception {
    public InsufficientBalanceException(double amount) {
        super("Insufficient balance. Tried to withdraw: " + amount);
    }
}

Number Programs

1. Check Armstrong Number

2. Reverse a Number

3. Check Palindrome Number

4. Swap Two Numbers Without Temp

5. GCD and LCM

6. Sum of Digits

String Programs

12. Count Vowels and Consonants

13. Check Anagram

14. Remove Whitespace

15. Count Words in String

Array Programs

16. Find Max and Min

17. Bubble Sort

18. Linear Search

19. Matrix Addition

20. Matrix Transpose

Recursion Programs

21. Tower of Hanoi

22. Power Function

23. Sum of Array (Recursive)

OOP Programs

24. Singleton Pattern

25. Custom Exception

Continue learning this concept

Number Programs

1. Check Armstrong Number

2. Reverse a Number

3. Check Palindrome Number

4. Swap Two Numbers Without Temp

5. GCD and LCM

6. Sum of Digits

String Programs

12. Count Vowels and Consonants

13. Check Anagram

14. Remove Whitespace

15. Count Words in String

Array Programs

16. Find Max and Min

17. Bubble Sort

18. Linear Search

19. Matrix Addition

20. Matrix Transpose

Recursion Programs

21. Tower of Hanoi

22. Power Function

23. Sum of Array (Recursive)

OOP Programs

24. Singleton Pattern

25. Custom Exception

Continue learning this concept