This collection of basic Java programs is designed for beginners preparing for interviews or practicing core concepts.
1. Read Number From Standard Input
Description: Reads an integer from the user and prints it.
Logic: Use Scanner class → read with nextInt().
Program:
import java.util.Scanner;
class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int num = sc.nextInt();
System.out.println("You entered: " + num);
}
}
Input:
5
Output:
You entered: 5
2. Get Input from the User
Description: Reads a string (name) from the user.
Logic: Use Scanner with nextLine().
Program:
import java.util.Scanner;
class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
String name = sc.nextLine();
System.out.println("Hello, " + name);
}
}
Input:
John
Output:
Hello, John
3. Multiply Two Floating-Point Numbers
Description: Multiply two decimal numbers.
Logic: Use float or double type with * operator.
Program:
class Main {
public static void main(String[] args) {
float a = 2.3f, b = 4.5f;
float product = a * b;
System.out.println("Product = " + product);
}
}
Input:
a = 2.3, b = 4.5
Output:
Product = 10.35
4. Swap Two Numbers
Description: Swap values of two integers using a temporary variable.
Logic: Store a in temp, assign b to a, then temp to b.
Program:
class Main {
public static void main(String[] args) {
int a = 5, b = 10;
int temp = a;
a = b;
b = temp;
System.out.println("a = " + a + ", b = " + b);
}
}
Input:
a = 5, b = 10
Output:
a = 10, b = 5
5. Add Two Binary Strings
Description: Adds two binary numbers represented as strings.
Logic: Convert to decimal → add → convert back to binary.
Program:
class Main {
public static void main(String[] args) {
String a = "1010", b = "1101";
int sum = Integer.parseInt(a, 2) + Integer.parseInt(b, 2);
System.out.println(Integer.toBinaryString(sum));
}
}
Input:
a = "1010", b = "1101"
Output:
10111
6. Add Two Complex Numbers
Description: Adds two complex numbers.
Logic: Add real parts and imaginary parts separately.
Program:
class Complex {
int real, imag;
Complex(int r, int i) { real = r; imag = i; }
Complex add(Complex c) {
return new Complex(this.real + c.real, this.imag + c.imag);
}
public static void main(String[] args) {
Complex c1 = new Complex(2, 3);
Complex c2 = new Complex(4, 5);
Complex result = c1.add(c2);
System.out.println("Sum = " + result.real + " + " + result.imag + "i");
}
}
Input:
(2+3i), (4+5i)
Output:
Sum = 6 + 8i
7. Check Even or Odd Integer
Description: Determine if a number is even or odd.
Logic: If n % 2 == 0 → Even, else Odd.
Program:
class Main {
public static void main(String[] args) {
int num = 7;
if (num % 2 == 0)
System.out.println("Even");
else
System.out.println("Odd");
}
}
Input:
7
Output:
Odd
8. Find Largest Among 3 Numbers
Description: Find the maximum of three numbers.
Logic: Use Math.max() or nested conditions.
Program:
class Main {
public static void main(String[] args) {
int a = 10, b = 20, c = 15;
int max = Math.max(a, Math.max(b, c));
System.out.println("Largest = " + max);
}
}
Input:
a=10, b=20, c=15
Output:
Largest = 20
9. Find LCM of 2 Numbers
Description: Find the Least Common Multiple (LCM) of two numbers.
Logic: Formula → LCM(a, b) = (a × b) / GCD(a, b).
Program:
class Main {
static int gcd(int a, int b) {
return (b == 0) ? a : gcd(b, a % b);
}
public static void main(String[] args) {
int a = 15, b = 20;
int lcm = (a * b) / gcd(a, b);
System.out.println("LCM = " + lcm);
}
}
Input:
a = 15, b = 20
Output:
LCM = 60
10. Find GCD or HCF of 2 Numbers
Description: Find the Greatest Common Divisor (GCD) of two numbers.
Logic: Use Euclidean Algorithm.
Program:
class Main {
static int gcd(int a, int b) {
return (b == 0) ? a : gcd(b, a % b);
}
public static void main(String[] args) {
int a = 36, b = 60;
System.out.println("GCD = " + gcd(a, b));
}
}
Input:
a = 36, b = 60
Output:
GCD = 12
11. Display All Prime Numbers from 1 to N
Description: Print all prime numbers up to a given number N.
Logic: A prime number is divisible only by 1 and itself.
Program:
class Main {
public static void main(String[] args) {
int n = 20;
for (int i = 2; i <= n; i++) {
boolean prime = true;
for (int j = 2; j * j <= i; j++) {
if (i % j == 0) {
prime = false;
break;
}
}
if (prime) System.out.print(i + " ");
}
}
}
Input:
n = 20
Output:
2 3 5 7 11 13 17 19
12. Check Leap Year
Description: Check whether a given year is a leap year.
Logic: Leap year → divisible by 400 OR divisible by 4 but not by 100.
Program:
class Main {
public static void main(String[] args) {
int year = 2024;
if ((year % 400 == 0) || (year % 4 == 0 && year % 100 != 0))
System.out.println("Leap Year");
else
System.out.println("Not Leap Year");
}
}
Input:
2024
Output:
Leap Year
13. Check Armstrong Number between Two Integers
Description: Armstrong number → sum of cubes of digits = original number.
Logic: Loop through numbers in range and check condition.
Program:
class Main {
public static void main(String[] args) {
int start = 100, end = 500;
for (int num = start; num <= end; num++) {
int sum = 0, temp = num;
while (temp > 0) {
int d = temp % 10;
sum += d * d * d;
temp /= 10;
}
if (sum == num) System.out.println(num);
}
}
}
Input:
100 to 500
Output:
153
370
371
407
14. Check whether the Input Number is a Neon Number
Description: Neon Number → sum of digits of square = original number.
Logic: Compute square → sum its digits → check equality.
Program:
class Main {
public static void main(String[] args) {
int n = 9, sq = n * n, sum = 0;
while (sq > 0) {
sum += sq % 10;
sq /= 10;
}
if (sum == n)
System.out.println("Neon Number");
else
System.out.println("Not Neon Number");
}
}
Input:
9
Output:
Neon Number
15. Check whether Input Character is Vowel or Consonant
Description: Determine if a given character is a vowel or consonant.
Logic: Compare character with 'a', 'e', 'i', 'o', 'u' (case insensitive).
Program:
class Main {
public static void main(String[] args) {
char ch = 'e';
if ("aeiouAEIOU".indexOf(ch) != -1)
System.out.println("Vowel");
else
System.out.println("Consonant");
}
}
Input:
e
Output:
Vowel
16. Find Factorial of a Number
Description: Calculate factorial of a given number.
Logic: Multiply numbers from 1 to n.
Program:
class Main {
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);
}
}
Input:
5
Output:
Factorial = 120
17. Find Even Sum of Fibonacci Series Till Number N
Description: Calculate sum of even Fibonacci numbers up to N.
Logic: Generate Fibonacci series → add only even terms.
Program:
class Main {
public static void main(String[] args) {
int n = 10, a = 0, b = 1, sum = 0;
while (b <= n) {
if (b % 2 == 0) sum += b;
int next = a + b;
a = b;
b = next;
}
System.out.println("Even Fibonacci Sum = " + sum);
}
}
Input:
N = 10
Output:
Even Fibonacci Sum = 10
18. Calculate Simple Interest
Description: Compute simple interest on given principal, rate, and time.
Logic: Formula → SI = (P × R × T) / 100.
Program:
class Main {
public static void main(String[] args) {
double p = 1000, r = 5, t = 2;
double si = (p * r * t) / 100;
System.out.println("Simple Interest = " + si);
}
}
Input:
P=1000, R=5%, T=2
Output:
Simple Interest = 100.0
19. Calculate Compound Interest
Description: Compute compound interest on given principal, rate, and time.
Logic: Formula → CI = P × (1 + R/100)^T – P.
Program:
class Main {
public static void main(String[] args) {
double p = 1000, r = 5, t = 2;
double ci = p * Math.pow((1 + r / 100), t) - p;
System.out.println("Compound Interest = " + ci);
}
}
Input:
P=1000, R=5%, T=2
Output:
Compound Interest = 102.5
20. Find the Perimeter of a Rectangle
Description: Calculate perimeter of a rectangle using length and width.
Logic: Formula → Perimeter = 2 × (length + width).
Program:
class Main {
public static void main(String[] args) {
int length = 10, width = 5;
int perimeter = 2 * (length + width);
System.out.println("Perimeter = " + perimeter);
}
}
Input:
Length = 10, Width = 5
Output:
Perimeter = 30
