Java Program to Print Prime Numbers

This Java program demonstrates how to calculate and print prime numbers. Whether you aim to print prime numbers from 1 to 100 in Java or want to understand the logic behind identifying a prime number in Java, this tutorial has you covered.

What is a Prime Number?

Prime numbers are unique natural numbers greater than 1, divisible only by 1 and themselves. Unlike composite numbers, they can't be divided by smaller natural numbers. Prime numbers are significant in fields like number theory, cryptography, and computer science.

Examples of prime numbers:

A few prime numbers are 2, 3, 5, 7, 11, and 13. These numbers can only be divided evenly by 1 or themselves, so they are prime numbers.

Prime Number Check Program in Java

Program:

public class PrimeNumbers {
  public static void main(String[] args) {
    int num = 20;  // Define the upper limit
    int count;  // Initialize counter for divisibility checks

    // Iterate from 1 up to 'num' to identify prime numbers
    for (int i = 1; i <= num; i++) {
      count = 0;  // Reset counter for each 'i'

      // Check for divisibility from 2 up to i/2
      for (int j = 2; j <= i / 2; j++) {
        if (i % j == 0) {
          count++;  // Increment if 'i' is divisible by 'j'
          break;  // Exit loop if a divisor is found
        }
      }

      // If the count is 0, 'i' is prime
      if (count == 0) {
        System.out.println(i);  // Output the prime number
      }
    }
  }
}

Program Output:

Execute this Java program to generate prime numbers. You will get the list of prime numbers up to 20 as follows:

1
2
3
5
7
11
13
17
19

Code Explanation:

Here is a line-by-line explanation of the prime no program in Java:

1. Class and Main Method: First, create a class named PrimeNumbers. Inside this class, declare the main() method.
2. Variable Initialization: Declare two integer variables num and count. Set num to 20, which is the upper limit for the prime number search.
3. Outer Loop: An outer loop runs from 1 to num. Initialize count to zero at the start of each iteration.
4. Inner Loop: An inner loop runs from 2 to i / 2 and checks if i is divisible by any number in this range. If a divisor is found, count increments by one, and the loop exits.
5. Prime Identification: After the inner loop, if count remains zero, the program considers i as a prime number and prints it.

Conclusion

This Java tutorial provides an efficient and straightforward way to calculate prime numbers up to a given limit. Whether you aim to print a list of prime numbers in Java or understand the core logic behind identifying them, this guide makes it accessible to everyone.