Prime Number Example in Java

Introduction

Prime Number is a classic Java console program that demonstrates the concept with complete source code and sample output. Calculation programs apply formulas to solve geometry, statistics and numeric problems.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself. A natural number (i.e. 1, 2, 3, 4, 5, 6, etc.) is called a prime number (or a prime) if it has exactly two positive divisors, 1 and the number itself. Natural numbers greater than 1 that are not prime are called composite. The number 12 is not a prime, as 12 items can be placed into 3 equal-size columns of 4 each (among other ways). 11 items cannot be all placed into several equal-size columns of more than 1 item each without some extra items leftover (a remainder). Therefore, the number 11 is a prime. Among the numbers 1 to 6, the numbers 2, 3, and 5 are the prime numbers, while 1, 4, and 6 are not prime.

Prime Number Example Program

import java.util.*;
 
class PrimeNumber{
	public static void main(String args[]){
		int num1, status = 1, num2 = 3;
		Scanner input = new Scanner(System.in);
		System.out.println("How many prime numbers do you need?");
		num1 = input.nextInt();
		if (num1 >= 1){
			System.out.println("First "+num1+" prime numbers are");
			System.out.println(2);
		}
		for ( int i = 2 ; i <=num1 ;  ){
			for ( int j = 2 ; j <= Math.sqrt(num2) ; j++ ){
				if ( num2%j == 0 ){
					status = 0;
					break;
				}
			}
			if ( status != 0 ){
				System.out.println(num2);
				i++;
			}
			status = 1;
			num2++;
		}         
	}
}

Sample Output

How many prime numbers do you need?
9
First 9 prime numbers are
2
3
5
7
11
13
17
19
23

When to use

Use these formulas in homework tools, engineering calculators or anywhere repeated numeric computation is needed.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. import java.util.*; imports a class used later in the program.

3. int num1, status = 1, num2 = 3; updates a variable used in the calculation or output.

4. A Scanner reads typed input from the keyboard (System.in).

5. A println / print call writes text to the console — part of the sample output below.

6. num1 = input.nextInt(); updates a variable used in the calculation or output.

7. The if statement runs the nested code only when the condition is true.

8. A println / print call writes text to the console — part of the sample output below.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.