How to Print Prime Numbers from 1 to 100 in Java [Solved]

Hello guys, today, I'll share with you a simple problem of writing a Java program to print prime numbers up to a given number like saying prime numbers from 1 to 100. It's one of the most common coding exercises for programmers learning in Java, as it gives you an opportunity to learn more about the essential operators in Java Programming. The key here is that you cannot use a library function which can simplify your job, you need to devise the algorithm for checking prime number by yourself. One of the most popular algorithms for generating prime is Sieve of Eratosthenes,  which we have discussed earlier, but in this post, we will take a simpler approach.

We'll first write a function to check whether a number is prime or not and then we loop through the first 100 numbers i.e. from 1 to 100 and print only those which passed the prime test.

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How to check if a number is prime or not in Java? Solution

A number is said to be prime if it's not divisible by any number other than itself like 2, 3, or 5. 1 is not counted as a prime number, so the lowest prime number is 2. 

One of the easiest ways to check whether a number is prime or not is to loop from 2 to the number itself and checks if it's divisible by any number in between or not.

You can do that check by using a modulus operator in Java, which returns zero if a number is perfectly divisible by another number. If the number you are checking is not divisible by anyone then it's a prime number otherwise, it's not a prime number.

But this logic can be further optimized to only loop through the square root of the number instead of the number itself, as shown in the below example. This will make the Java program fast for checking large prime numbers.

Here is a list of all prime numbers between 1 and 100:

An optimized way to generate Prime numbers from 1 to 100

And, here is our complete Java program which shows an optimized way to generate prime numbers in the range of 1 to 100. 

/**
 * Java Program to print prime numbers from 1 to 100
 *
 * @author Javin Paul
 */
public class PrimeNumberGenerator {

    public static void main(String args[]) {

        // print prime numbers from 1 - 100
        System.out.println("Prime numbers from 1 to 100 ");

        for (int i = 2; i <= 100; i++) {
            if (isPrime(i)) {
                System.out.println(i);
            }
        }
    }

    /*
     * An optimized to check if a number is prime or not.
     */
    public static boolean isPrime(int num) {
        if (num == 2 || num == 3) {
            return true;
        }

        if (num % 2 == 0 || num % 3 == 0) {
            return false;
        }

        for (int i = 3; i < Math.sqrt(num); i += 2) {
            if (num % i == 0 || num % Math.sqrt(num) == 0) {
                return false;
            }
        }
        return true;

    }
}

Output:
Prime numbers from 1 to 100 
2
3
5
7
11
13
17
19
23
29
31
37
41
43
47
53
59
61
67
71
73
79
83
89
97

That's all about how to print prime numbers in Java from 1 to 100.  Let me know if you find any bug in this program or you think if this program will not work in any specific scenario. This looks much more optimized than looping to the number itself. You can use this technique to solve other coding problems which are based on prime numbers. 

Knowing this trick is really useful for competitive programming as well. 

Here are a couple of more coding problems for practice:

• Top 10 Programming problems from Java Interviews? (article)
• Write a program to code insertion sort algorithm in Java (program)
• How to find a missing number in a sorted array? (solution)
• Write a program to check if a number is a power of two or not? (solution)
• How to calculate factorial using recursion and iteration? (solution)
• How do you reverse the word of a sentence in Java? (solution)
• How to find duplicate characters from String in Java? (solution)
• How to reverse an integer variable in Java? (solution)
• Write code to implement the Quicksort algorithm in Java? (algorithm)
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• How to check if a year is a leap year in Java? (answer)
• Write code to implement a Bubble sort algorithm in Java? (code)
• How to print the Pyramid pattern in Java? (solution)
• How to check if a given number is prime or not? (solution)
• How do you swap two integers without using a temporary variable? (solution)
• How to solve the FizzBuzz problem in Java? (solution)
• Write a program to print the highest frequency word from a text file? (solution)
• How to remove duplicate elements from ArrayList in Java? (solution)
• How to find if given String is a palindrome in Java? (solution)

Thanks for reading this article so far. If you like this article then please share it with your friends and colleagues. If you have any questions or doubt then please let us know and I'll try to find an answer for you. As always suggestions, comments, innovative and better answers are most welcome.

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