In my experience as a software developer, I’ve found identifying prime numbers to be a fundamental exercise for understanding control structures and loops in Java. Java is a popular and one of the most used languages, and the reason for its sunny day spotlight is providing features like object-oriented programming, platform independency, predefined libraries, etc. This led me to write a program focused on Prime Numbers From 1 To 100 in Java. This exercise is a practical way to get acquainted with Java’s capabilities for solving mathematical problems and serves as a solid foundation for more complex algorithmic challenges.

By tackling the problem of displaying Prime Numbers From 1 To 100 in Java, I have improved my algorithm development and Java programming skills. This article aims to share these insights with professionals looking to enhance their programming abilities. We will explore sections such as Introduction, Java Program, Prime Numbers in Given Input Range, and Conclusion, providing a step-by-step guide to this essential programming task. 

Let’s build a code for printing prime numbers from 1 to 100 and walk through it. Let’s start!

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Java Program

Before jumping to the code, we’ll understand the algorithm to check if a number is a prime number or not. At first, we need to loop over all the numbers from 1 to N and maintain a count of numbers that properly divides the given number. If the count is 2 then we can conclude that the given number is a prime, else it is not a prime. Here’s the code to do that.

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int n = 5; int c = 0; for(int i=1;i<=n;i++)   if(n%i==0)       c++; if(c==2)     System.out.println(n+” is a prime number”); else     System.out.println(n+” is not a prime number”);

In the above snippet n is the number which is to be checked if it is a prime number or not, c is a variable that stores the count of proper divisors. And we are looping over the range 1 to n and incrementing the count if we found a proper divisor.

And after coming out of the loop, we are checking if the count is 2 i.e.; there are only two proper divisors (1 and itself). If yes concluding it as a prime number, else a non-prime number. Talking about the time complexity of the above code it is a linear one, so it’s an O(n) complexity code.

Now that we were asked to print prime numbers from 1 to 100, we need to run this same algorithm for each number between 1 to 100 and store the prime number. And here’s the code to do that.

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ArrayList<Integer> a=new ArrayList<>();   for(int n=1; n<=100; n++){   int c = 0;   for (int i = 1; i <= n; i++)       if (n % i == 0)           c++;   if (c == 2)       a.add(n);   else       continue;   }   System.out.println(a);

In the above code, we’ve declared an ArrayList that stores all the prime numbers in the range of 1 to 100. Now we have two for loops first for loop is for looping over all the numbers between 1 to 100 and the second for loop is our previous prime number algorithm. After running the prime number algorithm for each number we are pushing it into ArrayList if it is a prime number.

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And after completing the loops we are printing our ArrayList which displays all the prime numbers between 1 to 100. Talking about the time complexity of the above code, we can see that there are two for loops. So it is an O(n²) complexity code.

We have hardcoded the range in the above code, what if we want to print prime numbers in the range given by the user input?

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Prime Numbers in Given Input Range

The whole algorithm will be almost similar to the above code, the only difference we make is taking user input for the lower limit and upper limit of the range.

Let’s build the code now!

Scanner sc=new Scanner(System.in); int lower=sc.nextInt(); int upper=sc.nextInt(); ArrayList<Integer> a=new ArrayList<>(); for(int n=lower; n<=upper; n++){ int c = 0; for (int i = 1; i <= n; i++) if (n % i == 0) c++; if (c == 2) a.add(n); else continue; } System.out.println(a);

In the above code, we are initializing a scanner for reading user input. We’ve declared two variables lower and upper and assigning those variables with user input. What we have to do is print all the prime numbers between the range [lower, upper]. Our previous algorithm does this task and appends all the prime numbers to the ArrayList.

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The following section explains how to find prime number between 1 to 100 in Java.

Program Logic:

The key method of the prime number program in Java includes a loop to check and find prime number between 1 to 100 in Java.

The main method calls the method “CheckPrime” to decide whether a number is prime or not in Java. For example, we have to divide an input number, say 15, from values 2 to 15 and check the remainder. But if the remainder is 0, the number is not prime.

Note that no number is divisible by more than half of itself. Hence, we must loop through just numToCheck/2. So, if the input is 15, its half is 7.5, and the loop will repeat through values 2 to 8.

If numToCheck is completely divisible by another number, the output is false, and the loop is broken.

But if numToCheck is prime, the output is true.

In the main method for how to find prime numbers from 1 to 100 in Java, check isPrime is TRUE and add it to primeNumFound String.

Here is the Java program to print prime numbers from 1 to 100.

public class primeNumFoundber {

    public static void main(String[] args) {

        int i;

        int num = 0;

        int maxCheck = 100; // The limit to find prime numbers is up to 100

        boolean isPrime = true;

        //Empty String

        String primeNumFound = “”;

        //Start loop#2 to maxCheck

        for (i = 2; i <= maxCheck; i++) {

            isPrime = CheckPrime(i);

            if (isPrime) {

             primeNumFound = primeNumFound + i + ” “;

            }

        }

        System.out.println(“Prime numbers from 1 to ” + maxCheck + “:”);

        // It prints prime numbers from 1 to maxCheck

    System.out.println(primeNumFound);

}

    public static boolean CheckPrime(int numToCheck) {

        int remainder;

        for (int i = 2; i <= numToCheck / 2; i++) {

            remainder = numToCheck % i;

            //if remainder gives 0 than numToCheckber is not prime number and breaks loop, else it continues the loop

         if (remainder == 0) {

             return false;

            }

        }

        return true;

}

}

Output:

The output of this Java program to print prime numbers from 1 to 100 would be:

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

Find Prime Number using While Loop in Java

1. In this program of calculating prime no between 1 to 100 in Java, the while loop is available in the constructor. If we instantiate the class, the constructor will be automatically executed.
2. Read the “n” value through scanner class object sc.Int(). “FindoutPrime class” is introduced in the class Prime as a new FindPrime(n); the constructor of “FindoutPrime” will be performed.

The while loop in Java iterates until i<=num is false. The remainder of the number/i=0 count will be incremented by 1 and “i” value is increased by 1. If the count=2, the code prints, “number is a prime number”.

Find Prime Number using While Loop in Java:

The program to find prime no between 1 to 100 in Java using “While Loop” is as below.

import java.util.Scanner;

class FindoutPrime

{

         FindoutPrime (int num)

         {

         int count=0,i=1;

         while(i<=num)

         {

           if(num%i==0)

           {

             count++;     

           }

         i++;

         }

         if(count==2)

           System.out.println(num+” is a prime number “);

         else

         System.out.println(num+” is not a prime number “);   

         }

}

class Prime

{

         public static void main(String arg[])  

         {

               System.out.println(“Enter a number “);

         Scanner sc=new Scanner(System.in);

         int n=sc.Int();

         new FindoutPrime (n);

         }

}

Output:

The output of the above program on how to find prime numbers from 1 to 100 in Java:

Enter a number

83

83 is a prime number

Output

Enter a number

80

80 is not a prime number

Find Prime Number using Recursion in Java:

Here are the steps to find prime numbers between 1 to 100 in Java using Recursion.

1. Firstly, read the entered number n.
2. The Prime class object will be created in the main method. This step calls the method CheckPrimeOrNot (n) with the help of object p.CheckeprimeOrNot(n);

iii. The method CheckPrimeOrNot (int num) will run and call itself as CheckPrimeOrNot (num). It executes until the condition if(i<=num) is false. But if the condition is false, this method returns the count and is assigned to the c variable. If count=2, the code prints “prime number” else, it prints “not a prime number”.

import java.util.Scanner;

class Prime

{     

         static int count=0,i=1;

         int CheckPrimeOrNot (int num)

         {

                     if(i<=num)

                     {

                       if(num%i==0)

                       {

                         count++;           

                       }

                       i++;

                     CheckPrimeOrNot (num);

                     }

         return count;

         }

         public static void main(String arg[])  

         {

               System.out.println(“Enter a number “);

         Scanner sc=new Scanner(System.in);

         int n=sc.Int();

         Prime p=new Prime();

         int c=p. CheckPrimeOrNot (n);

         if(c==2)

           System.out.println(“prime number “);

         else

           System.out.println(“not a prime number “);   

         }

}

Output:

The  output of prime numbers between 1 to 100 in Java.

Enter a number

8

not a prime number

These are the various methods to find the given range of prime numbers 1 to 100 java. You can implement several programming methods to find, or print prime numbers 1 to 100 java.

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Conclusion

The exploration of Prime Numbers From 1 To 100 in Java serves as a foundational exercise for professionals aiming to enhance their proficiency in Java programming. The process of creating a Java program to list numbers from 1 to 100 and then filtering those numbers to find primes demonstrates the combination of logic and mathematics. It highlights the importance of efficiency and optimization in code, especially when dealing with fundamental algorithms like prime number generation. As professionals navigate their careers, revisiting such fundamental exercises can offer fresh insights and refine problem-solving skills. Thus, mastering the art of identifying prime numbers within a specific range in Java is more than just an academic exercise; it is a stepping stone towards achieving greater complexity and sophistication in programming endeavors. 

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