Write a Java program to print all the prime numbers upto a number entered by the user.

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Algorithm

  1. Start
  2. x:=0
  3. If x<=1, go to step 4 or else go to step 9
  4. Print "Enter a number"
  5. Read x
  6. If x<=1, go to step 7 or else go to step 8
  7. Print "Invalid Input. Try Again."
  8. Go to step 3
  9. Print "The Prime Numbers Are:"
  10. j=2
  11. If j<=x, go to step 12 or else go to step 22
  12. a=j
  13. count=0
  14. i=2
  15. If  i<a, go to step 16 or else go to step 19
  16. If a%i==0, go to step 17 or else go to step 18
  17. count++
  18. i++. Go to step 15
  19. If count==0, go to step 20 or else go to step 21
  20. Print j
  21. j++. Go to step 11
  22. Stop

Program

Buffered Reader

import java.io.*;
class PrimeList
{
    int x;
    
    void accept() throws IOException
    {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        while(x<=1)
        {
            System.out.println("Enter a number");
            x=Integer.parseInt(br.readLine());
            if(x<=1)
            {
                System.out.println("Invalid Input. Try Again");
            }
        }
    }
    
    boolean isPrime(int a)
    {
        int count=0;
        for(int i=2;i<a;i++)
        {
            if(a%i==0)
            {
                count++;
            }
        }
        if(count==0)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
    
    void printList()
    {
        System.out.println("The Prime Numbers Are:");
        for(int j=2;j<=x;j++)
        {
            if(isPrime(j)==true)
            {
                System.out.println(j);
            }
        }
    }
    
    void main() throws IOException
    {
        accept();
        printList();
    }
}

Scanner

import java.util.*;
class PrimeList
{
    int x;
    
    void accept() 
    {
        Scanner sc=new Scanner(System.in);
        while(x<=1)
        {
            System.out.println("Enter a number");
            x=sc.nextInt();
            if(x<=1)
            {
                System.out.println("Invalid Input. Try Again");
            }
        }
    }
    
    boolean isPrime(int a)
    {
        int count=0;
        for(int i=2;i<a;i++)
        {
            if(a%i==0)
            {
                count++;
            }
        }
        if(count==0)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
    
    void printList()
    {
        System.out.println("The Prime Numbers Are:");
        for(int j=2;j<=x;j++)
        {
            if(isPrime(j)==true)
            {
                System.out.println(j);
            }
        }
    }
    
    void main() 
    {
        accept();
        printList();
    }
}

Documentation

Variable Description

 Data Type Variable Name Description
 intx To accept a number from the user. 
 intcount To count the number of factors except 1 and itself 
 int i To act as a counter in loop 
 intj To act as a counter in loop 
 inta To parse value to method. 

Method Description

 Return TypeMethod Name Description 
 voidaccept() To accept data from the user 
 booleanisPrime(int) To check if the argument is Prime or not 
 void printList() To print the list of  prime numbers 
 voidmain() To act as an entry gateway for the compiler 

Output

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