Java Program:

Example:

import java.util.Scanner;
public class PrimeNummber {

  public static boolean prime(int n) {
    if(n<2)return false;
    for(int i=2; i<=n/2; i++) {
        if(n%i==0) return false;
      }
      return true;
    }

  public static void main(String []args) {
    int num;
    Scanner userInput = new Scanner(System.in);
    System.out.print("Enter any +ve number : ");
    num = userInput.nextInt();
    
    System.out.println("Your prime numbers between 1 & "+num+" are :");
    for(int i=2; i<num; i++)
      if(prime(i))System.out.print(i+" ");
  }
}

Output:

  Enter any +ve number : 20
  Your prime numbers between 1 & 20 are :
  2 3 5 7 11 13 17 19

You should know first :

Prime numbers: Numbers greater than 1 & have only 2 factors ( 1 and itself )

Main Logic to check prime :

public static boolean prime(int n){
  if(n<2)return false;
  for(int i=2;i<=n/2;i++) {
    if(n%i==0) return false;
  }
  return true;
}

We are running the loop n/2 Since, any number can have only factors under its half, except itself

To print all primes :

for(int i=2; i<num; i++) {
  //check every number for prime [ if(prime(i)) ], if prime then print the number
}