// Using isLetterOrDigit() to check whether user_id or any string entered by user has a digit and/or letter and no other symbols or characters. 
import java.util.Scanner;
class test {
public static void main(String ar[]) {
	Scanner sc=new Scanner(System.in);
	boolean flag =false; // keep the switch/flag off in the beginning. 
	System.out.println("Enter user_id:");
	String user_id = sc.nextLine();
	// extract each character from user_id by charAt() method.
	for(int i=0;i<user_id.length();i++)
	{
		char ch = user_id.charAt(i); // get the 0th character, 1st character and so on..
		// if ch is a letter and/or digit, its valid for user_id. If not, its invalid.  
		if(! Character.isLetterOrDigit(ch)) // using ! to check the invlaid condition.
		{
			flag=true; // if any special symbol is found, set flag to true. 
			System.out.println("USER ID CAN ONLY CONTAIN LETTERS AND DIGITS.. ");// no need to check rest of characters now. so break the loop.
			break;
		}
	}
	if(flag==false) {// if flag was still holding old value (false), then the user_id is valid.
	System.out.println("VALID USER ID!!");
	}
}
}

/* OUTPUT
Enter user_id:
user_1
USER ID CAN ONLY CONTAIN LETTERS AND DIGITS..

Enter user_id:
user@123
USER ID CAN ONLY CONTAIN LETTERS AND DIGITS..

Enter user_id:
user1
VALID USER ID!!
*/