I need to create any simple compiler using java, that takes input only numbers

import java.io.InputStream;
import java.io.IOException;

public class Comp {
	private static char firstdigit;
	private static char seconddigit;
	private static int sum;
	
	public static void main(String[] args) throws IOException {

		firstdigit = (char) System.in.read();
		seconddigit = (char) System.in.read();
		
		try {
			expresion();
		}
		catch (ParseException e) {
			System.err.println(e.getMessage());
		}
	}

		private static void expresion()  throws IOException, ParseException{
			System.out.println(firstdigit + " "+ seconddigit );	
			System.out.println(sum);	
		}
		
		
		private static int sum() {
			sum =(firstdigit + seconddigit);
			return sum;
		}


		private static char seconddigit() throws IOException, ParseException{
			if(!Character.isDigit(seconddigit)) {
				throw new ParseException("Integer expected..");
			}
				return seconddigit;
			}
		private static char firstdigit() throws IOException, ParseException{
			if(!Character.isDigit(firstdigit)) {
				throw new ParseException("Integer expected..");
		}
			return firstdigit;

		}
		static class ParseException extends Exception {
			ParseException(String reason){
				super(reason);
			}
		}
}