Thread: Validating user input if it is in the correct format.

Hi, my prof. asks us to write a conversion program in from binary to decimal, decimal to binary, etc.. It should have a menu system and a method should validate if the user input is number and is in correct format depending on the conversion. I have no problem making the code for the conversions, my ONLY problem is the validation method. kindly guide me on what is wrong with my code and what I should do to correct it. thanks!

The validation method should validate if the user inputted the correct format depending on the conversion used.

import java.io.*;
import java.lang.*;
 
public class scope {
 
	static String num;
	static int choice;
 
	public static void main(String[] args) throws NumberFormatException, IOException{
 
		welcomeScreen();
		getUserInput();
 
 
 
 
 
 
	}
 
	public static void welcomeScreen(){
 
		System.out.println("		******************************************");
		System.out.println("		* Welcome to the Number System Converter *");
		System.out.println("		******************************************");
		System.out.println();
	}
 
	public static void getUserInput() throws NumberFormatException, IOException{
 
		BufferedReader bReader = new BufferedReader(new InputStreamReader(System.in));
 
		do{
			System.out.println("Please choose from ne of the following:");
			System.out.println();
			System.out.println("[1] Decimal to Binary");
			System.out.println("[2] Binary to Decimal");
			System.out.println("[3] Decimal to Hexadecimal");
			System.out.println("[4] Decimal to Octal");
			System.out.println("[5] Exit");
			System.out.println();
 
			choice = Integer.parseInt(bReader.readLine());
 
			switch(choice){
 
				case 1: System.out.println(validateNumber(num, choice)+ decimalToBinary(num)); break;
				case 2: System.out.println(); break;
				case 3: System.out.println(); break;
				case 4: System.out.println(); break;
 
			}
 
		}while(choice != 5);
 
	}
 
	public static boolean validateNumber(String num, int choice) throws IOException{
 
		BufferedReader bReader = new BufferedReader(new InputStreamReader(System.in));
 
		System.out.println("Please enter a number.");
		num = bReader.readLine();
 
		if(choice == 2){
			for(int i=0; i<num.length(); i++){
				char a=num.charAt(i);
				if(a== 0 || a== 1){
 
					return true;
			}	
 
			else{
 
				return false;
			}
 
		}
		}
 
		else if(choice == 1||choice == 3||choice == 4){
 
			try{
 
				int n = Integer.parseInt(num);
				System.out.println(n);
 
			} catch (NumberFormatException nfe){
 
				System.out.println("Please only enter numbers!");
 
			}
 
		}
		return false;
 
}
 
	public static String decimalToBinary(String num){
 
		int n = Integer.parseInt(num);
		String toBinary = Integer.toBinaryString(n);
		System.out.println("The answer is: " + toBinary);
 
		return toBinary;
 
 
	}
}

this is the output when I run the code.

		******************************************
		* Welcome to the Number System Converter *
		******************************************
 
Please choose from ne of the following:
 
[1] Decimal to Binary
[2] Binary to Decimal
[3] Decimal to Hexadecimal
[4] Decimal to Octal
[5] Exit
 
//I chose 1 to convert from decimal to binary conversion
 
1
 
//here is the problem, when i enter a number, it tells me the problem below.
Please enter a number.
 
1
1
Exception in thread "main" java.lang.NumberFormatException: null
	at java.lang.Integer.parseInt(Unknown Source)
	at java.lang.Integer.parseInt(Unknown Source)
	at scope.decimalToBinary(scope.java:101)
	at scope.getUserInput(scope.java:47)
	at scope.main(scope.java:12)

Originally Posted by Norm

Can you explain what happens when the code is execute? What does it do wrong?
Can you copy the console window from when the code is executed and paste it here?
Add some comments to it describing what the program is doing wrong and explain what the code should do.

edited it. the output if i run the code is there. i added comments in between the codes to let you know what i did.

can you explain it in english please? if you know what i mean..