New to Java

I only know how to do a program that converts only 4 numbers, but it to convert any amount of digits the number enters, I think you need to use a loop or something, but I'm not sure how. Could someone please help me? This is my code so far:

import javax.swing.JOptionPane;

public class bintodec {

	public static void main (String[] args) {
	
		String input;
		int number,digit1,digit2,digit3,digit4,result;
		
		input = JOptionPane.showInputDialog ("Enter a binary number.");
		
		number = Integer.parseInt(input);

		digit1 = ((number % 10000) - (number % 10000 % 1000)) / 1000;
		digit2 = ((number % 1000) - (number % 1000 % 100)) / 100;
		digit3 = ((number % 100) - (number % 100 % 10)) / 10;
		digit4 = (number % 10);
		
		result = (digit1 * 8) + (digit2 * 4) + (digit3 * 2) + (digit4 * 1);
		
		System.out.println ( "Binary number: " + input + "\nConverted Decimal Number: " + result);
		
		System.exit( 0 );
		
	} // main
	
} // bintodec

any help is super-appreciated.