Thread: I'm confused...

I was writing a code on how to say a number but my code was horrible so I asked for help. A lot helped but this code caught my attention:

import javax.swing.*;
public class word {
 
	private static final String[] simple= { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine",
											"ten", "evelen", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen",
											"eightteen", "nineteen" };
 
	private static final String[] tens= { null, null, "twenty", "thirty", "fourty", "fifty", "sixty", "seventy", "eighty", "ninety" };
 
	public static String say(int number) { 
		return say(number, true); //default true
 
	}
 
	private static String say(int number, boolean sayZero) {
		//simulate 123
		if (number < 0)				return "negative "+say(-number, sayZero); //is 123 < 0? False
 
		if (number == 0)			return sayZero?simple[number]:""; //is 123 < 0 ? False
 
		if (number < 20) 			return simple[number];  //is 123 < 0? false
 
		if (number < 100)			return tens[number/10]+say(number%10, false); //is 100 < 100? false
 
		if (number < 1000) 			return say(number/100, false)+" hundred "+say(number%100); //is 100 < 1000? true--execute say(1) hundred   
 
		if (number < 1000000) 		return say(number/1000, false)+" thousand"+say(number%1000, false);
 
		if (number < 1000000000)	return say(number/1000000, false)+" million "+say(number%1000000, false);
 
		return say(number/1000000000, false)+"billion "+say(number%1000000000, false);
	}
 
	public static void main(String[] args) {
 
		String say = JOptionPane.showInputDialog("Enter a number");
		int speak = Integer.parseInt(say);
		JOptionPane.showMessageDialog(null,say(speak));
 
	}
 
}

This code is 100% effective but confusing. So here are my questions:
How does the say() method work?
Why is it that it works even if its not on a loop?
What is the role of boolean here?