HumanizeNumber.java

import java.util.*;
/**
* Given any integer, print an English phrase that describes the integer (e.g., "One Thousand, Two Hundred Thirty Four").
*/
public class HumanizeNumber {
	
	public static String humanize(int number) {
		List<Integer> composites = splitIntoDecComposites(number);
		StringBuilder sb = new StringBuilder();
		for (Integer composite : composites) {
			if (composite == 0 ) continue;
			sb.append(wordFor(composite)).append(" ");
		}
		return sb.toString();
	}

	private static String wordFor(int i) {
		String s = Integer.toString(i);
		int leading = leadingDigit(i);
		switch(s.length()) {
			case 1:
				return wordForSmall(leading);
			case 2:
				return wordForTens(i);
			case 3:
				return wordForSmall(leading) + " Hundred";
			case 4:
				return wordForSmall(leading) + " Thousand";
			case 5:
				return wordForTens(leading * 10) + " Thousand";
			case 6:
				return wordForSmall(leading) + " Hundred Thousand";
			case 7:
				return wordForSmall(leading) + " Million";
			case 8:
				return wordForTens(leading * 10) + " Million";
			case 9:
				return wordForSmall(leading) + " Hundred Million";
			case 10:
				return wordForSmall(leading) + " Billion";
			default:
				throw new IllegalArgumentException("Unable to understand " + i);
		}
	}

	private static String wordForSmall(int i) {
		switch (i) {
			case 1: return "One";
			case 2: return "Two";
			case 3: return "Three";
			case 4: return "Four";
			case 5: return "Five";
			case 6: return "Six";
			case 7: return "Seven";
			case 8: return "Eight";
			case 9: return "Nine";
			default: throw new IllegalArgumentException(i + " exceeds range of small");
		}
	}
	private static String wordForTeen(int i) {
		switch (i) {
			case 10: return "Ten";
			case 11: return "Eleven";
			case 12: return "Twelve";
			case 13: return "Thirteen";
			case 14: return "Fourten";
			case 15: return "Fifteen";
			case 16: return "Sixteen";
			case 17: return "Seventeen";
			case 18: return "Eighteen";
			case 19: return "Nineteen";
			default: throw new IllegalArgumentException(i + " exceeds range of small");
		}
	}
	private static String wordForTens(int i) {
		switch (i) {
			case 10: return "Ten";
			case 20: return "Twenty";
			case 30: return "Thirty";
			case 40: return "Forty";
			case 50: return "Fifty";
			case 60: return "Sixty";
			case 70: return "Seventy";
			case 80: return "Eighty";
			case 90: return "Ninety";
			default: throw new IllegalArgumentException(i + " exceeds range of small");
		}
	}

	private static List<Integer> splitIntoDecComposites(int number) {
		List<Integer> composites = new ArrayList<Integer>();
		String text = Integer.toString(number);
		for (int i =0; i < text.length(); i++) {
			Integer sigDigit = Integer.parseInt(Character.toString(text.charAt(i)));
			int decScale = text.length() - i - 1;
			composites.add(sigDigit * (int) Math.pow(10, decScale)); 
		}
		return composites;
	}

	private static int leadingDigit(int i) {
		String text = Integer.toString(i);
		return Integer.parseInt(Character.toString(text.charAt(0)));
	}

	public static void main(String... args) {
		for (int i = 1; i < 40; i++) {
			int number = (int)Math.pow(2,i);
			System.out.println(number + " " + humanize(number));
		}
	}

}