ParseNum.java

public class ParsingNumbers {
	static String expression;
	static int currentCharPosition;
	static char Look;
	static double result;
	// parse integer or double of any length.
	public static double parseNum() {
		double number = 0.0;
		int digit = 0;
		double scale = 1;
		// parse integer part
		if (Look != '.') {
			while (Character.isDigit(Look)) {
				digit = Character.getNumericValue(Look);
				number = number * 10 + digit;
				nextChar();
			}
		}
		// parse decimal part
		if (Look == '.') {
			nextChar(); // eat the '.'
			while (Character.isDigit(Look)) {
				scale = scale * 0.1;
				digit = Character.getNumericValue(Look);
				number = number + (scale * digit);
				nextChar();
			}
		}
		return number;
	}
	// advance the next char from input.
	private static void nextChar() {
		if (currentCharPosition < expression.length()) {
			Look = expression.charAt(currentCharPosition);
			currentCharPosition += 1;
		} else {
			Look = ' '; // mark end of input.
		}
	}
	// test
	public static void main(String[] args) {
		expression = "1985.35";
		nextChar(); // start the current char.
		System.out.println("Number Result: " + parseNum());
	}
}