DecimalParser.java

import java.io.InputStream;
import java.io.InputStreamReader;

class DecimalParser {
	public static void main(String args[]) throws Exception {
		if (args.length==0) {
			System.out.println("Usage: java NumberParser value");
			System.exit(1);
		}
		double val = MyParseDecimal(args[0]);
		System.out.println("Value="+val);
	}

	enum StateDecimal { SSTART, SINTEGER, SDECIMAL, SEND };

	public static double MyParseDecimal(String s) throws Exception {
		
		StateDecimal state;

		state = StateDecimal.SSTART;

		int i = 0;
		double divider = 10;
		double value = 0;

		while ( i < s.length() && state != StateDecimal.SEND) {
			char ch = s.charAt(i);
			switch (state) {
				case SSTART: 
				if (Character.isDigit(ch)) {
					state = StateDecimal.SINTEGER;
					value = Character.getNumericValue(ch);	
					i++;
				}
				else {
					throw new Exception("Bad format");
				}
				break;
				case SINTEGER:
				if (Character.isDigit(ch)) {
					state = StateDecimal.SINTEGER;
					value = 10.0*value + Character.getNumericValue(ch);
					i++;
				}
				else if (ch == '.') {
					state = StateDecimal.SDECIMAL;
					i++;	
				}
				break;
				case SDECIMAL:
				if (Character.isDigit(ch)) {
					value = value + Character.getNumericValue(ch)/divider;
					divider = divider * 10;
					i++;
				}
				else {
					throw new Exception("Bad format");
				}
				break;
			}
			
		}

		return value;


	}

}