ParseTest.java

public class ParseTest
{

    public static void main(String[] args)
    {
        if ( args.length == 1 )
            System.out.println( ParseTest.parseInt( args[0] ) );
        else
            System.out.println( "Please supply an argument!" );
    }

    public static int parseInt(String str)
    {
        int result = 0;
        boolean isNegative = str.charAt( 0 ) == '-';
        String parsedStr = isNegative ? str.substring( 1 ) : str;
        int place = parsedStr.length() - 1;

        for ( int i = 0; i < parsedStr.length(); i++ )
        {
            char currentChar = parsedStr.charAt( i );

            if ( !Character.isDigit( currentChar ) )
                throw new NumberFormatException("Not an integer.");

            result += Character.getNumericValue( currentChar ) * Math.pow( 10, place );
            place--;
        }

        if ( isNegative )
            result = -result;

        return result;
    }
}