System.exit(0) error in program.

Question

Any help appreciated. Let me first put my code here:
Converts money using CurrencyConverter, there's another class I haven't posted here!! (which calculates conversion)
import java.util.* ;
public class CurrencyConverterTester
  public static void main(String[] args)
    Scanner in = new Scanner(System.in) ;
    System.out.println("How many euros is one dollar?") ;
    String input = in.nextLine() ;
    double rate = Double.parseDouble(input) ;
    CurrencyConverter myconverter = new CurrencyConverter(rate);
    System.out.println("Dollar value (Q to quit)") ;
    input = in.nextLine() ;
    while (true) {
      if (input == "Q"){
        System.exit(0);
        break;}
      else {
      System.out.println(input + " dollar = " +  myconverter.convert(Double.parseDouble(input)) + " euros");
      input = in.nextLine();
      continue;}   
}The issue is that the program won't terminate when I enter Q as my input?? I'm guessing it might have something to do my conversion from string to double in the 'else' statement, but not sure. Also, if you can suggest a more simpler method for the last 'while' statement, that would be helpful as well. Thanks. (noob to java)

Answer 1

paulcw wrote:
Curse Gosling for making + syntactic sugar for string append, but not overloading any other operators.@Paul
What did you expect from a duck ;-)
Seriously, yep, maybe it would have been better for noobs if the syntax of Java Strings was consistent with other kinds of Object... or maybe it would have been better if == was compiler-magiced into stringA.equals(stringB)... and the left and right shift operators have promise, and of course that way lies MADNESS, as witnessed in C++ ;-)
@OP... You are (IMHO) using the break, continue and especially System.exit() inappropriately... see my comments in your code...
    System.out.println("Dollar value (Q to quit)") ;
    input = in.nextLine() ;
    // while-true-loops are a "fairly usual" way of doing this kind of stuff,
    // but that doesn't make them "the preferred option". Typically you would
    // explore more "vanilla approaches" (which don't rely on the break and
    // continue statements)... leaping straight to the "tricky approach" means
    // you haven't (most often out of sheer laziness) thought it through... and
    // ALL good computer programs are thoroughly thought through (yep, try
    // saying that three times fast, especially after three beers).
    while (true) {
      // You don't compare Strings using ==
      if ( "Q".equalsIgnoreCase(input) ) {
        // Either of the following statements will do the job. The call to exit
        // will exit the JVM, therfore the break statement cannot be reached,
        // so it is superflous, so it's presence is just a bit confusing. Having
        // said that, "real programmers" don't use System.exe routinely, only in
        // the case of an emergency, such as handling a fatal-error, such as an
        // out-of-memory condition... an even then it's usually indicative of a
        // poor "system design", because it terminates the JVM which is running
        // this program without giving anything else in the program a chance to
        // clean-up after itself... like ask the user if they want to save there
        // work, or whatever.
        // I would use break (if anything) instead of System.exit
        // ... and if I wrote the Java tutorials, exit wouldn't be mentioned at
        // all until both "normal" flow control, and exception handling had both
        // been thoroughly covered.
        System.exit(0);
        break;
      } else {
        // I would break this line up, probably into three lines, simply becuase
        // it's syntatically "a long line".
        // Also the name "myconverter" doesn't tell what the class is/does.
        // IMHO, currencyConverter would be a better (more meaningful) name.
        // HERE'S HOW I WOULD WRITE IT
        // double dollars = Double.parseDouble(input);
        // double euros = currencyConverter.convert(dollars);
        // System.out.println(input + " dollar = " + euros + " euros");
        System.out.println(input + " dollar = " +  myconverter.convert(Double.parseDouble(input)) + " euros");
        input = in.nextLine();
        // This continue statement is superflous. continue says "go back to the
        // top of loop, reevaluate the loop-condition (true in this case) and
        // (if the condition is still true) "Play it again Sam".
        // ... which is exactly what will happen without this continue statement
        // and hence (IMHO) your code is easier to follow without it, simply
        // because another programmer may waste there time trying to figure out
        // WHY that continue statement is present.
        continue;
    }*ALSO:* The format of that code totally sucks. Braces all over the place; improper indentation. No wonder you're struggling to read your own code. Please read and abide the [The Java Code Conventions|http://java.sun.com/docs/codeconv/] (at least until you have the requisite experience to formulate credible and compelling arguments as to why your "style" is superior to the standard, and that's no small ask). Yes this is *important*... trust me on this (for now)... especially if you are going to ask for help on the forums... You're effectively wasting our time asking us to decipher your code because you are too lasy to format it correctly... and I for one find that "self entitled" attitude ugly, and offensive... Help us help you... you know?
And BTW.... Here's how I would actually do it.... no funky while-true, break, or continue... just a plain-ole'-while-woop....
package forums;
import java.util.Scanner;
public class KRC
  private static final Scanner SCANNER = new Scanner(System.in);
  public static void main(String[] args) {
    try {
      String input = null;
      while ( !"Q".equalsIgnoreCase(input=enterDollarAmount()) ) {
        System.out.println(input + " dollars is " +  Double.parseDouble(input) + " euros.");
        System.out.println();
    } catch (Exception e) {
      e.printStackTrace();
  private static String enterDollarAmount() {
    System.out.print("Please enter an amount in dollars : ");
    return SCANNER.nextLine();
}Edited by: corlettk on 25/10/2009 10:21 ~~ Distant Monarching Forum Markup!