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Newsgroups: comp.lang.java.programmer
From: Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at>
Subject: Re: Call by Result
References: <nkc3v61jg7i21l7n482ce561dq9fdogg3m@4ax.com>
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Date: 10 Jun 2011 07:33:12 GMT
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Gene Wirchenko <genew@ocis.net> wrote:
>      I am writing a simple preprocessor.  I have a few spots where a
> string needs to be parsed.  I want to call something like this:
>           String ReturnString="";
>           boolean DidItWork=GetString(ReturnString);
>           if (!DidItWork)
>              // too bad
> It is not acceptable to have a special String value mean failure.  I
> want the method to be able to return any potential string.

There's three basic ways to do it:

1) Return null to indicate failure. (Unless you consider null to
   also be a valid String-value - That typically depends on the
   context and I didn't read your wording as being explicit on that.)
    String retVal = getString();
    if (retVal == null) { /* too bad */ }
    This won't work, though, if you need to return a String both
    on failure and success.

2) instead of the string, pass a mutable container of a string:
    String[] stringContainer = new String[1];
    boolean didItWork = getString(stringContainer);
    if (didItWork) { /* stringContainer[0] has the string */ }

  and within GetString:
    public boolean getString(String[] strCont) {
            strCont[0] = "blah"; return true;
    }

3) encode the String: e.g.: prepend a particular text to "success"-
    strings, and a different text to "failure"-strings:

    public String getString(String[] strCont) {
        if (ok()) { return "+" + okMessage; }
        else      { return "-" + errMessage; }
    }
  use:
    String retVal = getString();
    if (retVal == null || retVal.length() == 0) { /* internal error */ } else
    if ( retVal[0] == '+' ) { 
        // All's well that ends well:  retVal.subString(1)
    } else if ( retVal[0] == '-' ) {
        // Uh-Oh!:  retVal.subString(1)
    } else {  /* internal error */ }


PS: Surely, someone will soon point out coding-conventions about
  upper-/lower-casing different kinds of identifiers. I dare to agree
  in advance.  (Btw., I changed to conformant casing in my examples.)