JAVA - How to access variables inside a FOR loop, from outside the for loop

Question

My code is to add RSS feeds to a list - and the code originally was only to pull one feed from the first position in a list, and add this object to another list.

This was the original code:

public static List<Feed> getFeedsFromXml(String xml) {
      Pattern feedPattern = Pattern.compile("<feed>\\s*<name>\\s*([^<]*)</name>\\s*<uri>\\s*([^<]*)</uri>\\s*</feed>");


      Matcher feedMatch = feedPattern.matcher(xml);
      while (feedMatch.find()) {
          String feedName = feedMatch.group(1);
          String feedURI = feedMatch.group(2);
          feeds.add(new Feed(feedName, feedURI));
      }

      return feeds;
}

@POST
@Consumes(MediaType.APPLICATION_XML)
@Produces(MediaType.APPLICATION_XML)
public String addXmlFeed() throws IOException
{
    int i = 0;
    String stringXml = "<feed><name>SMH Top Headlines</name><uri>http://feeds.smh.com.au/rssheadlines/top.xml</uri></feed><feed><name>UTS Library News</name>";
    getFeedsFromXml(stringXml);
    Feed f = (Feed) feeds.get(0);
    feedList.add(f);
    String handler = "You have successfully added: \n"; 
    String xmlStringReply = "" + f + "\n";

    feedList.save(feedFile);
    return handler + xmlStringReply;

}

Everything was going well, and then I decided to implement a for loop for handling the adding of more than one feed to the list, and I tried the following (only the code for the second method in question):

@POST
@Consumes(MediaType.APPLICATION_XML)
@Produces(MediaType.APPLICATION_XML)
public String addXmlFeed() throws IOException
{
    int i = 0;
    String stringXml = "<feed><name>SMH Top Headlines</name><uri>http://feeds.smh.com.au/rssheadlines/top.xml</uri></feed><feed><name>UTS Library News</name>";
    getFeedsFromXml(stringXml);
    for (Feed feed: feeds)
    {
        Feed f = (Feed) feeds.get(i++);
        feedList.add(f);
        String handler = "You have successfully added: \n"; 
        String xmlStringReply = "" + f + "\n";
    }

    feedList.save(feedFile);
    return handler + xmlStringReply;

}

Now I'm sure this is a basic problem, but now in the line:

    return handler + xmlStringReply;

handler and xmlStringReply cannot be resolved to a variable as they are within the FOR LOOP.

Is there any easy way around this?

assylias · Accepted Answer · 2012-08-30 09:23:55Z

12

The scope of those 2 variables is limited to the for loop. To access them outside the loop, you need to increase their scope by declaring them before the loop:

String handler = ""; 
String xmlStringReply = "";
for (Feed f: feeds) {
    feedList.add(f);
    handler = "You have successfully added: \n"; 
    xmlStringReply = "" + f + "\n";
}

feedList.save(feedFile);
return handler + xmlStringReply;

Also, your current code overwrites the value of your strings at each loop, whereas you probably meant to concatenate the values. In that case, you could use a StringBuilder instead of string concatenation:

StringBuilder xmlStringReply = new StringBuilder("You have successfully added: \n");
for (Feed f: feeds) {
    feedList.add(f);
    xmlStringReply.append(f + "\n");
}

feedList.save(feedFile);
return xmlStringReply.toString();

edited Aug 30, 2012 at 9:23

answered Aug 30, 2012 at 8:37

assylias

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9 Comments

Stefan Neubert Over a year ago

Note that you might not want to overwrite the values, but append some information to the Strings instead.

assylias Over a year ago

@StefanNeubert Yes good point - I had missed the absence of +.

Infinitexistence Over a year ago

@assylias thank you also for a clear & concise response, both of the upper responses were very helpful, although I have accepted yours as it explains in more detail, which was extremely useful. All the best!

assylias Over a year ago

@PetarMinchev Yes it seems... Whatever! I +1'ed you FWIW ;-)

Petar Minchev Over a year ago

@assylias - Thanks, I have upvoted you too:) Lol, I did not see the i++ at all.

|

Orabîg · Accepted Answer · 2012-08-30 08:47:47Z

2

The question you need to answer is "what do I want to return if I add several feeds ?".

Maybe you'd like to return "You have successfully added : feed1 feed2 feed3\n"

In that case, the code is :

            StringBuilder response = new StringBuilder( "You have successfully added: ");
            for (Feed feed: feeds)
                {
                    feedList.add(feed);
                    response.append(f.toString()).append(" ");
                }
            feedList.save(feedFile);
            return response.toString();

By the way, your feedand fvariables are just the same and redondant !

Don't write :

int i = 0;    
for (Feed feed: feeds)
{
    Feed f = (Feed) feeds.get(i++);
    feedList.add(f);
}

but

for (Feed feed: feeds)
{
    feedList.add(feed);
}

answered Aug 30, 2012 at 8:47

Orabîg

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1 Comment

Infinitexistence Over a year ago

This makes sense, the reason it was coded as the prior, was that initially, the functional requirement was that I only pull the FIRST feed from the list, and add it, and now the next functional requirement was to then add a mechanism for accepting multiple feeds. I will investigate your response.

Adeel Ansari · Accepted Answer · 2012-08-30 09:10:35Z

2

You need to accumulate the result into a variable. I am using StringBuilder because it makes string concatenation efficient.

@POST
@Consumes(MediaType.APPLICATION_XML)
@Produces(MediaType.APPLICATION_XML)
public String addXmlFeed() throws IOException
{
    String stringXml = "<feed><name>SMH Top Headlines</name><uri>http://feeds.smh.com.au/rssheadlines/top.xml</uri></feed><feed><name>UTS Library News</name>";
    getFeedsFromXml(stringXml);

    StringBuilder replyBuilder = new StringBuilder("You have successfully added: \n");
    for (Feed feed : feeds)
    {
        feedList.add(feed);

        String xmlStringReply = feed  + "\n";
        reployBuilder.append(xmlStringReply); 
    }

    feedList.save(feedFile);
    return replyBuilder.toString();    
}

edited Aug 30, 2012 at 9:10

Adeel Ansari

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answered Aug 30, 2012 at 8:39

Petar Minchev

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3 Comments

Infinitexistence Over a year ago

@Peter Minchev, excellent, very helpful thankyou. Just one question, in your code, I assume the String variable "xmlStringReply" should be declared outside of the FOR loop as well as String variable "Handler". Is this correct?

assylias Over a year ago

@AlexM No it does not have to because it is only use within the loop.

Petar Minchev Over a year ago

@AlexM - Handler should be outside, but xmlStringReply there is no need.

Adeel Ansari · Accepted Answer · 2012-08-30 09:14:06Z

2

Because, now they became out of scope.

Beside the original error -- you can easily fix that using other suggestions, I would like to suggest that you should not make feeds as instance variable. I can see your method getFeedsFromXml() is returning the list. So, I think it would have been better if you define that variable inside that method. And then, call the method like,

List<Feed> feeds = getFeedsFromXml(stringXml);

Or in case, this doesn't give you the desired behaviour, then you should rename the method to something, loadFeedsFromXml(). Making that as instance variable may result in threading issues.

Now, trying to improve on your looping,

StringBuilder xmlStringReply = new StringBuilder("You have successfully added: \n");
for (Feed feed: feeds) {
    feedList.add(feed);
    xmlStringReply.append(f + "\n");
}

feedList.save(feedFile);
return xmlStringReply.toString();

Moreover, I found that your feedList is also a instance variable. And this again can cause threading issues, as it doesn't sound immutable or stateless. Synchronising the methods will give you performance issues. See if you can make it local to this method. A rule of thumb is to keep variable scope as narrow as possible.

edited Aug 30, 2012 at 9:14

answered Aug 30, 2012 at 8:39

Adeel Ansari

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3 Comments

Adeel Ansari Over a year ago

Hey, folks. What's wrong with this answer? I think you folks can't wait.

Nick Over a year ago

I downvoted it because the original answer didn't make sense. I've removed the downvote now.

Adeel Ansari Over a year ago

@Nick: To me the original answer made a lot of sense, but yes may not be very clear for the beginner. "Because, now they became out of scope", was my original answer. Anyway, thanks for revoking the negative cast.

Robert · Accepted Answer · 2012-08-30 13:41:46Z

1

A good rule of thumb is to view scope like this:

 { //This is a constructor

  int i;

} // This is a deconstructor

anything that is created / instantiated between the curlies only lives inside the curlies. Whenever your working with variables and loops:

for(int i = 0; i < 10; i++){

 //some code here
 } // after this curly i is no longer in scope or accessible.

answered Aug 30, 2012 at 13:41

Robert

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