JAVA - 如何从for循环外部访问FOR循环内的变量

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?

Answer 1

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();

Answer 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 .

Answer 3

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();    
}

Answer 4

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 feed and f variables 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);
}

Answer 5

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.