Cannot assign a value to final variable

Question

Here's my code:

public String getGPA() {
        String gpa = null;
        final String responseBody1;
        new Thread(new Runnable(){
            public void run()
            {
                try {
                    HttpGet httpGet_gpa = new HttpGet("https://somedomain.com" + getRegId() + "&format=P");
                    HttpResponse response = httpClient.execute(httpGet_gpa);
                    responseBody1 = EntityUtils.toString(response.getEntity());
                } catch (ClientProtocolException cpe) {
                    // Ignore
                } catch (IOException ioe) {
                    // Ignore
                }
            }
        }).start();
        Document gpa_doc = Jsoup.parse(responseBody1);
        Element p = gpa_doc.select("p").first();
        gpa = p.text().replace("Your overall GPA (Grade Point Average) to date is:", "");
        return gpa;
    }

I'm getting this error: cannot assign a value to final variable responseBody1. What does this mean? I tried removing the final modifier but it tells me I need to make it final, but when I do i get that error. Any clues on why this is happening and how to fix it?

Answer 1

That's the problem related to anonymous classes. Since Java's black magic creates a copy of the environment in which the class is instantiated to pass variables to the inner class, you must declare them as final. Changing responseBody1 wouldn't change the original variable but a local reference to it (and since String is immutable you won't obtain what you think).

You should pass through a wrapper or a method of the outer class.

Answer 2

The final attribute means that you pledge not to modify the variable. That's why you can't modify your variable.

If you are forced to use a final, it's super easy : just do like so

String newString = responseBody1;

And then modify newString as you will

edit:

Just realized that it's not what you want. You can use a function that you will call in your anonymous class

Update :

private static String responseBody1;

public String getGPA() {
    new Thread(new Runnable(){
        public void run()
        {
           fetchResponse();
        }
    }).start();
    Document gpa_doc = Jsoup.parse(responseBody1);
    Element p = gpa_doc.select("p").first();
    gpa = p.text().replace("Your overall GPA (Grade Point Average) to date is:", "");
    responseBody1 = null;
    return gpa;
}

public static void fetchResponse(){
try {
       HttpGet httpGet_gpa = new HttpGet("https://somedomain.com" + getRegId() + "&format=P");
       HttpResponse response = httpClient.execute(httpGet_gpa);
       responseBody1 = EntityUtils.toString(response.getEntity());
    } catch (ClientProtocolException cpe) {
    } catch (IOException ioe) {}
}

Now I'm not sure if it has to be a static function and a static responseBody1. But I'm guessing so.

Answer 3

If you create an anonymous class that implements an interface (such as Runnable), any local variables that it accesses must be final, and their values have to be determined at that point. The anonymous class isn't allowed to change those variables in a way that the outer method could see the changes. (This is a feature of Java; other languages allow outside local variables to be changed.)

I can't recommend a solution, because it looks to me like your method won't work anyway. You create a new thread and use start() on it, and it looks like you're expecting this thread to modify responseBody1. But your method then tries to use the value of responseBody1. You cannot tell whether the thread has gotten to the point where it modifies responseBody1, since you don't do anything to wait for the thread or synchronize with it in any way. Something else about the design will need to change.

However, a possible solution is to define a new class that contains a String member:

static class StringHolder {
    String responseBody1;
}

Then create a new StringHolder and assign a final local variable to this. Although the reference to the StringHolder can't be changed, the responseBody1 member can still be changed by the thread.