Sybex 829 errata. Ch 13 Concurrency review question.

[Anil Philip]

tb864585.JaSE17SG.c13.04

    static void f6() {
        ScheduledExecutorService  service =   // w1
                Executors.newSingleThreadScheduledExecutor();
        service.scheduleWithFixedDelay(() -> {
            System.out.println("Open Zoo");
        }, 0, 1, TimeUnit.MINUTES);
        var result = service.submit(() ->   // w3
                System.out.println("Wake Staff"));
        System.out.println(result.get());   // w4 <<<<<<< compile error here
    }
    public static void main(String[] args) {
        f6();
    }

After fixing the errors as in the solution, there is another compile error on w4.

java: unreported exception java.lang.InterruptedException; must be caught or declared to be thrown

[Ron McLeod]

According to the documentation Future#get throws InterruptedException and ExecutionException.

[Anil Philip]

Ron McLeod wrote:According to the documentation Future#get throws InterruptedException and ExecutionException.

I agree. The issue is that should have been another option in the question - a further action to get it to compile

[Ron McLeod]

Maybe, but without knowing the question, its hard to comment any further.

[Anil Philip]

Ron McLeod wrote:Maybe, but without knowing the question, its hard to comment any further.

original question:

Which lines need to be changed to make the code compile? (Choose all that apply.)


ExecutorService service =   // w1
   Executors.newSingleThreadScheduledExecutor();
service.scheduleWithFixedDelay(() -˃ {
   System.out.println("Open Zoo");
   return null;   // w2
}, 0, 1, TimeUnit.MINUTES);
var result = service.submit(() -˃   // w3
   System.out.println("Wake Staff"));
System.out.println(result.get());   // w4

Original answer is w1 and w2 only.
I think it should be w4 also.

[Ron McLeod]

Hmm - I'm not sure what is meant by to make the code compile.  Obviously to successfully compile the code it needs to be placed in a method inside a class. If the method declares that it throws Exception (or the specific exceptions), line w4 would not need to be changed.

void runTest() throws Exception {
    ScheduledExecutorService service = // w1
            Executors.newSingleThreadScheduledExecutor();
    service.scheduleWithFixedDelay(() -> {
        System.out.println("Open Zoo");
    }, 0, 1, TimeUnit.MINUTES);
    var result = service.submit(() -> // w3
    System.out.println("Wake Staff"));
    System.out.println(result.get()); // w4
}

[Anil Philip]

Ron McLeod wrote:Hmm - I'm not sure what is meant by to make the code compile.  Obviously to successfully compile the code it needs to be placed in a method inside a class. If the method declares that it throws Exception (or the specific exceptions), line w4 would not need to be changed.

void runTest() throws Exception {
    ScheduledExecutorService service = // w1
            Executors.newSingleThreadScheduledExecutor();
Do you have a copy of the book? You can look at it. 
    service.scheduleWithFixedDelay(() -> {
        System.out.println("Open Zoo");
    }, 0, 1, TimeUnit.MINUTES);
    var result = service.submit(() -> // w3
    System.out.println("Wake Staff"));
    System.out.println(result.get()); // w4
}

[Paul Clapham]

Ron McLeod wrote:Hmm - I'm not sure what is meant by to make the code compile.  Obviously to successfully compile the code it needs to be placed in a method inside a class. If the method declares that it throws Exception (or the specific exceptions), line w4 would not need to be changed.

Or alternatively, you could delete line w4... or turn it into a comment... or do any number of things to it to make the code compile.

[Jeanne Boyarsky]

In the book, it is common to have code snippets. It says in the intro to assume any surrounding code is correct. So you can assume imports/class/method signature. I made just the two changes the explanation suggests in the book and it compiles. Remember that you are assuming the method signature isn't the problem so no need to handle the exception:

import java.util.concurrent.*;
public class MyClass {
    public static void main(String args[]) throws Exception {
        ScheduledExecutorService service = // w1
            Executors.newSingleThreadScheduledExecutor();
        service.scheduleWithFixedDelay(() -> {
         System.out.println("Open Zoo");   // w2
       }, 0, 1, TimeUnit.MINUTES);
      var result = service.submit(() -> // w3
     System.out.println("Wake Staff"));
     System.out.println(result.get()); // w4
    }
}

[Anil Philip]

Jeanne,
Do you not get an error on w4?
As I reported in my OP, w4 errors out

java: unreported exception java.lang.InterruptedException; must be caught or declared to be thrown

Jeanne Boyarsky wrote:In the book, it is common to have code snippets. It says in the intro to assume any surrounding code is correct. So you can assume imports/class/method signature. I made just the two changes the explanation suggests in the book and it compiles. Remember that you are assuming the method signature isn't the problem so no need to handle the exception:

import java.util.concurrent.*;
public class MyClass {
    public static void main(String args[]) throws Exception {
        ScheduledExecutorService service = // w1
            Executors.newSingleThreadScheduledExecutor();
        service.scheduleWithFixedDelay(() -> {
         System.out.println("Open Zoo");   // w2
       }, 0, 1, TimeUnit.MINUTES);
      var result = service.submit(() -> // w3
     System.out.println("Wake Staff"));
     System.out.println(result.get()); // w4
    }
}

[Mike Simmons]

Note that in Jeanne's code, the main() method now says "throws Exception".  Did you add that?  Her code has only one method, so she needed "throws Exception" only on that one method.  In the code you showed where main() calls f6(), you would need to put "throws Exception" on both methods.

[Anil Philip]

Mike Simmons wrote:Note that in Jeanne's code, the main() method now says "throws Exception".  Did you add that?  Her code has only one method, so she needed "throws Exception" only on that one method.  In the code you showed where main() calls f6(), you would need to put "throws Exception" on both methods.

The actual question in the test bank is like in my post here https://coderanch.com/p/3546743
There is no wrapping method.
IMHO this is an incorrectly worded question.
The compiler does point to this line as producing an error and it is marked as w4 in the question.
Otherwise, it should not have been marked such.

[Mike Simmons]

This was addressed by Jeanne's comment.  They often show code snippets, without the containing class, maybe without a containing method, and maybe without mention of some exceptions that need to be added in throws statements to make the code compile.  In a book, they're trying to save printing space by not including some details that the reader can figure out without showing them.  Otherwise every example is longer, and it's more expensive and heavy to carry around.

In this case, w1 and w2 were things that needed to be fixed in the code shown.  w4 is something that can be fixed outside the code shown.  And for stuff like that, we're normally supposed to assume that outside code is written in a way to prevent errors.  That may seem a subtle point, but that's the logic being used here.

[Ron McLeod]

Anil Philip wrote:The actual question in the test bank is like in my post here https://coderanch.com/p/3546743
There is no wrapping method.

Sure, but if the code was not wrapped in a method, it would not compile at all; even with line w4 commented-out.
error: <identifier> expected    service.scheduleWithFixedDelay(() -> {                                  ^ error: illegal start of type    service.scheduleWithFixedDelay(() -> {                                   ^ error: illegal start of type    }, 0, 1, TimeUnit.MINUTES);     ^ error: <identifier> expected    }, 0, 1, TimeUnit.MINUTES);      ^ error: <identifier> expected    }, 0, 1, TimeUnit.MINUTES);                             ^ error: 'var' is not allowed here    var result = service.submit(() -> // w3
I don't have the book, but Jeanne's comment regarding that was:

Jeanne Boyarsky wrote:In the book, it is common to have code snippets. It says in the intro to assume any surrounding code is correct. So you can assume imports/class/method signature.

[Jeanne Boyarsky]

Anil Philip wrote:Jeanne,
Do you not get an error on w4?
As I reported in my OP, w4 errors out

java: unreported exception java.lang.InterruptedException; must be caught or declared to be thrown

The question is which lines need to be changed to make the code compile. Changing lines w1 and w2 as described in the explanation cause it to compile. The question doesn't ask whether fixing produces an exception.

This question is correct as is.