Java 8 How do I work with exception throwing methods in streams

Question

Suppose I have a class and a method  class A     void foo   throws Exception                     Now I would like to call foo for each instance of A delivered by a stream like   void bar   throws Exception     Stream lt A gt  as         as forEach a - gt  a foo         Question  How do I properly handle the exception  The code does not compile on my machine because I do not handle the possible exceptions that can be thrown by foo    The throws Exception of bar seems to be useless here  Why is that

User · Answer

I suggest to use Google Guava Throwables class

propagate(Throwable throwable)

Propagates throwable as-is if it is an instance of RuntimeException or Error, or else as a last resort, wraps it in a RuntimeException and then propagates.**

void bar() {
    Stream<A> as = ...
    as.forEach(a -> {
        try {
            a.foo()
        } catch(Exception e) {
            throw Throwables.propagate(e);
        }
    });
}

UPDATE:

Now that it is deprecated use:

void bar() {
    Stream<A> as = ...
    as.forEach(a -> {
        try {
            a.foo()
        } catch(Exception e) {
            Throwables.throwIfUnchecked(e);
            throw new RuntimeException(e);
        }
    });
}

User · Answer

You can wrap and unwrap exceptions this way   class A       void foo   throws Exception           throw new Exception              interface Task       void run   throws Exception     static class TaskException extends RuntimeException       private static final long serialVersionUID   1L      public TaskException Exception e            super e            void bar   throws Exception         Stream lt A gt  as   Stream generate   - gt new A           try           as forEach a - gt  wrapException    - gt  a foo         or a  foo instead of    - gt  a foo         catch  TaskException e            throw  Exception e getCause             static void wrapException Task task        try           task run          catch  Exception e            throw new TaskException e

User · Answer

More readable way   class A     void foo   throws MyException                     Just hide it in a RuntimeException to get it past forEach      void bar   throws MyException         Stream lt A gt  as             try             as forEach a - gt                  try                     a foo                    catch MyException e                      throw new RuntimeException e                                         catch RuntimeException e              throw  MyException  e getCause                  Although at this point I won t hold against someone if they say skip the streams and go with a for loop  unless    you re not creating your stream using Collection stream    i e  not straight forward translation to a for loop  you re trying to use parallelstream

User · Answer

If all you want is to invoke foo  and you prefer to propagate the exception as is  without wrapping   you can also just use Java s for loop instead  after turning the Stream into an Iterable with some trickery    for  A a    Iterable lt A gt   as  iterator       a foo        This is  at least  what I do in my JUnit tests  where I don t want to go through the trouble of wrapping my checked exceptions  and in fact prefer my tests to throw the unwrapped original ones

User · Answer

You need to wrap your method call into another one  where you do not throw checked exceptions  You can still throw anything that is a subclass of RuntimeException   A normal wrapping idiom is something like   private void safeFoo final A a        try           a foo          catch  Exception ex            throw new RuntimeException ex              Supertype exception Exception is only used as example  never try to catch it yourself   Then you can call it with  as forEach this  safeFoo

User · Answer

You might want to do one of the following    propagate checked exception  wrap it and propagate unchecked exception  or catch the exception and stop propagation    Several libraries let you do that easily  Example below is written using my NoException library      Propagate checked exception as forEach Exceptions sneak   consumer A  foo        Wrap and propagate unchecked exception as forEach Exceptions wrap   consumer A  foo    as forEach Exceptions wrap MyUncheckedException  new  consumer A  foo        Catch the exception and stop propagation  using logging handler for example  as forEach Exceptions log   consumer Exceptions sneak   consumer A  foo

User · Answer

This question may be a little old  but because I think the  right  answer here is only one way which can lead to some issues hidden Issues later in your code  Even if there is a little Controversy  Checked Exceptions exist for a reason   The most elegant way in my opinion can you find was given by Misha here  Aggregate runtime exceptions in Java 8 streams by just performing the actions in  futures   So you can run all the working parts and collect not working Exceptions as a single one  Otherwise you could collect them all in a List and process them later   A similar approach comes from Benji Weber  He suggests to create an own type to collect working and not working parts   Depending on what you really want to achieve a simple mapping between the input values and Output Values occurred Exceptions may also work for you   If you don t like any of these ways consider using  depending on the Original Exception  at least an own exception

[java] Java 8: How do I work with exception throwing methods in streams?

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