How to cast objects with different classes in a Java 8 stream?

Question

Use Case

I'm using an 3rd party library where there are two very similar classes that don't implement an interface. The code currently loops through a list of items to find the first occurrence of an object using one of these classes and then converts it to a stream where it is processed. It would be nice if I could convert this code to use a stream and have it chained to the rest of my code.

Current Code

    for (Component3Choice component: components) {
        if (component instanceof OptionalComponent3Bean) {
            OptionalComponent3Bean section = (OptionalComponent3Bean) component;

            entryStream = section.getSection().getEntry().stream()
            break;
        }
        else if (component instanceof RequiredComponent3Bean) {
            RequiredComponent3Bean section = (RequiredComponent3Bean) component;

            entryStream = section.getSection().getEntry().stream();
            break;
        }
    }
    ... do something with the stream ...

Desired Code

components.stream()
  .filter(entry -> entry instanceof OptionalComponent3Bean 
                     || entry instanceof RequiredComponent3Bean)
  .findFirst()
  .map( {{ cast entry }} )
  .map( castedEntry.getSection().getEntry())
  ... continue on with my processing

Question

Is it possible cast the entry based on the previous filter in the stream?

Can you modify OptionalComponent3Bean & RequiredComponent3Bean to implement an interface (for instance HasSection) which would define getSection()? — Olivier Grégoire
– Olivier Grégoire, Commented Mar 11, 2015 at 14:03
Unfortunately, I don't have access to the source code for that library. :( — Pete
– Pete, Commented Mar 11, 2015 at 14:04

Giulio Franco · Accepted Answer · 2015-03-11 14:10:48Z

3

No, nothing can save you from bad design, which is what it seems you're fighting against.

You can force a common interface by means of a wrapper, if you need to replicate boilerplate similar to this one in many places. Otherwise, I guess the best you can do is

static private IDontKnow getStream(Component3Choice c3c) {
  if (c3c instanceof OptionalComponent3Bean) {
    return ((OptionalComponent3Bean)c3c).getStream();
  } else if (c3c instanceof RequiredComponent3Bean) {
    return ((RequiredComponent3Bean)c3c).getStream();
  } else {
    return null;
  }
}

components.stream()
  .map(x -> getStream(x))
  .filter(x -> x!=null)
  .findFirst()
  .map(x -> x.getEntry().stream());
  ... continue on with yout processing

answered Mar 11, 2015 at 14:10

Giulio Franco

3,25017 silver badges18 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Pete Over a year ago

Looks good as this isolates the logic. if and when the library gets updated and hopefully fixed, it'll be easier to transition.

Matt Campbell Over a year ago

Chiming in re another case. Wanting to stream members of a Set to .joining() but that looks for CharSequence instances like String, not Longs, etc.. This is what I did: setOfLongObjects.stream().map(it -> ""+it).collect(Collectors.joining(","))

Eran · Accepted Answer · 2015-03-11 14:04:46Z

1

Not the prettiest code, but you could do :

components.stream()
          .filter(entry -> entry instanceof OptionalComponent3Bean 
                     || entry instanceof RequiredComponent3Bean)
          .map(entry -> {
                 if ((entry instanceof OptionalComponent3Bean)
                   return ((OptionalComponent3Bean) entry).getSection().getEntry().stream(); 
                 else
                   return ((RequiredComponent3Bean) entry).getSection().getEntry().stream();
                        })
          .findFirst();

This would return an Optional<Stream<Something>>.

Note that findFirst must be the last operation, since it's terminal.

answered Mar 11, 2015 at 14:04

Eran

395k57 gold badges726 silver badges793 bronze badges

Collectives™ on Stack Overflow

How to cast objects with different classes in a Java 8 stream?

Use Case

Current Code

Desired Code

Question

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

Use Case

Current Code

Desired Code

Question

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related