handle exceptions in stream java 8

Question

I have a string:

"1, 2, 3 , -4"

it is split by ", ". I have a function to convert a number to a enum type which works fine. I want to use java 8 to convert this string to a list of enum objects.

Pattern pattern = Pattern.compile(", ");
List<f> fList = pattern.splitAsStream(str)
  .map(s -> {
    try {
      return this.getEnumObject(Integer.valueOf(s), f.class);
    }
    catch (NoEleException e) {
      e.printStackTrace();
    }
  })
  .collect(Collectors.toList());

This gives me an error:

missing return type.

How could I fix it?

If you're suppressing the exception, you need to return some value. What value is up to you. — shmosel
– shmosel, Commented Nov 15, 2017 at 23:39
So what would you like to go into your list in the event that some of the numbers don't resolve to an enum object? — Misha
– Misha, Commented Nov 15, 2017 at 23:57
@misha it will never happen..but if it happens, just a null object is fine. — user3369592
– user3369592, Commented Nov 16, 2017 at 0:01
if you are convinced it will never happen, you can throw new RuntimeException(e); in the catch block. — Misha
– Misha, Commented Nov 16, 2017 at 0:08

Ousmane D. · Accepted Answer · 2017-11-15 23:39:24Z

6

Currently, if an exception occurs no result will be returned hence the compilation error. You'll need to return a value after the catch block .

answered Nov 15, 2017 at 23:39

Ousmane D.

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1 Comment

StvnBrkdll Over a year ago

that will not give the result he is looking for, what would that value be?

Jean-Baptiste Yunès · Accepted Answer · 2017-11-16 07:46:35Z

Basically to ways of managing this:

catching the exception and return some value or encapsulate values in Optionals and filter accordingly
Throwing a RuntimeException which chains the original one

In the first case we use Optional to put something into the stream on error, and then manage these empty values further in the stream:

pattern.splitAsStream(str)
.map(s -> {
  try {
    return Optional.of(this.getEnumObject(Integer.valueOf(s), f.class));
  }
  catch (NoEleException e) {
    e.printStackTrace();
    return Optional.empty();
  }
 })
.filter(Optional::isPresent) // remove empty optionals
.map(Optional::get) // unwrap them
.collect(Collectors.toList());

In the second case the stream is stopped and you can then try to catch the RuntimeException and unchain the original one:

pattern.splitAsStream(str)
.map(s -> {
  try {
    return Optional.of(this.getEnumObject(Integer.valueOf(s), f.class));
  }
  catch (NoEleException e) {
    e.printStackTrace();
    throw new RuntimeException(e); // stop the stream
  }
 })
.collect(Collectors.toList());

Julian Rubin · Accepted Answer · 2017-11-15 23:41:58Z

0

You can create Null Object like MissingElement, return it in catch and them filter it out after map.

answered Nov 15, 2017 at 23:41

Julian Rubin

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Comments

user514949 · Accepted Answer · 2021-04-04 15:06:45Z

0

If you are certain that this won't happen you could return null in the catch and filter for non null before collecting:

Pattern pattern = Pattern.compile(", ");
List<f> fList = pattern.splitAsStream(str)
  .map(s -> {
    try {
      return this.getEnumObject(Integer.valueOf(s), f.class);
    }
    catch (Exception e) {
      return null;
    }
  })
  .filter(Objects::nonNull)
  .collect(Collectors.toList());

answered Apr 4, 2021 at 15:06

user514949

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