Handling exception in Lambda Expression

Question

What will be the equivalent lambda expression for below code?

List<String> List = new ArrayList<String>();
for (String var: alreadyList) {
     try {
         if (getNumber(var) == num) {
             filteredList.add(var);
         }
     } catch(NullPointerException exception) {
           throw exception;
     }
 }

getNumber method throws Nullpointerexception and the above code is also in a method which throws the same exception to a caller.

This is usual lambda expression but how to throw the Nullpointerexception in it?

List<String> List = alreadyList.stream()
            .filter(var -> getNumber(var) == num)
            .collect(Collectors.toList());

The original code is strange. As you can see, it just re-throws the exception. In other words, it doesn't really do anything. — Magnilex
– Magnilex, Commented Aug 6, 2018 at 13:44
NullPointerException is an unchecked exception and so you don't need to catch / throw it explicitly at all (and arguably shouldn't.) So the short answer is the stream based code you have is already the equivalent of what's above, the code above just has some needless exception catching in it. — Michael Berry
– Michael Berry, Commented Aug 6, 2018 at 13:48

GBlodgett · Accepted Answer · 2018-08-06 13:54:38Z

3

Catching a NPE is bad practice. NPE is an unchecked exception. From the docs:

If an argument is null, the method might throw a NullPointerException, which is an unchecked exception.

Instead why not add another filter to the stream:

List<String> List = alreadyList.stream()
        .filter(e -> e!= null)
        .filter(var -> getNumber(var) == num)
        .collect(Collectors.toList());

edited Aug 6, 2018 at 13:54

answered Aug 6, 2018 at 13:48

GBlodgett

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1 Comment

Nikolas Over a year ago

Agreed that Catching an NPE is bad practice.

Andrei Prakhov · Accepted Answer · 2018-08-06 13:44:15Z

2

It is bad practice to catch NullPointerException. You can use Optional class to avoid it and do necessary action only if object is present. You can throw other exceptions from your getNumber() method without any problems

answered Aug 6, 2018 at 13:44

Andrei Prakhov

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Comments

Nikolas · Accepted Answer · 2018-08-06 14:00:09Z

1

Never catch the unchecked exceptions including the NullPointerException. The way you do is just rethrowing it. Either filter out the null values out.

List<String> List = alreadyList.stream()
                               .filter(Objects::nonNull)
                               .filter(var -> getNumber(var) == num)
                               .collect(Collectors.toList());

The code you have provided:

List<String> List = alreadyList.stream()
                               .filter(var -> getNumber(var) == num)
                               .collect(Collectors.toList());

Will already throw the NullPointerException in case getNumber(var) would fail. Your try-catch does not do anything different.

answered Aug 6, 2018 at 14:00

Nikolas

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2 Comments

GBlodgett Over a year ago

Is there a difference between Objects::nonNull and (c -> c != null) ?

Nikolas Over a year ago

@GBlodgett: Nope in the behavior. Objects::nonNull is just a static wrapper used as a shortcut and a reference which is generally accepted standard of filtering null values out. See its implementation since the line 264. Moreover in c -> c != null you make easier an mistake forgetting the !.

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