如何在 Java 8 中执行空检查

Question

How to write below code in java 8

List<String> names = service.serviceCall();

if(names != null) { // **how to do this null check with java 8**
    names.forEach(System.out::println);
}

Answer 1

You can do:

Optional.ofNullable(names).orElse(Collections.emptyList()).forEach(System.out::println);

or

Optional.ofNullable(names).ifPresent(n -> n.forEach(System.out::println));

or

Stream.of(names)
    .filter(Objects::nonNull)
    .flatMap(Collection::stream)
    .forEach(System.out::println);

but don't. Look at all all that extra stuff you'd have to write.

Just use a plain old null check, like you have in your code already.

Answer 2

As the Answer by Andy Turner suggest, writing a plain null check is best here.

Objects.nonNull

But Java 8 does offer one minor improvement: methods on the Objects utility class for the null check.

• Objects.isNull
• Objects.nonNull

So your code would like this.

List<String> names = service.serviceCall();

if( Objects.nonNull( names ) ) 
{   
    names.forEach( System.out::println );
}

Objects.requireNonNullElseGet

Another method on Objects can return an alternate object if that suits your situation. For example, if your service call returns null list, perhaps you should substitute an empty list. Call Objects.requireNonNullElseGet .

The lambda passed is handled in a lazy manner, not executed unless needed.

List<String> names = 
        Objects.requireNonNullElseGet
        ( 
            service.serviceCall() , 
            () -> new ArrayList<>( 0 )  // In Java 9 and later, use `List.of()`.
        )
;
names.forEach( System.out::println );