Get rid of the list null-checks

During development of the legacy Java applications, we still have to deal with null. It’s possible to avoid it completly when we’re designing application from the scratch, applying proper code constructions, static code analysis and we’re consistent during code reviews. Nevertheless in majority of the cases we will encounter null in daily projects. We may even expect them in the method inputs and we have to be prepared for it. With the functional programming in Java we can deal with them in quite elegant way, but I often see people are not using features available nowadays.

Let’s have a look at the following code snippet:

private List<MyType> getList(final List<MyType> list) {
    if (list != null || list.isEmpty()) {
        return Collections.emptyList();
    }

    return list
            .stream()
            .filter(...)
            .collect(Collectors.toList());
}

In this example list can be null, so we have to perform null-check. We can also verify if the list is not empty once we’re sure it’s not null and return empty collection in that case. If the condition is not met, we’re converting list to the stream and start processing it.

We can use CollectionUtils from Apache Commons library to perform exactly the same check if we have it in our project dependencies:

private List<MyType> getList(final List<MyType> list) {
    if (CollectionUtils.isEmpty(list)) {
        return Collections.emptyList();
    }

    return list
            .stream()
            .filter(...)
            .collect(Collectors.toList());
}

We can try to figure out, how to achieve the same goal with functional programming concepts in Java and write the same logic within the single, fluent stream. We can call Optional.ofNullable(...) method because we know that our method argument can be null. Next, once we’ve got an Optional type, we can call .orElseGet(...) method and provide an object, which we want to return, when nullable value will be null. In this case, it will be an empty list. After that, we can fluently proceed with the stream processing.

private List<MyType> getList(final List<MyType> list) {
    return Optional
            .ofNullable(list)
            .orElseGet(Collections::emptyList)
            .stream()
            .filter(...)
            .collect(Collectors.toList());
}

Code snippet above looks much more elegant, it’s shorter, more concise and we have achieved our goal within a single stream without any additional if statements and without breaking chain of method invocations.