Filter values only if not null using lambda in Java8

Solution 1:

In this particular example, I think @Tagir is 100% correct get it into one filter and do the two checks. I wouldn't use Optional.ofNullable the Optional stuff is really for return types not to be doing logic... but really neither here nor there.

I wanted to point out that java.util.Objects has a nice method for this in a broad case, so you can do this:

    cars.stream()
        .filter(Objects::nonNull)

Which will clear out your null objects. For anyone not familiar, that's the short-hand for the following:

    cars.stream()
        .filter(car -> Objects.nonNull(car))

To partially answer the question at hand to return the list of car names that starts with "M":

    cars.stream()
        .filter(car -> Objects.nonNull(car))
        .map(car -> car.getName())
        .filter(carName -> Objects.nonNull(carName))
        .filter(carName -> carName.startsWith("M"))
        .collect(Collectors.toList());

Once you get used to the shorthand lambdas you could also do this:

    cars.stream()
        .filter(Objects::nonNull)
        .map(Car::getName)        // Assume the class name for car is Car
        .filter(Objects::nonNull)
        .filter(carName -> carName.startsWith("M"))
        .collect(Collectors.toList());

Unfortunately once you .map(Car::getName) you'll only be returning the list of names, not the cars. So less beautiful but fully answers the question:

    cars.stream()
        .filter(car -> Objects.nonNull(car))
        .filter(car -> Objects.nonNull(car.getName()))
        .filter(car -> car.getName().startsWith("M"))
        .collect(Collectors.toList());

Solution 2:

You just need to filter the cars that have a null name:

requiredCars = cars.stream()
                   .filter(c -> c.getName() != null)
                   .filter(c -> c.getName().startsWith("M"));

Solution 3:

The proposed answers are great. Just would like to suggest an improvement to handle the case of null list using Optional.ofNullable, new feature in Java 8:

List<String> carsFiltered = Optional.ofNullable(cars)
                    .orElseGet(Collections::emptyList)
                    .stream()
                    .filter(Objects::nonNull)
                    .collect(Collectors.toList());

So, the full answer will be:

List<String> carsFiltered = Optional.ofNullable(cars)
                    .orElseGet(Collections::emptyList)
                    .stream()
                    .filter(Objects::nonNull) //filtering car object that are null
                    .map(Car::getName) //now it's a stream of Strings
                    .filter(Objects::nonNull) //filtering null in Strings
                    .filter(name -> name.startsWith("M"))
                    .collect(Collectors.toList()); //back to List of Strings