Strategies to Avoid NullPointerExceptions for Null-Safe Stream Operations in Java

NullPointerExceptions, or NPEs for short, are a common source of frustration for Java developers. They occur when an attempt is made to access an object or invoke a method on an object that is null. While Java 14 introduced the NullPointerException.getMessage() method to provide more detailed information about the cause of the NPE, preventing these exceptions in the first place is always the best approach. In this article, we'll explore strategies to avoid NullPointerExceptions when working with streams in Java and try to answer “How to avoid a NullPointerException while using stream API?”

Understanding the Stream Paradigm

Before we dive into specific techniques for avoiding NPEs, it’s essential to understand how streams work in Java. Streams were introduced in Java 8 as a way to process sequences of data in a functional and declarative manner. They provide a powerful tool for working with collections, but they also introduce the possibility of NPEs due to their lazy evaluation nature.

NullPointerException can occur in two ways, either creating the stream object or processing the stream.

1. Creating the Stream Object

When a null object is used while creating a stream object, the stream method throws a NullPointerException.

public static void main(String[] args) {

    List<String> list = null;

    Stream<String> stream = list.stream();
}

This code throws a NullPointerException because the list variable is null. Hence, a check is needed to avoid this NPE. There are a few other ways to control the nullability of collections.

• With null check

<T> Stream<T> getStream(Collection<T> coll) {
  return coll == null ? Stream.empty() : coll.stream();
}

• With Optional (Java 8+)

<T> Stream<T> getStream(Collection<T> coll) {
    return Optional.ofNullable(coll).map(Collection::stream).orElseGet(Stream::empty);
}

• With Stream.ofNullable (Java 9+)

<T> Stream<T> getStream(Collection<T> collection) {
    return Stream.ofNullable(collection).flatMap(Collection::stream);
}

• With MultiMap instead of flatMap (Java 16+)

<T> Stream<T> getStream(Collection<T> collection) {
    return Stream.ofNullable(collection).mapMulti(Iterable::forEach);
}

All of the above methods help to convert nullable collections to streams without having a NullPointerException. If you are looking for a generic method to use while converting nullable collections into streams, those are the methods that you are looking for.

2. Processing the Stream Object

When there is a null object in the stream or a null value of an object that is in the stream, throwing a NullPointerException can be the case. filter() method can be used to prevent the NullPointerException while processing the stream data.

public static void nullExample() {
    List<String> list = Arrays.asList("String", null, "Example");
    list.stream().forEach(item -> System.out.println(item.length()));
}

nullExample() method throws a NullPointerException because the second element of the list is null. When the length method is called to print, we see a NullPointerException. The below function can be chained to the methods that are explained above and all null items in the collections can be filtered and NPE can be prevented.

.filter(Objects::nonNull)

Here is an example getStream method to prevent NullPointerException that occurs because of null collections or null items in collections.

<T> Stream<T> getStream(Collection<T> collection) {
    return Stream.ofNullable(collection)
                 .flatMap(Collection::stream).filter(Objects::nonNull);
}

If there is a nullable value that is processed in the stream, filter() method can be used to filter the null values for that field.

public static void nullExample() {
    List<User> users = getUsers();
    users.stream().filter(Objects::nonNull)
         .filter(user -> user.getAddress() != null)
         .forEach(user -> System.out.println(user.getAddress()));
}

NullPointerExceptions can be a source of frustration and bugs in Java programs, but by understanding how streams work and employing the strategies mentioned above, you can minimize their occurrence in your code.

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