Nullpointerexception in Collectors.toMap with null entry values

java

null-pointer-exception

collectors-tomap

map-handling

byAnton Shumikhin·Sep 18, 2024

Avoid NullPointerException in Collectors.toMap() by using a merge function for null values. Here’s a quick-fix:

Map<Key, Value> map = list.stream()
    .collect(Collectors.toMap(
        Object::getKey,                      // Key extractor
        Object::getValue,                    // Value extractor
        (val1, val2) -> Objects.requireNonNullElse(val1, val2)));  //Handle Null. Null? Not on my watch!

We're using Objects.requireNonNullElse() to substitute nulls with non-null values.

Deep dive into the issue

The Collectors.toMap() method can throw NullPointerException when dealing with null values. Here we delve into the reasons behind this behavior and the workarounds.

Unmasking the culprit

The traditional way of using Collectors.toMap(), can lead to an explosion when it encounters null values. Instead, use a custom collector that handles null values with suave. This involves:

Creating a new HashMap: To start with a shiny new map
Defining a merge function: To solve the null conundrum
Using HashMap::putAll: To put in values without tripping over null values.

Head-on collision of keys

When a stream is collected to a map, there can be cases of duplicate keys. If not managed properly, values might get replaced silently. Avoid this by managing the collision of keys using a merge function.

Extra care with nulls

Working with null values isn't straightforward. Here are some strategies to help you tame the null beast:

Use Collectors.collectingAndThen to apply a transformer over collected data
Preprocess the Stream with .map() to ensure nulls are replaced with a default value
Implement a custom collector, toMapNullFriendly, factoring in performance while handling nulls

Keeping maps safe from null

Dealing with nulls in maps ushers in a new set of challenges regarding data integrity and performance optimization.

Keeping data intact

To maintain data integrity, special measures need to be put in place when handling nulls to avoid loss of information. Be sure to check for duplicate keys and choose a suitable merge function.

Efficient key-value mapping

Optimized key-value mapping involves using the Map.merge() method and the custom toMapWithNullValues method which allows specifying the desired Map type, such as HashMap, LinkedHashMap, etc.

Testing for peace of mind

Use JUnit and assertj to validate your map collections. This aids in ensuring that null handling is effective and bug-free.

Adding finishing touches

Sometimes a map might require additional modifications after collecting its elements. For this, employ Collectors.collectingAndThen() to perform additional transformations.