This page wants to point out mistakes that are easily made when using JDK11 features. Mostly the pitfalls have their origin in reasonable expectations that aren't meat by the new APIs.
Overview
| Pitfall | Beware ... |
|---|---|
List.of / List.copyOf as a replacement for Arrays.asList and alike |
null is not allowed, contains(null) => NPE |
collect(toUnmodifiableList()) instead of collect(toList()) |
null is not allowed |
Stream.forEach as a replacement for for/while loops |
forEach does not guarantee order, consider forEachOrdered |
Map.of to create maps with few entries |
null is not allowed and map does not retain order of entries, key must also never be null when reading using contains, get, getOrDefault |
Set.of for deduplication |
does not allow null or duplicates, contains(null) => NPE |
While List.of lets you create effectivly immutable Lists
they also have the special semantic of not allowing null values
or the factory function will throw a NullPointerException.
Use List.of with care and can only replace Arrays.asList and co
when the list is initialised with constants that are clearly not null.
Same is true for List.copyOf and its cousins Set.copyOf and Map.copyOf.
Consistent with the immutable lists created by List.of
the unmodifiable list created by a collector do also not allow null values.
If there is a chance for null values in the stream use collect(toList()).
In JDK16 Stream.toList() is added which results in an unmodifiable list which does allow null values.
While the name forEach is easily understood as the proper replacement for a for-each loop
the javadoc of Stream.forEach states: The behavior of this operation is explicitly nondeterministic.
The proper replacement for ordered iteration is called Stream.forEachOrdered.
Consistent with List.of the Map.of has special semantics of not allowing null references
for both keys and values.
A quick lock at the implementation MapN appears to preserve the order of key-value pairs.
This is misleading. Maps created by Map.of do not provide a determenistic order.
To create a map with fixed entry order still a class like LinkedHashMap is required.
Besides not allowing null values unmodifiable sets also do not allow duplicate values.