| The [double-brace initialization pattern][dbi] should be avoided—especially in |
| non-static contexts. |
| |
| The double-brace pattern uses an instance-initializer in an anonymous inner |
| class to express the initialization of a class (often a collection) in a single |
| step. |
| |
| Inner classes in a non-static context are terrific sources of memory leaks! If |
| you pass the collection somewhere that retains it, the entire instance you |
| created it from can no longer be garbage collected. Even if it is completely |
| unreachable. And if someone serializes the map? Yep, the entire creating |
| instance goes along for the ride (or if that fails, serializing the map fails, |
| which is also awfully strange). All this is completely nonobvious. |
| |
| Luckily, there are more readable and more performant alternatives in the factory |
| methods and builders for `ImmutableList`, `ImmutableSet`, and `ImmutableMap`. |
| |
| The `List.of`, `Set.of`, and `Map.of` static factories |
| [added in Java 9](http://openjdk.java.net/jeps/269) are also a good choice. |
| |
| That is, prefer this: |
| |
| ```java |
| ImmutableList.of("Denmark", "Norway", "Sweden"); |
| ``` |
| |
| Not this: |
| |
| ```java |
| new ArrayList<>() { |
| { |
| add("Denmark"); |
| add("Norway"); |
| add("Sweden"); |
| } |
| }; |
| ``` |
| |
| TIP: Neither the guava immutable collections nor the static factory methods |
| added in a JDK 9 support `null` elements. The double-brace pattern is still best |
| avoided for collections that contain null. Consider using `Arrays.asList` to |
| initialize `List`s and `Set`s with `null` values, and refactoring `Map` |
| initializers into a helper method. |
| |
| [dbi]: https://stackoverflow.com/questions/1958636/what-is-double-brace-initialization-in-java |
