RFR: 6323374: (coll) Optimize Collections.unmodifiable* and synchronized*

[Michael Hixson]

On Tue, 16 Feb 2021 21:57:43 GMT, Ian Graves <igraves at openjdk.org> wrote:

> Modify the `unmodifiable*` methods in `java.util.Collections` to be idempotent. That is, when given an immutable collection from `java.util.ImmutableCollections` or `java.util.Collections`, these methods will return the reference instead of creating a new immutable collection that wraps the existing one.

src/java.base/share/classes/java/util/Collections.java line 1473:

> 1471:     public static <K,V> Map<K,V> unmodifiableMap(Map<? extends K, ? extends V> m) {
> 1472:         if(m.getClass() == UnmodifiableMap.class ||
> 1473:            m.getClass() == ImmutableCollections.Map1.class ||

(I'm not a reviewer.)

I think this causes a change in behavior to this silly program.

var map1 = Map.of(1, 2);
var map2 = Collections.unmodifiableMap(map1);

try {
  System.out.println("map1 returned " + map1.entrySet().contains(null));
} catch (NullPointerException e) {
  System.out.println("map1 threw");
}

try {
  System.out.println("map2 returned " + map2.entrySet().contains(null));
} catch (NullPointerException e) {
  System.out.println("map2 threw");
}

With JDK 15 the output is:
> map1 threw
> map2 returned false

With this change I think the output will be:
> map1 threw
> map2 threw

It seems unlikely that anyone will be bit by this, but it is a change to behavior and it wasn't called out in the Jira issue, so I felt it was worth mentioning.

I think it is just this one specific case that changes -- only `Map1`, and only `entrySet().contains(null)`.  Other sub-collections like `keySet()` and `values()` and `subList(...)` already throw on `contains(null)` for both the `ImmutableCollections.*` implementation and the `Collections.umodifiable*` wrapper.  `MapN`'s `entrySet().contains(null)` already returns `false` for both.

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PR: https://git.openjdk.java.net/jdk/pull/2596