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

[liach]

On Tue, 16 Feb 2021 23:18:55 GMT, Claes Redestad <redestad 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 1130:
> 
>> 1128:     public static <T> Set<T> unmodifiableSet(Set<? extends T> s) {
>> 1129:         if(s.getClass() == UnmodifiableSet.class ||
>> 1130:            s.getClass() == ImmutableCollections.Set12.class ||
> 
> These might be problematic: `ImmutableCollections.Set*` differs in behavior subtly from `UnmodifiableSet`, e.g., `set.contains(null)` will throw an NPE. I'd limit the check and optimization to `UnmodifiableSet` here

No? This unmodifiable set here just delegates call to the backing field `c`, so all exceptions from `c`'s calls are just delegated, aren't they? The NPE will still be thrown; it's just that the stack trace will be different (i.e. one more level of call in the stacktrace)

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