8369184: SimpleTimeZone equals() Returns True for Unequal Instances with Different hashCode Values by naotoj · Pull Request #27660 · openjdk/jdk

naotoj · 2025-10-06T21:36:44Z

Fixing the equals/hashCode contract in the SimpleTimeZone class. The current implementation includes DST rule fields in hash code computation even for zones that do not observe DST, while equals() always considers them. Also correcting the example code in the class description, where it used 20-year-old obsolete "America/Los_Angeles" rule.

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Issue

JDK-8369184: SimpleTimeZone equals() Returns True for Unequal Instances with Different hashCode Values (Bug - P3)

Reviewers

Justin Lu (@justin-curtis-lu - Committer) Review applies to a2c988d8
Roger Riggs (@RogerRiggs - Reviewer) Review applies to aafdf18b
Iris Clark (@irisclark - Reviewer) Review applies to aafdf18b
Pavel Rappo (@pavelrappo - Reviewer)

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bridgekeeper · 2025-10-06T21:38:33Z

👋 Welcome back naoto! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2025-10-06T21:38:51Z

@naotoj This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8369184: SimpleTimeZone equals() Returns True for Unequal Instances with Different hashCode Values

Reviewed-by: prappo, jlu, rriggs, iris

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 58 new commits pushed to the master branch:

ac73e68: 8368984: Extra slashes in Cipher transformation leads to NSPE instead of NSAE
79bcc7b: 8369257: PPC: compiler/whitebox/RelocateNMethodMultiplePaths.java fails with assertion
927aa3f: 8369178: G1: Use NMethodMarkingScope and ThreadsClaimTokenScope in G1RootProcessor
... and 55 more: https://git.openjdk.org/jdk/compare/23a65644ae63b271ca99c55a2a60a192c4e4dfb8...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

openjdk · 2025-10-06T21:39:34Z

@naotoj The following labels will be automatically applied to this pull request:

core-libs
i18n

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing lists. If you would like to change these labels, use the /label pull request command.

mlbridge · 2025-10-06T21:43:20Z

Webrevs

justin-curtis-lu

LGTM

justin-curtis-lu · 2025-10-07T18:41:14Z

test/jdk/java/util/TimeZone/SimpleTimeZoneEqualsHashCodeTest.java

Include equals in the test name as well? Perhaps HashCodeEqualsTest or something along those lines.

Renamed. I left SimpleTimeZone as the test is specific for this class.

justin-curtis-lu · 2025-10-07T18:54:34Z

src/java.base/share/classes/java/util/SimpleTimeZone.java

    {
-        return startMonth ^ startDay ^ startDayOfWeek ^ startTime ^
-            endMonth ^ endDay ^ endDayOfWeek ^ endTime ^ rawOffset;
+        int hash = 31 * getID().hashCode() + rawOffset;


int hash = Objects.hash(getID(), rawOffset, useDaylight); return !useDaylight ? hash : 31 * hash + Objects.hash( startMonth, startDay, startDayOfWeek, startTime, endMonth, endDay, endDayOfWeek, endTime);

Seems reasonable to use Objects.hash here. Could save some lines if wanted?

Initially, I thought so, but decided to avoid extra array allocation/copy. But on second thought, the instances of this class would seldom be used as hash keys so not that performance critical. Either way is fine with me

RogerRiggs

LGTM

justin-curtis-lu · 2025-10-08T17:24:19Z

src/java.base/share/classes/java/util/SimpleTimeZone.java

@@ -112,15 +112,15 @@
 * <pre><code>


We can consider replacing <pre><code> with @snippet so that the code example can be copied by readers. Although I understand if leaving it for another time as to not invalidate the existing approvals.

I think, that's out of scope for this PR.

test/jdk/java/util/TimeZone/SimpleTimeZoneEqualsHashCodeTest.java

pavelrappo · 2025-10-08T17:44:43Z

test/jdk/java/util/TimeZone/SimpleTimeZoneEqualsHashCodeTest.java

+        stz.setEndRule(NOVEMBER, 8, -SUNDAY, 7_200_000);
+        assertNotEquals(STZ_WITH_DST, stz);
+        // from the contract point, hash codes may be the same
+        assertNotEquals(STZ_WITH_DST.hashCode(), stz.hashCode());


Like you said, from !a.equals(b) it does not follow that a.hashCode() != b.hashCode(). Unless you want hash code to specifically have this additional property, I'd remove this assertion.

I wanted to test the implementation which takes DST related fields into consideration for hash code calculation. Added some comments for clarity

pavelrappo · 2025-10-08T17:47:40Z

test/jdk/java/util/TimeZone/SimpleTimeZoneEqualsHashCodeTest.java

+        stz.setEndRule(NOVEMBER, 8, -SUNDAY, 7_200_000);
+        assertTrue(stz.useDaylightTime());
+        assertNotEquals(STZ_WITHOUT_DST, stz);
+        assertNotEquals(STZ_WITHOUT_DST.hashCode(), stz.hashCode());


Ditto on equals-hashCode contract.

Co-authored-by: Pavel Rappo <32523691+pavelrappo@users.noreply.github.com>

naotoj · 2025-10-09T16:19:01Z

Thanks for the reviews!
/integrate

openjdk · 2025-10-09T16:21:00Z

Going to push as commit 37b725d.
Since your change was applied there have been 82 commits pushed to the master branch:

7c75cb3: 8367376: Bad ButtonUI prevents other components from updating when system changes desktop properties
9891cd6: 8369313: java/net/httpclient/TimeoutBasic.java should accept that HttpTimeoutException should be somewhere down the cause chain
48d21b5: 8369434: java/net/httpclient/AltServiceUsageTest.java fails intermittently
... and 79 more: https://git.openjdk.org/jdk/compare/23a65644ae63b271ca99c55a2a60a192c4e4dfb8...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-10-09T16:21:08Z

@naotoj Pushed as commit 37b725d.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

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openjdk bot added core-libs core-libs-dev@openjdk.org i18n i18n-dev@openjdk.org labels Oct 6, 2025

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Co-authored-by: Pavel Rappo <32523691+pavelrappo@users.noreply.github.com>

openjdk bot removed the ready Pull request is ready to be integrated label Oct 8, 2025

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