Add generic Pair utility class with full test coverage for DSA convenience.

[adi1103sh]

While solving DSA problems on LeetCode (especially under time constraints), it's common to require a Pair structure for representing coordinates, graph edges, mapping logic, etc.

In the absence of a native Pair class in Java (unlike C++ STL's std::pair), this often leads to redundant and repeated creation of custom Pair classes during contests or interviews.

This PR introduces a reusable, generic Pair<F, S> utility class to address that gap. Key highlights:

✅ Supports Comparable

🔁 Includes common utilities like swap() and makePair()

✅ Covers equals(), hashCode(), compareTo(), toString()

🧪 Comes with comprehensive TestNG test suite with full coverage

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
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• Commit message must refer to an issue

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Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/25020/head:pull/25020
$ git checkout pull/25020

Update a local copy of the PR:
$ git checkout pull/25020
$ git pull https://git.openjdk.org/jdk.git pull/25020/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 25020

View PR using the GUI difftool:
$ git pr show -t 25020

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/25020.diff

[bridgekeeper]

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[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

@adi1103sh The following label will be automatically applied to this pull request:

• core-libs

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[adi1103sh]

/signed

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[openjdk]

@adi1103sh Please do not rebase or force-push to an active PR as it invalidates existing review comments. Note for future reference, the bots always squash all changes into a single commit automatically as part of the integration. See OpenJDK Developers’ Guide for more information.

[wenshao]

Suggested change

	return "(" + String.valueOf(first) + ", " + String.valueOf(second) + ")";
	return "(" + first + ", " + second + ")";

StringConcation will handle null, no need to call String.valueOf here

[adi1103sh]

THANKS for pointing out, pushed a new patch with your suggested change.

[adi1103sh]

Hey @wenshao
It would be so nice of you, if you could review #25059

[adi1103sh]

After using record for couple of days, I believe making Pair class is re-inventing the wheel, just for the sake of mutability.
If someone has any suggestion will collaborate.

[openjdk]

@adi1103sh Please do not rebase or force-push to an active PR as it invalidates existing review comments. Note for future reference, the bots always squash all changes into a single commit automatically as part of the integration. See OpenJDK Developers’ Guide for more information.