[java] Keys: enforce CharSequence contract in charAt()

[seethinajayadileep]

💥 What does this PR do?

Updates Keys.charAt(int index) to validate the index and throw
IndexOutOfBoundsException when the index is not 0.

Previously, the method returned the null character ('\u0000') for any
index other than 0, which silently violated the CharSequence contract.

This change ensures:

• length() returns 1
• charAt(0) returns the key code
• charAt(index != 0) throws IndexOutOfBoundsException

This aligns the implementation with expected CharSequence behavior.

---

🔧 Implementation Notes

Keys represents a single Unicode PUA character. Since its length is
always 1, only index 0 is valid.

Instead of returning '\u0000' for invalid indexes (which can mask bugs
and produce incorrect comparisons), the method now explicitly throws
IndexOutOfBoundsException.

Alternative considered:

• Keeping the previous behavior (returning '\u0000') — rejected because
  it silently hides incorrect usage and breaks contract expectations.

---

💡 Additional Considerations

• No behavioral change for valid usage (charAt(0)).
• Improves correctness when used in string comparisons or iteration.
• No impact on toString(), subSequence(), or chord() behavior.

---

🔄 Types of changes

• Bug fix (backwards compatible)

[qodo-code-review]

Review Summary by Qodo

Enforce CharSequence contract in Keys.charAt() bounds checking

🐞 Bug fix

Walkthroughs

Description

• Enforce bounds checking in Keys.charAt() method
• Throw IndexOutOfBoundsException for invalid indexes
• Comply with CharSequence contract requirements
• Prevent silent violations and incorrect comparisons

File Changes

1. java/src/org/openqa/selenium/Keys.java 🐞 Bug fix +3/-3

Add bounds validation to Keys.charAt() method

• Changed charAt(int index) to validate index parameter
• Now throws IndexOutOfBoundsException when index != 0
• Removed silent return of null character ('\u0000')
• Ensures compliance with CharSequence interface contract

java/src/org/openqa/selenium/Keys.java

---

[qodo-code-review]

Code Review by Qodo

🐞 Bugs (0) 📘 Rule violations (1) 📎 Requirement gaps (0)

1. KeysTest expects charAt(10) ☑ 📘 Rule violation ⛯ Reliability

Description

Keys.charAt(int) now throws IndexOutOfBoundsException for any index other than 0, but the
existing unit test still asserts that charAt(10) returns 0. This functional change needs
corresponding test updates/additions to avoid incorrect expectations and failing CI.

Code

java/src/org/openqa/selenium/Keys.java[R143-146]

+    if (index != 0) {
+      throw new IndexOutOfBoundsException("Index: " + index + ", Length: 1");
   }
-    return 0;
+    return keyCode;

Evidence

PR Compliance ID 5 requires updating/adding tests for functional changes. The PR changes
Keys.charAt to throw on index != 0, while the existing unit test still expects a 0 return
value for a non-zero index, so tests must be updated to match the new contract.

AGENTS.md
java/src/org/openqa/selenium/Keys.java[141-147]
java/test/org/openqa/selenium/KeysTest.java[37-40]

Agent prompt

The issue below was found during a code review. Follow the provided context and guidance below and implement a solution

## Issue description
`Keys.charAt(int)` now throws for `index != 0`, but `KeysTest.charAtOtherPositionReturnsZero()` still asserts a `0` return value, which no longer matches the new behavior.
## Issue Context
The production change enforces the `CharSequence` contract by throwing `IndexOutOfBoundsException` for invalid indices. The unit test suite should be updated to reflect this and prevent CI failures.
## Fix Focus Areas
- java/test/org/openqa/selenium/KeysTest.java[32-41]
- java/src/org/openqa/selenium/Keys.java[141-147]

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[cgoldberg]

@seethinajayadileep I just approved the workflows to run for this PR, but we are having some issues so you will likely see some unrelated test failures in CI.