Refactor Sets/Maps/Lists MethodHandle dispatch for full branch coverage on any JDK

[Copilot]

On JDK 8, the Set/Map/List.of(...) MethodHandles are null at class load time, making the invokeExact branch unreachable in tests. On JDK 9+, the null-guard branch is never hit. Neither branch achieves full coverage under a single JDK version.

Approach

Extract the MethodHandle dispatch logic from each public factory method into a package-private helper that accepts the handle as an explicit parameter. The public method becomes a one-liner delegate.

Before:

public static <E> Set<E> ofSet() {
    if (of0MethodHandle == null) {
        return emptySet();
    }
    try {
        return (Set<E>) of0MethodHandle.invokeExact();
    } catch (Throwable e) {
        return emptySet();
    }
}

After:

public static <E> Set<E> ofSet() {
    return ofSet0(of0MethodHandle);
}

static <E> Set<E> ofSet0(MethodHandle methodHandle) {
    if (methodHandle == null) {
        return emptySet();
    }
    try {
        return (Set<E>) methodHandle.invokeExact();
    } catch (Throwable e) {
        return emptySet();
    }
}

Tests can now exercise both branches unconditionally by injecting null or a type-incompatible handle:

@Test
void testOfSet0WithNull() {
    assertEquals(emptySet(), ofSet0(null));
}

@Test
void testOfSet0WithWrongMethodHandle() {
    MethodHandle wrongMethodHandle = findPublicStatic(Collections.class, "emptySet");
    assertEquals(emptySet(), ofSet0(wrongMethodHandle));
}

Changes

• Sets.java — extracted ofSet0–ofSet10 and ofSetElements helpers
• Maps.java — extracted ofMap0–ofMap10 and ofMapEntries helpers
• Lists.java — extracted ofList0–ofList10 and ofListElements helpers
• SetsTest / MapsTest / ListsTest — added null + wrong-handle tests for every helper (114 tests total)

[sonarqubecloud]

Quality Gate failed

Failed conditions
B Maintainability Rating on New Code (required ≥ A)

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[codecov]

Codecov Report

❌ Patch coverage is 98.14815% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../src/main/java/io/microsphere/collection/Sets.java	94.44%	2 Missing ⚠️

Files with missing lines	Coverage Δ	Complexity Δ
...src/main/java/io/microsphere/collection/Lists.java	97.67% <100.00%> (+40.91%)	38.00 <35.00> (+24.00)
.../src/main/java/io/microsphere/collection/Maps.java	97.67% <100.00%> (+43.62%)	38.00 <35.00> (+24.00)
.../src/main/java/io/microsphere/collection/Sets.java	94.18% <94.44%> (+46.88%)	37.00 <35.00> (+24.00)

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