Quickfix for java:S6880 produces code on which tests fail

asm0dey · May 3, 2026, 10:18am

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Programming language you’re coding in: Java
Is connected mode used:
- SonarQube Cloud, SonarQube Server, or SonarQube Community Build? (if one of the latter two, which version?): no/ I don’t know

And a thorough description of the problem / question:

In the following code:


    @SuppressWarnings("unchecked")
    static Map<String, Object> deepMerge(Map<String, Object> base, Map<String, Object> overlay) {
        var out = new LinkedHashMap<>(base);
        for (var e : overlay.entrySet()) {
            var k = e.getKey();
            var v = e.getValue();
            var existing = out.get(k);
            if (existing instanceof Map<?, ?> em && v instanceof Map<?, ?> vm) {
                out.put(k, deepMerge((Map<String, Object>) em, (Map<String, Object>) vm));
            } else if (existing instanceof List<?> el && v instanceof List<?> vl) {
                var combined = new ArrayList<Object>(el);
                combined.addAll(vl);
                out.put(k, combined);
            } else {
                out.put(k, v);
            }
        }
        return out;
    }

all the tests work correctly, however when I apply the suggested quick fix, changing code to

    @SuppressWarnings("unchecked")
    static Map<String, Object> deepMerge(Map<String, Object> base, Map<String, Object> overlay) {
        var out = new LinkedHashMap<>(base);
        for (var e : overlay.entrySet()) {
            var k = e.getKey();
            var v = e.getValue();
            var existing = out.get(k);
            switch (existing) {
                case Map<?, ?> em when v instanceof Map<?, ?> vm -> {
                    out.put(k, deepMerge((Map<String, Object>) em, (Map<String, Object>) vm));
                }
                case List<?> el when v instanceof List<?> vl -> {
                    var combined = new ArrayList<Object>(el);
                    combined.addAll(vl);
                    out.put(k, combined);
                }
                default -> {
                    out.put(k, v);
                }
            }
        }
        return out;
    }

tests start to fail with

java.lang.NullPointerException
	at java.base/java.util.Objects.requireNonNull(Objects.java:220)
	at io.guessit.config.ConfigLoader.deepMerge(ConfigLoader.java:106)
	at io.guessit.config.ConfigLoader.load(ConfigLoader.java:25)
	at io.guessit.Guessit.<init>(Guessit.java:20)
	at io.guessit.Guessit.withOptions(Guessit.java:24)
	at io.guessit.Guessit.parse(Guessit.java:29)
	at io.guessit.parity.YmlParityTest.ymlParity(YmlParityTest.java:45)
	at java.base/java.lang.reflect.Method.invoke(Method.java:565)
	at java.base/java.util.Optional.ifPresent(Optional.java:178)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:186)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:186)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:186)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:186)
	at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:197)
	at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:197)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1716)
	at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:803)
	at java.base/java.util.stream.ReferencePipeline$7$1FlatMap.accept(ReferencePipeline.java:293)
	at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:197)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.Iterator.forEachRemaining(Iterator.java:133)
	at java.base/java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1939)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:570)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
	at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:153)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:176)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:265)
	at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:632)
	at java.base/java.util.stream.ReferencePipeline$7$1FlatMap.accept(ReferencePipeline.java:293)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:1024)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:570)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
	at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:153)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:176)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:265)
	at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:632)
	at java.base/java.util.stream.ReferencePipeline$7$1FlatMap.accept(ReferencePipeline.java:293)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1716)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:570)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
	at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:153)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:176)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:265)
	at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:632)
	at java.base/java.util.stream.ReferencePipeline$7$1FlatMap.accept(ReferencePipeline.java:293)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1716)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:570)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
	at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:153)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:176)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:265)
	at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:632)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1604)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1604)

This happens because this change changes the semantics: switch(existing) throws NPE is existing is null.
Since we start with a (usually) empty out (var out = new LinkedHashMap<>(base);) existing is usually null, so this code will usually fail.

alban.auzeill · May 5, 2026, 8:14am

Hi Pasha,

You are right: instanceof is null-safe while a switch expression without a case null arm throws a NullPointerException, so the quickfix was silently changing the semantics of the code.
I have created the bug ticket SONARJAVA-6341 to track the fix.

Thank you for the detailed report and the clear reproducer,

Alban

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Quickfix for java:S6880 produces code on which tests fail

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