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Remove null checks before instanceof only for side-effect-free expressions #972

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Remove null checks before instanceof only for side-effect-free expressions #972

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be5ee63
RemoveRedundantNullCheckBeforeInstanceof: only simplify side-effect-f…
martinfrancois Aug 9, 2026
aba6f51
Review fixes: require both compared operands to be side-effect free
timtebeek Aug 11, 2026
728edcf
Trim commentary
timtebeek Aug 11, 2026
af7f05a
Consolidate side-effect regression tests
martinfrancois Aug 16, 2026
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19 changes: 11 additions & 8 deletions src/main/java/org/openrewrite/staticanalysis/RemoveRedundantNullCheckBeforeInstanceof.java
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,7 @@
import java.util.Set;

import static java.util.Collections.singleton;
import static org.openrewrite.staticanalysis.SideEffects.mayHaveSideEffects;

@EqualsAndHashCode(callSuper = false)
@Value
Expand Down Expand Up @@ -90,15 +91,17 @@ public J visitBinary(J.Binary binary, ExecutionContext ctx) {
}

private boolean isRedundantNullCheck(J.Binary nullCheck, J.InstanceOf instanceOf) {
if (nullCheck.getOperator() == J.Binary.Type.NotEqual) {
if (J.Literal.isLiteralValue(nullCheck.getLeft(), null)) {
return SemanticallyEqual.areEqual(nullCheck.getRight(), instanceOf.getExpression());
}
if (J.Literal.isLiteralValue(nullCheck.getRight(), null)) {
return SemanticallyEqual.areEqual(nullCheck.getLeft(), instanceOf.getExpression());
}
if (nullCheck.getOperator() != J.Binary.Type.NotEqual) {
return false;
}
Expression checked = J.Literal.isLiteralValue(nullCheck.getLeft(), null) ? nullCheck.getRight() :
J.Literal.isLiteralValue(nullCheck.getRight(), null) ? nullCheck.getLeft() : null;
if (checked == null || !SemanticallyEqual.areEqual(checked, instanceOf.getExpression())) {
return false;
}
return false;
// The rewrite evaluates once what was evaluated twice, so both occurrences must be side-effect
// free; they can differ, as `SemanticallyEqual` matches a static field access against its qualified form
return !mayHaveSideEffects(checked) && !mayHaveSideEffects(instanceOf.getExpression());
}
});
}
Expand Down
101 changes: 95 additions & 6 deletions ...est/java/org/openrewrite/staticanalysis/RemoveRedundantNullCheckBeforeInstanceofTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -87,35 +87,124 @@ void foo(Object obj) {
);
}


@Issue("https://github.com/openrewrite/rewrite-static-analysis/issues/953")
@Test
void removeRedundantNullCheckWithMethodInvocation() {
void doNotChangeWhenNullCheckedExpressionIsMethodInvocation() {
rewriteRun(
//language=java
java(
"""
class A {
void foo() {
void direct() {
if (getValue() != null && getValue() instanceof String) {
System.out.println("String value");
}
if (null != getValue() && getValue() instanceof String) {
System.out.println("String value");
}
}

void chained(boolean enabled) {
if (enabled && getValue() != null && getValue() instanceof String) {
System.out.println("String value");
}
}

String getValue() {
return "test";
}
}
"""
)
);
}

@Test
void removeRedundantNullCheckInChainedCondition() {
rewriteRun(
//language=java
java(
"""
class A {
void foo(boolean enabled, Object obj) {
if (enabled && obj != null && obj instanceof String) {
System.out.println("String value");
}
}
}
""",
"""
class A {
void foo(boolean enabled, Object obj) {
if (enabled && obj instanceof String) {
System.out.println("String value");
}
}
}
"""
)
);
}

@Issue("https://github.com/openrewrite/rewrite-static-analysis/issues/953")
@Test
void doNotChangeWhenConstructorCall() {
rewriteRun(
//language=java
java(
"""
class A {
void foo() {
if (new StringBuilder() != null && new StringBuilder() instanceof CharSequence) {
System.out.println("CharSequence value");
}
}
}
"""
)
);
}

@Issue("https://github.com/openrewrite/rewrite-static-analysis/issues/953")
@Test
void doNotChangeWhenArrayIndexHasSideEffect() {
rewriteRun(
//language=java
java(
"""
class A {
Object[] values = new Object[2];
int i;

void foo() {
if (getValue() instanceof String) {
if (values[i++] != null && values[i++] instanceof String) {
System.out.println("String value");
}
}
}
"""
)
);
}

String getValue() {
return "test";
@Issue("https://github.com/openrewrite/rewrite-static-analysis/issues/953")
@Test
void doNotChangeWhenOnlyTheNullCheckedOperandHasSideEffects() {
rewriteRun(
//language=java
java(
"""
class A {
static Integer count = 1;

A getInstance() {
return this;
}

void foo() {
if (getInstance().count != null && count instanceof Integer) {
System.out.println("Integer value");
}
}
}
"""
Expand Down
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