Remove parentheses from single unnamed lambda parameters (#1054)

timtebeek · web-flow · commit 8ad8d92ac020 · 2026-04-09T13:07:00.000+02:00
When `ReplaceUnusedVariablesWithUnderscore` renames a single
parenthesized lambda parameter to `_`, also strip the parentheses
so `(_) -&gt; ...` becomes `_ -&gt; ...`.
diff --git a/src/main/java/org/openrewrite/java/migrate/lang/ReplaceUnusedVariablesWithUnderscore.java b/src/main/java/org/openrewrite/java/migrate/lang/ReplaceUnusedVariablesWithUnderscore.java
@@ -70,22 +70,31 @@ public J.Try.Catch visitCatch(J.Try.Catch _catch, ExecutionContext ctx) {
             @Override
             public J.Lambda visitLambda(J.Lambda lambda, ExecutionContext ctx) {
                 J.Lambda l = super.visitLambda(lambda, ctx);
+                boolean willRename = false;
                 for (J param : l.getParameters().getParameters()) {
                     if (param instanceof J.VariableDeclarations) {
                         for (J.VariableDeclarations.NamedVariable namedVariable : ((J.VariableDeclarations) param).getVariables()) {
-                            renameVariableIfUnusedInContext(namedVariable, l.getBody());
+                            if (renameVariableIfUnusedInContext(namedVariable, l.getBody())) {
+                                willRename = true;
+                            }
                         }
                     }
                 }
+                if (willRename && l.getParameters().isParenthesized() &&
+                        l.getParameters().getParameters().size() == 1) {
+                    l = l.withParameters(l.getParameters().withParenthesized(false));
+                }
                 return l;
             }
 
-            private void renameVariableIfUnusedInContext(J.VariableDeclarations.NamedVariable variable, J context) {
+            private boolean renameVariableIfUnusedInContext(J.VariableDeclarations.NamedVariable variable, J context) {
                 if (!UNDERSCORE.equals(variable.getName().getSimpleName()) &&
                         VariableReferences.findRhsReferences(context, variable.getName()).isEmpty() &&
                         !usedInModifyingUnary(variable.getName(), context)) {
                     doAfterVisit(new RenameVariable<>(variable, UNDERSCORE));
+                    return true;
                 }
+                return false;
             }
 
             private boolean usedInModifyingUnary(J.Identifier identifier, J context) {
diff --git a/src/test/java/org/openrewrite/java/migrate/lang/ReplaceUnusedVariablesWithUnderscoreTest.java b/src/test/java/org/openrewrite/java/migrate/lang/ReplaceUnusedVariablesWithUnderscoreTest.java
@@ -174,6 +174,37 @@ void example() {
         );
     }
 
+    @Test
+    void replaceUnusedParenthesizedLambdaParameter() {
+        rewriteRun(
+          //language=java
+          java(
+            """
+              import java.util.stream.Stream;
+              import java.util.stream.Collectors;
+
+              class Test {
+                  void example() {
+                      Stream.of("a", "b", "c")
+                          .collect(Collectors.toMap(String::toUpperCase, (item) -> "NODATA"));
+                  }
+              }
+              """,
+            """
+              import java.util.stream.Stream;
+              import java.util.stream.Collectors;
+
+              class Test {
+                  void example() {
+                      Stream.of("a", "b", "c")
+                          .collect(Collectors.toMap(String::toUpperCase, _ -> "NODATA"));
+                  }
+              }
+              """
+          )
+        );
+    }
+
     @Test
     void doNotReplaceUsedLambdaParameter() {
         rewriteRun(

Original file line number	Diff line number	Diff line change
`@@ -70,22 +70,31 @@ public J.Try.Catch visitCatch(J.Try.Catch _catch, ExecutionContext ctx) {`
`70`	`70`	`@Override`
`71`	`71`	`public J.Lambda visitLambda(J.Lambda lambda, ExecutionContext ctx) {`
`72`	`72`	`J.Lambda l = super.visitLambda(lambda, ctx);`
	`73`	`+ boolean willRename = false;`
`73`	`74`	`for (J param : l.getParameters().getParameters()) {`
`74`	`75`	`if (param instanceof J.VariableDeclarations) {`
`75`	`76`	`for (J.VariableDeclarations.NamedVariable namedVariable : ((J.VariableDeclarations) param).getVariables()) {`
`76`		`- renameVariableIfUnusedInContext(namedVariable, l.getBody());`
	`77`	`+ if (renameVariableIfUnusedInContext(namedVariable, l.getBody())) {`
	`78`	`+ willRename = true;`
	`79`	`+ }`
`77`	`80`	`}`
`78`	`81`	`}`
`79`	`82`	`}`
	`83`	`+ if (willRename && l.getParameters().isParenthesized() &&`
	`84`	`+ l.getParameters().getParameters().size() == 1) {`
	`85`	`+ l = l.withParameters(l.getParameters().withParenthesized(false));`
	`86`	`+ }`
`80`	`87`	`return l;`
`81`	`88`	`}`
`82`	`89`
`83`		`- private void renameVariableIfUnusedInContext(J.VariableDeclarations.NamedVariable variable, J context) {`
	`90`	`+ private boolean renameVariableIfUnusedInContext(J.VariableDeclarations.NamedVariable variable, J context) {`
`84`	`91`	`if (!UNDERSCORE.equals(variable.getName().getSimpleName()) &&`
`85`	`92`	`VariableReferences.findRhsReferences(context, variable.getName()).isEmpty() &&`
`86`	`93`	`!usedInModifyingUnary(variable.getName(), context)) {`
`87`	`94`	`doAfterVisit(new RenameVariable<>(variable, UNDERSCORE));`
	`95`	`+ return true;`
`88`	`96`	`}`
	`97`	`+ return false;`
`89`	`98`	`}`
`90`	`99`
`91`	`100`	`private boolean usedInModifyingUnary(J.Identifier identifier, J context) {`