Java 25: Unused variable to underscore (#844)

* initial

* Implementation

* polish

* Update ReplaceUnusedVariablesWithUnderscore.java

* Apply formatter and inline named visitor

* Adopt `VariableReferences.findRhsReferences`

* Leverage existing `RenameVariable`

* Add additional tests and handling

* Override individual visit methods and only evaluate params

* Simplify logic

* Remove generic type

---------

Co-authored-by: Tim te Beek <[email protected]>

Author: JohannisK

.sdkmanrc

@@ -1,3 +1,3 @@
 # Enable auto-env through the sdkman_auto_env config
 # Add key=value pairs of SDKs to use below
-java=21.0.5-tem
+java=21.0.7-tem

src/main/java/org/openrewrite/java/migrate/lang/ReplaceUnusedVariablesWithUnderscore.java

@@ -0,0 +1,111 @@
+/*
+ * Copyright 2025 the original author or authors.
+ * <p>
+ * Licensed under the Moderne Source Available License (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://docs.moderne.io/licensing/moderne-source-available-license
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openrewrite.java.migrate.lang;
+
+import lombok.EqualsAndHashCode;
+import lombok.Value;
+import org.openrewrite.ExecutionContext;
+import org.openrewrite.Preconditions;
+import org.openrewrite.Recipe;
+import org.openrewrite.TreeVisitor;
+import org.openrewrite.java.JavaIsoVisitor;
+import org.openrewrite.java.RenameVariable;
+import org.openrewrite.java.search.SemanticallyEqual;
+import org.openrewrite.java.search.UsesJavaVersion;
+import org.openrewrite.java.tree.J;
+import org.openrewrite.java.tree.Statement;
+import org.openrewrite.staticanalysis.VariableReferences;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+
+@EqualsAndHashCode(callSuper = false)
+@Value
+public class ReplaceUnusedVariablesWithUnderscore extends Recipe {
+
+    private static final String UNDERSCORE = "_";
+
+    @Override
+    public String getDisplayName() {
+        return "Replace unused variables with underscore";
+    }
+
+    @Override
+    public String getDescription() {
+        return "Replace unused variable declarations with underscore (_) for Java 22+. " +
+                "This includes unused variables in enhanced for loops, catch blocks, " +
+                "and lambda parameters where the variable is never referenced.";
+    }
+
+    @Override
+    public TreeVisitor<?, ExecutionContext> getVisitor() {
+        return Preconditions.check(new UsesJavaVersion<>(25), new JavaIsoVisitor<ExecutionContext>() {
+            @Override
+            public J.ForEachLoop visitForEachLoop(J.ForEachLoop forLoop, ExecutionContext ctx) {
+                J.ForEachLoop l = super.visitForEachLoop(forLoop, ctx);
+                Statement variable = l.getControl().getVariable();
+                if (variable instanceof J.VariableDeclarations) {
+                    for (J.VariableDeclarations.NamedVariable namedVariable : ((J.VariableDeclarations) variable).getVariables()) {
+                        renameVariableIfUnusedInContext(namedVariable, l.getBody());
+                    }
+                }
+                return l;
+            }
+
+            @Override
+            public J.Try.Catch visitCatch(J.Try.Catch _catch, ExecutionContext ctx) {
+                J.Try.Catch c = super.visitCatch(_catch, ctx);
+                for (J.VariableDeclarations.NamedVariable namedVariable : c.getParameter().getTree().getVariables()) {
+                    renameVariableIfUnusedInContext(namedVariable, c.getBody());
+                }
+                return c;
+            }
+
+            @Override
+            public J.Lambda visitLambda(J.Lambda lambda, ExecutionContext ctx) {
+                J.Lambda l = super.visitLambda(lambda, ctx);
+                for (J param : l.getParameters().getParameters()) {
+                    if (param instanceof J.VariableDeclarations) {
+                        for (J.VariableDeclarations.NamedVariable namedVariable : ((J.VariableDeclarations) param).getVariables()) {
+                            renameVariableIfUnusedInContext(namedVariable, l.getBody());
+                        }
+                    }
+                }
+                return l;
+            }
+
+            private void 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));
+                }
+            }
+
+            private boolean usedInModifyingUnary(J.Identifier identifier, J context) {
+                return new JavaIsoVisitor<AtomicBoolean>() {
+                    @Override
+                    public J.Unary visitUnary(J.Unary unary, AtomicBoolean atomicBoolean) {
+                        if (unary.getOperator().isModifying() &&
+                                SemanticallyEqual.areEqual(identifier, unary.getExpression())) {
+                            atomicBoolean.set(true);
+                        }
+                        return super.visitUnary(unary, atomicBoolean);
+                    }
+                }.reduce(context, new AtomicBoolean(false)).get();
+            }
+        });
+    }
+}

src/main/resources/META-INF/rewrite/java-version-25.yml

@@ -28,6 +28,7 @@ recipeList:
   - org.openrewrite.java.migrate.UpgradeToJava21
   - org.openrewrite.java.migrate.UpgradeBuildToJava25
   - org.openrewrite.java.migrate.lang.MigrateProcessWaitForDuration
+  - org.openrewrite.java.migrate.lang.ReplaceUnusedVariablesWithUnderscore
   - org.openrewrite.java.migrate.util.MigrateInflaterDeflaterToClose
   - org.openrewrite.java.migrate.AccessController
   - org.openrewrite.java.migrate.RemoveSecurityPolicy