Java 25: Unused variable to underscore

.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 +1,91 @@
+/*
+ * 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.staticanalysis.VariableReferences;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.function.Function;
+
+@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.VariableDeclarations.NamedVariable visitVariable(J.VariableDeclarations.NamedVariable variable, ExecutionContext ctx) {
+                J.VariableDeclarations.NamedVariable v = super.visitVariable(variable, ctx);
+                if (UNDERSCORE.equals(v.getSimpleName())) {
+                    return v;
+                }
+                if (replaceIfUnusedInContext(v, J.ForEachLoop.class, J.ForEachLoop::getBody) ||
+                        replaceIfUnusedInContext(v, J.Try.Catch.class, J.Try.Catch::getBody) ||
+                        replaceIfUnusedInContext(v, J.Lambda.class, J.Lambda::getBody)) {
+                    doAfterVisit(new RenameVariable<>(variable, UNDERSCORE));
+                }
+                return v;
+            }
+
+            private <T extends J> boolean replaceIfUnusedInContext(
+                    J.VariableDeclarations.NamedVariable variable, Class<T> contextClass, Function<T, J> bodyExtractor) {
+                T context = getCursor().firstEnclosing(contextClass);
+                return context != null &&
+                        VariableReferences.findRhsReferences(bodyExtractor.apply(context), variable.getName()).isEmpty() &&
+                        !usedInModifyingUnary(context, variable.getName());
+            }
+
+            private <T extends J> boolean usedInModifyingUnary(T context, J.Identifier name) {
+                return new JavaIsoVisitor<AtomicBoolean>() {
+                    @Override
+                    public J.Unary visitUnary(J.Unary unary, AtomicBoolean atomicBoolean) {
+                        if (unary.getOperator().isModifying() &&
+                                SemanticallyEqual.areEqual(name, 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