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claude
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Add OpenRewrite recipe for JEP 512 instance main methods (#852)
* Add OpenRewrite recipe for JEP 512 instance main methods Implements a migration recipe to convert `public static void main(String[] args)` to instance `void main()` when the args parameter is unused, as introduced in JEP 512 (Java 21+). The recipe: - Detects main methods with unused String[] args parameter - Removes public and static modifiers - Removes the unused parameter - Preserves formatting and annotations - Only applies to Java 21+ codebases 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <[email protected]> * Apply formatter * Inline named visitor * Adopt `VariableReferences.findRhsReferences` * Use ListUtils.filter instead of manual ArrayList construction Refactored the modifier filtering logic to use OpenRewrite's ListUtils.filter and ListUtils.mapFirst utilities for cleaner and more idiomatic code. * Condense * Suppress warnings for confusing main methods * Further simplifications for String arrays --------- Co-authored-by: Claude <[email protected]>
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‎src/main/java/org/openrewrite/java/migrate/lang/MigrateMainMethodToInstanceMain.java‎

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/*
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* Copyright 2025 the original author or authors.
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* <p>
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* Licensed under the Moderne Source Available License (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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* <p>
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* https://docs.moderne.io/licensing/moderne-source-available-license
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* <p>
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package org.openrewrite.java.migrate.lang;
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import org.openrewrite.ExecutionContext;
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import org.openrewrite.Preconditions;
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import org.openrewrite.Recipe;
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import org.openrewrite.TreeVisitor;
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import org.openrewrite.java.JavaIsoVisitor;
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import org.openrewrite.java.search.UsesJavaVersion;
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import org.openrewrite.java.tree.J;
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import org.openrewrite.java.tree.JavaType;
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import org.openrewrite.java.tree.TypeUtils;
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import org.openrewrite.staticanalysis.VariableReferences;
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import static java.util.Collections.emptyList;
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public class MigrateMainMethodToInstanceMain extends Recipe {
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@Override
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public String getDisplayName() {
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return "Migrate `public static void main(String[] args)` to instance `void main()`";
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}
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@Override
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public String getDescription() {
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return "Migrate `public static void main(String[] args)` method to instance `void main()` method when the `args` parameter is unused, as supported by JEP 512 in Java 25+.";
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}
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@Override
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public TreeVisitor<?, ExecutionContext> getVisitor() {
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return Preconditions.check(new UsesJavaVersion<>(25), new JavaIsoVisitor<ExecutionContext>() {
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@Override
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public J.MethodDeclaration visitMethodDeclaration(J.MethodDeclaration method, ExecutionContext ctx) {
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J.MethodDeclaration md = super.visitMethodDeclaration(method, ctx);
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// Check if this is a main method: public static void main(String[] args)
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if (!"main".equals(md.getSimpleName()) ||
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md.getReturnTypeExpression() == null ||
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md.getReturnTypeExpression().getType() != JavaType.Primitive.Void ||
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!md.hasModifier(J.Modifier.Type.Public) ||
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!md.hasModifier(J.Modifier.Type.Static) ||
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md.getParameters().size() != 1 ||
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!(md.getParameters().get(0) instanceof J.VariableDeclarations) ||
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md.getBody() == null) {
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return md;
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}
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// Check if parameter is String[] type
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J.VariableDeclarations param = (J.VariableDeclarations) md.getParameters().get(0);
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JavaType paramType = param.getType();
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if (!TypeUtils.isOfClassType(paramType, "java.lang.String") || !(paramType instanceof JavaType.Array)) {
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return md;
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}
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// Remove the parameter if unused
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J.Identifier variableName = param.getVariables().get(0).getName();
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if (VariableReferences.findRhsReferences(md.getBody(), variableName).isEmpty()) {
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md = md.withParameters(emptyList());
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}
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return md.withReturnTypeExpression(md.getReturnTypeExpression().withPrefix(md.getModifiers().get(0).getPrefix()))
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.withModifiers(emptyList());
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}
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});
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}
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}

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

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- org.openrewrite.github.SetupJavaUpgradeJavaVersion:
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minimumJavaMajorVersion: 25
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- org.openrewrite.java.migrate.io.ReplaceSystemOutWithIOPrint
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- org.openrewrite.java.migrate.lang.MigrateMainMethodToInstanceMain
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- org.openrewrite.java.migrate.lang.MigrateProcessWaitForDuration
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- org.openrewrite.java.migrate.lang.ReplaceUnusedVariablesWithUnderscore
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- org.openrewrite.java.migrate.util.MigrateInflaterDeflaterToClose

‎src/test/java/org/openrewrite/java/migrate/lang/MigrateMainMethodToInstanceMainTest.java‎

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/*
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* Copyright 2025 the original author or authors.
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* <p>
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* Licensed under the Moderne Source Available License (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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* <p>
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* https://docs.moderne.io/licensing/moderne-source-available-license
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* <p>
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package org.openrewrite.java.migrate.lang;
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import org.junit.jupiter.api.Test;
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import org.openrewrite.DocumentExample;
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import org.openrewrite.test.RecipeSpec;
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import org.openrewrite.test.RewriteTest;
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import static org.openrewrite.java.Assertions.java;
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import static org.openrewrite.java.Assertions.javaVersion;
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@SuppressWarnings("ConfusingMainMethod")
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class MigrateMainMethodToInstanceMainTest implements RewriteTest {
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@Override
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public void defaults(RecipeSpec spec) {
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spec.recipe(new MigrateMainMethodToInstanceMain())
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.allSources(s -> s.markers(javaVersion(25)));
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}
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@DocumentExample
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@Test
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void migrateMainMethodWithUnusedArgs() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public static void main(String[] args) {
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System.out.println("Hello, World!");
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}
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}
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""",
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"""
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class Application {
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void main() {
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System.out.println("Hello, World!");
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}
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}
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"""
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)
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);
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}
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@Test
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void retainArgsWhenUsed() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public static void main(String[] args) {
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if (args.length > 0) {
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System.out.println("Args provided: " + args[0]);
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}
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}
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}
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""",
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"""
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class Application {
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void main(String[] args) {
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if (args.length > 0) {
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System.out.println("Args provided: " + args[0]);
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}
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}
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}
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"""
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)
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);
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}
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@Test
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void retainArgsWhenUsedInMethodCall() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public static void main(String[] args) {
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processArgs(args);
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}
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private static void processArgs(String[] args) {
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// Process arguments
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}
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}
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""",
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"""
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class Application {
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void main(String[] args) {
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processArgs(args);
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}
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private static void processArgs(String[] args) {
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// Process arguments
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}
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}
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"""
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)
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);
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}
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@Test
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void migrateMainMethodWithEmptyBody() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public static void main(String[] args) {
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}
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}
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""",
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"""
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class Application {
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void main() {
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}
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}
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"""
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)
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);
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}
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@Test
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void doNotMigrateNonMainMethod() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public static void notMain(String[] args) {
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System.out.println("Not a main method");
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}
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}
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"""
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)
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);
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}
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@Test
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void doNotMigrateMainWithDifferentSignature() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public static int main(String[] args) {
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return 0;
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}
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}
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"""
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)
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);
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}
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@Test
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void doNotMigratePrivateMain() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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private static void main(String[] args) {
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System.out.println("Private main");
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}
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}
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"""
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)
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);
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}
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@Test
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void doNotMigrateInstanceMain() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public void main(String[] args) {
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System.out.println("Already instance main");
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}
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}
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"""
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)
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);
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}
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@Test
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void migrateMainWithComplexUnusedArgs() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public static void main(String[] arguments) {
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System.out.println("Starting application...");
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new Application().run();
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}
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void run() {
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System.out.println("Running...");
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}
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}
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""",
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"""
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class Application {
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void main() {
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System.out.println("Starting application...");
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new Application().run();
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}
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void run() {
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System.out.println("Running...");
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}
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}
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"""
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)
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);
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}
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@Test
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void doNotMigrateMainWithMultipleParameters() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public static void main(String first, String[] args) {
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System.out.println("Invalid main method");
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}
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}
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"""
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)
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);
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}
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@Test
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void doNotMigrateMainWithNoParameters() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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public static void main() {
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System.out.println("No parameters");
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}
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}
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"""
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)
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);
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}
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@Test
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void migrateMainWithAnnotations() {
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//language=java
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rewriteRun(
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java(
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"""
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class Application {
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@SuppressWarnings("unused")
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public static void main(String[] args) {
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System.out.println("Hello!");
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}
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}
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""",
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"""
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class Application {
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@SuppressWarnings("unused")
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void main() {
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System.out.println("Hello!");
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}
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}
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"""
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)
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);
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}
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}

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