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Replace Collections.emptyXXX with Immutable Static Factory Methods (#1045)
* Replace Collections.emptyXXX with Immutable Static Factory Methods Closes #1044 * Add Java 9+ precondition and use JavaTemplate for Collections.emptyXXX recipes * Add all util recipes to JavaUtilAPIs and reference it from best practices * Fix ReplaceMathRandomWithThreadLocalRandom recipe class name --------- Co-authored-by: Tim te Beek <[email protected]>
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‎src/main/java/org/openrewrite/java/migrate/util/MigrateCollectionsEmptyList.java‎

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/*
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* Copyright 2026 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.util;
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import lombok.Getter;
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import org.openrewrite.*;
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import org.openrewrite.java.JavaIsoVisitor;
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import org.openrewrite.java.JavaTemplate;
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import org.openrewrite.java.MethodMatcher;
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import org.openrewrite.java.search.UsesJavaVersion;
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import org.openrewrite.java.search.UsesMethod;
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import org.openrewrite.java.tree.J;
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public class MigrateCollectionsEmptyList extends Recipe {
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private static final MethodMatcher EMPTY_LIST = new MethodMatcher("java.util.Collections emptyList()");
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@Getter
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final String displayName = "Prefer `List.of()`";
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@Getter
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final String description = "Prefer `List.of()` instead of using `Collections.emptyList()` in Java 9 or higher.";
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@Override
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public TreeVisitor<?, ExecutionContext> getVisitor() {
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TreeVisitor<?, ExecutionContext> check = Preconditions.and(new UsesJavaVersion<>(9),
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new UsesMethod<>(EMPTY_LIST));
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return Preconditions.check(check, new JavaIsoVisitor<ExecutionContext>() {
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@Override
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public J.MethodInvocation visitMethodInvocation(J.MethodInvocation method, ExecutionContext ctx) {
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J.MethodInvocation m = super.visitMethodInvocation(method, ctx);
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if (EMPTY_LIST.matches(m)) {
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maybeRemoveImport("java.util.Collections");
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maybeAddImport("java.util.List");
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return JavaTemplate.builder("List.of()")
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.contextSensitive()
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.imports("java.util.List")
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.build()
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.apply(updateCursor(m), m.getCoordinates().replace());
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}
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return m;
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}
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});
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}
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}

‎src/main/java/org/openrewrite/java/migrate/util/MigrateCollectionsEmptyMap.java‎

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/*
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* Copyright 2026 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.util;
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import lombok.Getter;
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import org.openrewrite.*;
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import org.openrewrite.java.JavaIsoVisitor;
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import org.openrewrite.java.JavaTemplate;
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import org.openrewrite.java.MethodMatcher;
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import org.openrewrite.java.search.UsesJavaVersion;
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import org.openrewrite.java.search.UsesMethod;
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import org.openrewrite.java.tree.J;
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public class MigrateCollectionsEmptyMap extends Recipe {
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private static final MethodMatcher EMPTY_MAP = new MethodMatcher("java.util.Collections emptyMap()");
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@Getter
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final String displayName = "Prefer `Map.of()`";
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@Getter
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final String description = "Prefer `Map.of()` instead of using `Collections.emptyMap()` in Java 9 or higher.";
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@Override
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public TreeVisitor<?, ExecutionContext> getVisitor() {
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TreeVisitor<?, ExecutionContext> check = Preconditions.and(new UsesJavaVersion<>(9),
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new UsesMethod<>(EMPTY_MAP));
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return Preconditions.check(check, new JavaIsoVisitor<ExecutionContext>() {
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@Override
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public J.MethodInvocation visitMethodInvocation(J.MethodInvocation method, ExecutionContext ctx) {
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J.MethodInvocation m = super.visitMethodInvocation(method, ctx);
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if (EMPTY_MAP.matches(m)) {
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maybeRemoveImport("java.util.Collections");
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maybeAddImport("java.util.Map");
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return JavaTemplate.builder("Map.of()")
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.contextSensitive()
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.imports("java.util.Map")
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.build()
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.apply(updateCursor(m), m.getCoordinates().replace());
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}
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return m;
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}
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});
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}
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}

‎src/main/java/org/openrewrite/java/migrate/util/MigrateCollectionsEmptySet.java‎

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/*
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* Copyright 2026 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.util;
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import lombok.Getter;
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import org.openrewrite.*;
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import org.openrewrite.java.JavaIsoVisitor;
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import org.openrewrite.java.JavaTemplate;
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import org.openrewrite.java.MethodMatcher;
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import org.openrewrite.java.search.UsesJavaVersion;
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import org.openrewrite.java.search.UsesMethod;
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import org.openrewrite.java.tree.J;
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public class MigrateCollectionsEmptySet extends Recipe {
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private static final MethodMatcher EMPTY_SET = new MethodMatcher("java.util.Collections emptySet()");
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@Getter
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final String displayName = "Prefer `Set.of()`";
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@Getter
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final String description = "Prefer `Set.of()` instead of using `Collections.emptySet()` in Java 9 or higher.";
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@Override
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public TreeVisitor<?, ExecutionContext> getVisitor() {
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TreeVisitor<?, ExecutionContext> check = Preconditions.and(new UsesJavaVersion<>(9),
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new UsesMethod<>(EMPTY_SET));
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return Preconditions.check(check, new JavaIsoVisitor<ExecutionContext>() {
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@Override
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public J.MethodInvocation visitMethodInvocation(J.MethodInvocation method, ExecutionContext ctx) {
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J.MethodInvocation m = super.visitMethodInvocation(method, ctx);
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if (EMPTY_SET.matches(m)) {
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maybeRemoveImport("java.util.Collections");
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maybeAddImport("java.util.Set");
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return JavaTemplate.builder("Set.of()")
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.contextSensitive()
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.imports("java.util.Set")
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.build()
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.apply(updateCursor(m), m.getCoordinates().replace());
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}
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return m;
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}
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});
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}
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}

‎src/main/resources/META-INF/rewrite/examples.yml‎

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language: java
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---
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type: specs.openrewrite.org/v1beta/example
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recipeName: org.openrewrite.java.migrate.util.MigrateCollectionsEmptyList
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examples:
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- description: '`MigrateCollectionsEmptyListTest#emptyList`'
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sources:
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- before: |
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import java.util.*;
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class Test {
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List<String> list = Collections.emptyList();
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}
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after: |
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import java.util.List;
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class Test {
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List<String> list = List.of();
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}
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language: java
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---
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type: specs.openrewrite.org/v1beta/example
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recipeName: org.openrewrite.java.migrate.util.MigrateCollectionsEmptySet
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examples:
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- description: '`MigrateCollectionsEmptySetTest#emptySet`'
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sources:
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- before: |
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import java.util.*;
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class Test {
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Set<String> set = Collections.emptySet();
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}
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after: |
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import java.util.Set;
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class Test {
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Set<String> set = Set.of();
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}
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language: java
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---
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type: specs.openrewrite.org/v1beta/example
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recipeName: org.openrewrite.java.migrate.util.MigrateCollectionsEmptyMap
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examples:
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- description: '`MigrateCollectionsEmptyMapTest#emptyMap`'
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sources:
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- before: |
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import java.util.*;
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class Test {
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Map<String, String> map = Collections.emptyMap();
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}
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import java.util.Map;
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class Test {
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Map<String, String> map = Map.of();
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}
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language: java
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---
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type: specs.openrewrite.org/v1beta/example
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recipeName: org.openrewrite.java.migrate.util.MigrateCollectionsSingletonList
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examples:
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- description: '`MigrateCollectionsSingletonListTest#singletonList`'

‎src/main/resources/META-INF/rewrite/java-best-practices.yml‎

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- org.openrewrite.java.migrate.lang.var.UseVarForGenericMethodInvocations
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- org.openrewrite.java.migrate.lang.var.UseVarForPrimitive
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# Prefer modern Java collection factories and utilities
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- org.openrewrite.java.migrate.util.UseEnumSetOf
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- org.openrewrite.java.migrate.util.UseListOf
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- org.openrewrite.java.migrate.util.UseMapOf
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- org.openrewrite.java.migrate.util.UseSetOf
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- org.openrewrite.java.migrate.util.JavaUtilAPIs
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# Static analysis: bug prevention
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- org.openrewrite.staticanalysis.ReplaceWeekYearWithYear
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- org.openrewrite.staticanalysis.RemoveHashCodeCallsFromArrayInstances

‎src/main/resources/META-INF/rewrite/java-util-apis.yml‎

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preconditions:
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- org.openrewrite.Singleton
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recipeList:
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- org.openrewrite.java.migrate.util.IteratorNext
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- org.openrewrite.java.migrate.util.ListFirstAndLast
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- org.openrewrite.java.migrate.util.MigrateCollectionsEmptyList
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- org.openrewrite.java.migrate.util.MigrateCollectionsEmptyMap
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- org.openrewrite.java.migrate.util.MigrateCollectionsEmptySet
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- org.openrewrite.java.migrate.util.MigrateCollectionsSingletonList
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- org.openrewrite.java.migrate.util.MigrateCollectionsSingletonMap
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- org.openrewrite.java.migrate.util.MigrateCollectionsSingletonSet
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- org.openrewrite.java.migrate.util.MigrateCollectionsUnmodifiableList
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- org.openrewrite.java.migrate.util.MigrateCollectionsUnmodifiableSet
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- org.openrewrite.java.migrate.util.MigrateStringReaderToReaderOf
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- org.openrewrite.java.migrate.util.OptionalNotEmptyToIsPresent
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- org.openrewrite.java.migrate.util.OptionalNotPresentToIsEmpty
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- org.openrewrite.java.migrate.util.OptionalStreamRecipe
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- org.openrewrite.java.migrate.util.RemoveFinalizerFromZip
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- org.openrewrite.java.migrate.util.ReplaceMathRandomWithThreadLocalRandomRecipe
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- org.openrewrite.java.migrate.util.ReplaceStreamCollectWithToList
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- org.openrewrite.java.migrate.util.StreamFindFirst
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- org.openrewrite.java.migrate.util.UseEnumSetOf
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- org.openrewrite.java.migrate.util.UseListOf
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- org.openrewrite.java.migrate.util.UseLocaleOf
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- org.openrewrite.java.migrate.util.UseMapOf
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- org.openrewrite.java.migrate.util.UseSetOf

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