Error prone validation to avoid redundant modifiers

README.md

@@ -175,6 +175,7 @@ Safe Logging can be found at [github.com/palantir/safe-logging](https://github.c
 - `ReverseDnsLookup`: Calling address.getHostName may result in an unexpected DNS lookup.
 - `ReadReturnValueIgnored`: The result of a read call must be checked to know if EOF has been reached or the expected number of bytes have been consumed.
 - `FinalClass`: A class should be declared final if all of its constructors are private.
+- `RedundantModifier`: Avoid using redundant modifiers.
 
 ### Programmatic Application
 

baseline-error-prone/src/main/java/com/palantir/baseline/errorprone/RedundantModifier.java

@@ -0,0 +1,118 @@
+/*
+ * (c) Copyright 2019 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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 com.palantir.baseline.errorprone;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.fixes.SuggestedFixes;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.google.errorprone.matchers.Matchers;
+import com.sun.source.tree.ClassTree;
+import com.sun.source.tree.MethodTree;
+import com.sun.source.tree.Tree;
+import java.util.Locale;
+import javax.lang.model.element.Modifier;
+
+/**
+ * In a future change we may want to validate against unnecessary modifiers based on encapsulating
+ * component visibility, for example there's no reason to allow a public constructor for a private
+ * class.
+ */
+@AutoService(BugChecker.class)
+@BugPattern(
+        name = "RedundantModifier",
+        link = "https://github.com/palantir/gradle-baseline#baseline-error-prone-checks",
+        linkType = BugPattern.LinkType.CUSTOM,
+        providesFix = BugPattern.ProvidesFix.REQUIRES_HUMAN_ATTENTION,
+        severity = BugPattern.SeverityLevel.ERROR,
+        summary = "Avoid using redundant modifiers")
+public final class RedundantModifier extends BugChecker
+        implements BugChecker.ClassTreeMatcher, BugChecker.MethodTreeMatcher {
+
+    private static final Matcher<ClassTree> STATIC_ENUM_OR_INTERFACE = Matchers.allOf(
+            Matchers.anyOf(Matchers.kindIs(Tree.Kind.ENUM), Matchers.kindIs(Tree.Kind.INTERFACE)),
+            Matchers.hasModifier(Modifier.STATIC));
+
+    private static final Matcher<MethodTree> PRIVATE_ENUM_CONSTRUCTOR = Matchers.allOf(
+            Matchers.methodIsConstructor(),
+            Matchers.enclosingClass(Matchers.kindIs(Tree.Kind.ENUM)),
+            Matchers.hasModifier(Modifier.PRIVATE));
+
+    private static final Matcher<MethodTree> STATIC_FINAL_METHOD = Matchers.allOf(
+            Matchers.isStatic(),
+            Matchers.hasModifier(Modifier.FINAL));
+
+    private static final Matcher<MethodTree> UNNECESSARY_INTERFACE_METHOD_MODIFIERS = Matchers.allOf(
+            Matchers.enclosingClass(Matchers.kindIs(Tree.Kind.INTERFACE)),
+            Matchers.not(Matchers.isStatic()),
+            Matchers.not(Matchers.hasModifier(Modifier.DEFAULT)),
+            Matchers.anyOf(Matchers.hasModifier(Modifier.PUBLIC), Matchers.hasModifier(Modifier.ABSTRACT)));
+
+    private static final Matcher<MethodTree> UNNECESSARY_FINAL_METHOD_ON_FINAL_CLASS = Matchers.allOf(
+            Matchers.not(Matchers.isStatic()),
+            Matchers.enclosingClass(Matchers.allOf(
+                    Matchers.kindIs(Tree.Kind.CLASS),
+                    Matchers.hasModifier(Modifier.FINAL))),
+            Matchers.allOf(
+                    Matchers.hasModifier(Modifier.FINAL),
+                    Matchers.not(Matchers.hasAnnotation(SafeVarargs.class))));
+
+    @Override
+    public Description matchClass(ClassTree tree, VisitorState state) {
+        if (STATIC_ENUM_OR_INTERFACE.matches(tree, state)) {
+            return buildDescription(tree)
+                    .setMessage(tree.getKind().name().toLowerCase(Locale.ENGLISH)
+                            + "s are static by default. The 'static' modifier is unnecessary.")
+                    .addFix(SuggestedFixes.removeModifiers(tree, state, Modifier.STATIC))
+                    .build();
+        }
+        return Description.NO_MATCH;
+    }
+
+    @Override
+    public Description matchMethod(MethodTree tree, VisitorState state) {
+        if (PRIVATE_ENUM_CONSTRUCTOR.matches(tree, state)) {
+            return buildDescription(tree)
+                    .setMessage("Enum constructors are private by default. The 'private' modifier is unnecessary.")
+                    .addFix(SuggestedFixes.removeModifiers(tree, state, Modifier.PRIVATE))
+                    .build();
+        }
+        if (STATIC_FINAL_METHOD.matches(tree, state)) {
+            return buildDescription(tree)
+                    .setMessage("Static methods cannot be overridden. The 'final' modifier is unnecessary.")
+                    .addFix(SuggestedFixes.removeModifiers(tree, state, Modifier.FINAL))
+                    .build();
+        }
+        if (UNNECESSARY_INTERFACE_METHOD_MODIFIERS.matches(tree, state)) {
+            return buildDescription(tree)
+                    .setMessage("Interface methods are public and abstract by default. "
+                            + "The 'public' and 'abstract' modifiers are unnecessary.")
+                    .addFix(SuggestedFixes.removeModifiers(tree, state, Modifier.PUBLIC, Modifier.ABSTRACT))
+                    .build();
+        }
+        if (UNNECESSARY_FINAL_METHOD_ON_FINAL_CLASS.matches(tree, state)) {
+            return buildDescription(tree)
+                    .setMessage("Redundant 'final' modifier on an instance method of a final class.")
+                    .addFix(SuggestedFixes.removeModifiers(tree, state, Modifier.FINAL))
+                    .build();
+        }
+        return Description.NO_MATCH;
+    }
+}

baseline-error-prone/src/test/java/com/palantir/baseline/errorprone/RedundantModifierTest.java

@@ -0,0 +1,180 @@
+/*
+ * (c) Copyright 2019 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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 com.palantir.baseline.errorprone;
+
+import com.google.errorprone.BugCheckerRefactoringTestHelper;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.parallel.Execution;
+import org.junit.jupiter.api.parallel.ExecutionMode;
+
+@Execution(ExecutionMode.CONCURRENT)
+class RedundantModifierTest {
+
+    @Test
+    void fixEnumConstructor() {
+        fix()
+                .addInputLines(
+                        "Test.java",
+                        "public enum Test {",
+                        "  INSTANCE(\"str\");",
+                        "  private final String str;",
+                        "  private Test(String str) {",
+                        "    this.str = str;",
+                        "  }",
+                        "}"
+                )
+                .addOutputLines(
+                        "Test.java",
+                        "public enum Test {",
+                        "  INSTANCE(\"str\");",
+                        "  private final String str;",
+                        "  Test(String str) {",
+                        "    this.str = str;",
+                        "  }",
+                        "}"
+                ).doTest(BugCheckerRefactoringTestHelper.TestMode.TEXT_MATCH);
+    }
+
+    @Test
+    @SuppressWarnings("checkstyle:RegexpSinglelineJava")
+    void fixStaticEnum() {
+        fix()
+                .addInputLines(
+                        "Enclosing.java",
+                        "public class Enclosing {",
+                        "  public static enum Test {",
+                        "    INSTANCE",
+                        "  }",
+                        "}"
+                )
+                .addOutputLines(
+                        "Enclosing.java",
+                        "public class Enclosing {",
+                        "  public enum Test {",
+                        "    INSTANCE",
+                        "  }",
+                        "}"
+                ).doTest(BugCheckerRefactoringTestHelper.TestMode.TEXT_MATCH);
+    }
+
+    @Test
+    void fixStaticInterface() {
+        fix()
+                .addInputLines(
+                        "Enclosing.java",
+                        "public class Enclosing {",
+                        "  public static interface Test {",
+                        "  }",
+                        "}"
+                )
+                .addOutputLines(
+                        "Enclosing.java",
+                        "public class Enclosing {",
+                        "  public interface Test {",
+                        "  }",
+                        "}"
+                ).doTest(BugCheckerRefactoringTestHelper.TestMode.TEXT_MATCH);
+    }
+
+    @Test
+    void fixInterfaceMethods() {
+        fix()
+                .addInputLines(
+                        "Enclosing.java",
+                        "public class Enclosing {",
+                        "  public interface Test {",
+                        "    public int a();",
+                        "    int b();",
+                        "    abstract int c();",
+                        "    public abstract int d();",
+                        "  }",
+                        "}"
+                )
+                .addOutputLines(
+                        "Enclosing.java",
+                        "public class Enclosing {",
+                        "  public interface Test {",
+                        "    int a();",
+                        "    int b();",
+                        "    int c();",
+                        "    int d();",
+                        "  }",
+                        "}"
+                ).doTest(BugCheckerRefactoringTestHelper.TestMode.TEXT_MATCH);
+    }
+
+    @Test
+    void fixFinalClassModifiers() {
+        fix()
+                .addInputLines(
+                        "Test.java",
+                        "public final class Test {",
+                        "  public final void a() {}",
+                        "  private final void b() {}",
+                        "  final void c() {}",
+                        "  @SafeVarargs public final void d(Object... value) {}",
+                        "}"
+                )
+                .addOutputLines(
+                        "Test.java",
+                        "public final class Test {",
+                        "  public void a() {}",
+                        "  private void b() {}",
+                        "  void c() {}",
+                        // SafeVarargs is a special case
+                        "  @SafeVarargs public final void d(Object... value) {}",
+                        "}"
+                ).doTest(BugCheckerRefactoringTestHelper.TestMode.TEXT_MATCH);
+    }
+
+    @Test
+    void fixStaticFinalMethod() {
+        fix()
+                .addInputLines(
+                        "Test.java",
+                        "public class Test {",
+                        "  public static final int a() {",
+                        "    return 1;",
+                        "  }",
+                        "  private static final int b() {",
+                        "    return 1;",
+                        "  }",
+                        "  static final int c() {",
+                        "    return 1;",
+                        "  }",
+                        "}"
+                )
+                .addOutputLines(
+                        "Test.java",
+                        "public class Test {",
+                        "  public static int a() {",
+                        "    return 1;",
+                        "  }",
+                        "  private static int b() {",
+                        "    return 1;",
+                        "  }",
+                        "  static int c() {",
+                        "    return 1;",
+                        "  }",
+                        "}"
+                ).doTest(BugCheckerRefactoringTestHelper.TestMode.TEXT_MATCH);
+    }
+
+    private BugCheckerRefactoringTestHelper fix() {
+        return BugCheckerRefactoringTestHelper.newInstance(new RedundantModifier(), getClass());
+    }
+}

changelog/@unreleased/pr-1010.v2.yml

@@ -0,0 +1,5 @@
+type: improvement
+improvement:
+  description: Error prone validation to avoid redundant modifiers
+  links:
+  - https://github.com/palantir/gradle-baseline/pull/1010

gradle-baseline-java-config/resources/checkstyle/checkstyle.xml

@@ -370,10 +370,6 @@
             <property name="format" value="\bIOUtils\.toString\("/>
             <property name="message" value="Prefer Guava''s [CharStreams,Files,Resources].toString to avoid charset/stream closing issues."/>
         </module>
-        <module name="RegexpSinglelineJava">
-            <property name="format" value="static enum"/>
-            <property name="message" value="Redundant ''static'' modifier."/>
-        </module>
         <module name="RegexpSinglelineJava">
             <property name="format" value="\/\/TODO|\/\/ TODO(?!\([^()\s]+\): )"/>
             <property name="message" value="TODO format: // TODO(#issue): explanation"/>

gradle-baseline-java/src/main/groovy/com/palantir/baseline/extensions/BaselineErrorProneExtension.java

@@ -34,6 +34,7 @@ public class BaselineErrorProneExtension {
             "PreferSafeLoggableExceptions",
             "PreferSafeLoggingPreconditions",
             "ReadReturnValueIgnored",
+            "RedundantModifier",
             "Slf4jLevelCheck",
             "Slf4jLogsafeArgs",
             "StrictUnusedVariable",