Use constants for arrays initialized with zero length. #1555

Fixes `ZeroLengthArrayInitialization` inspection violation

Description:
>Reports on allocations of arrays with known lengths of zero. Since array lengths in Java are non-modifiable, it is almost always possible to share zero-length arrays, rather than repeatedly allocating new zero-length arrays. Such sharing may provide useful optimizations in program runtime or footprint. Note that this inspection does not report zero-length arrays allocated as static final fields, as it is assumed that those arrays are being used to implement array sharing.

src/main/java/com/puppycrawl/tools/checkstyle/api/AbstractFileSetCheck.java

@@ -24,6 +24,8 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.SortedSet;
 
+import org.apache.commons.lang3.ArrayUtils;
+
 import com.puppycrawl.tools.checkstyle.Utils;
 
 /**
@@ -39,7 +41,7 @@ public abstract class AbstractFileSetCheck
     private MessageDispatcher messageDispatcher;
 
     /** The file extensions that are accepted by this filter */
-    private String[] fileExtensions = {};
+    private String[] fileExtensions = ArrayUtils.EMPTY_STRING_ARRAY;
 
     /** Collects the error messages */
     private final LocalizedMessages messageCollector = new LocalizedMessages();

src/main/java/com/puppycrawl/tools/checkstyle/checks/coding/IllegalInstantiationCheck.java

@@ -22,6 +22,8 @@ package com.puppycrawl.tools.checkstyle.checks.coding;
 import java.util.Set;
 import java.util.StringTokenizer;
 
+import org.apache.commons.lang3.ArrayUtils;
+
 import antlr.collections.AST;
 
 import com.google.common.collect.Sets;
@@ -101,7 +103,7 @@ public class IllegalInstantiationCheck
     @Override
     public int[] getAcceptableTokens() {
         // Return an empty array to not allow user to change configuration.
-        return new int[] {};
+        return ArrayUtils.EMPTY_INT_ARRAY;
     }
 
     @Override

src/main/java/com/puppycrawl/tools/checkstyle/checks/coding/IllegalTokenCheck.java

@@ -19,6 +19,8 @@
 
 package com.puppycrawl.tools.checkstyle.checks.coding;
 
+import org.apache.commons.lang3.ArrayUtils;
+
 import com.puppycrawl.tools.checkstyle.Utils;
 import com.puppycrawl.tools.checkstyle.api.Check;
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
@@ -74,7 +76,7 @@ public class IllegalTokenCheck
 
     @Override
     public int[] getRequiredTokens() {
-        return new int[0];
+        return ArrayUtils.EMPTY_INT_ARRAY;
     }
 
     @Override

src/main/java/com/puppycrawl/tools/checkstyle/checks/coding/IllegalTokenTextCheck.java

@@ -21,6 +21,8 @@ package com.puppycrawl.tools.checkstyle.checks.coding;
 
 import java.util.regex.Pattern;
 
+import org.apache.commons.lang3.ArrayUtils;
+
 import com.puppycrawl.tools.checkstyle.Utils;
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
 import com.puppycrawl.tools.checkstyle.checks.AbstractFormatCheck;
@@ -75,7 +77,7 @@ public class IllegalTokenTextCheck
 
     @Override
     public int[] getDefaultTokens() {
-        return new int[0];
+        return ArrayUtils.EMPTY_INT_ARRAY;
     }
 
     @Override
@@ -85,7 +87,7 @@ public class IllegalTokenTextCheck
 
     @Override
     public int[] getRequiredTokens() {
-        return new int[0];
+        return ArrayUtils.EMPTY_INT_ARRAY;
     }
 
     @Override

src/main/java/com/puppycrawl/tools/checkstyle/checks/coding/MagicNumberCheck.java

@@ -408,7 +408,7 @@ public class MagicNumberCheck extends Check {
      */
     public void setIgnoreNumbers(double... list) {
         if (list.length == 0) {
-            ignoreNumbers = new double[0];
+            ignoreNumbers = ArrayUtils.EMPTY_DOUBLE_ARRAY;
         }
         else {
             ignoreNumbers = new double[list.length];

src/main/java/com/puppycrawl/tools/checkstyle/checks/coding/SimplifyBooleanExpressionCheck.java

@@ -19,6 +19,8 @@
 
 package com.puppycrawl.tools.checkstyle.checks.coding;
 
+import org.apache.commons.lang3.ArrayUtils;
+
 import com.puppycrawl.tools.checkstyle.api.Check;
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
@@ -58,7 +60,7 @@ public class SimplifyBooleanExpressionCheck
     public int[] getAcceptableTokens() {
         // Return empty list to prevent user changing tokens in the
         // configuration.
-        return new int[] {};
+        return ArrayUtils.EMPTY_INT_ARRAY;
     }
 
     @Override

src/main/java/com/puppycrawl/tools/checkstyle/checks/coding/UnnecessaryParenthesesCheck.java

@@ -19,6 +19,8 @@
 
 package com.puppycrawl.tools.checkstyle.checks.coding;
 
+import org.apache.commons.lang3.ArrayUtils;
+
 import com.puppycrawl.tools.checkstyle.api.Check;
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
@@ -186,7 +188,7 @@ public class UnnecessaryParenthesesCheck extends Check {
     @Override
     public int[] getRequiredTokens() {
         // Check can work with any of acceptable tokens
-        return new int[] {};
+        return ArrayUtils.EMPTY_INT_ARRAY;
     }
 
     @Override

src/test/java/com/puppycrawl/tools/checkstyle/checks/DescendantTokenCheckTest.java

@@ -260,8 +260,7 @@ public class DescendantTokenCheckTest extends BaseCheckTestSupport {
         checkConfig.addAttribute("maximumNumber", "1");
         checkConfig.addAttribute("maximumMessage", "What are you doing?");
 
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         verify(checkConfig,
                getPath("coding" + File.separator + "InputReturnFromFinallyCheck.java"),

src/test/java/com/puppycrawl/tools/checkstyle/checks/SuppressWarningsHolderTest.java

@@ -43,8 +43,7 @@ public class SuppressWarningsHolderTest extends BaseCheckTestSupport {
     public void testOnComplexAnnotations() throws Exception {
         Configuration checkConfig = createCheckConfig(SuppressWarningsHolder.class);
 
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         verify(checkConfig, getPath("InputSuppressWarningsHolder.java"), expected);
     }

src/test/java/com/puppycrawl/tools/checkstyle/checks/annotation/SuppressWarningsCheckTest.java

@@ -23,6 +23,7 @@ import static com.puppycrawl.tools.checkstyle.checks.annotation.SuppressWarnings
 
 import java.io.File;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.junit.Test;
 
 import com.puppycrawl.tools.checkstyle.BaseCheckTestSupport;
@@ -788,9 +789,7 @@ public class SuppressWarningsCheckTest extends BaseCheckTestSupport {
     public void testUncheckedInConstant() throws Exception {
         DefaultConfiguration checkConfig = createCheckConfig(SuppressWarningsCheck.class);
 
-        String[] expected = {
-
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         verify(checkConfig, getPath("annotation" + File.separator
                 + "SuppressWarningsConstants.java"), expected);
@@ -800,8 +799,7 @@ public class SuppressWarningsCheckTest extends BaseCheckTestSupport {
     public void testValuePairAnnotation() throws Exception {
         DefaultConfiguration checkConfig = createCheckConfig(SuppressWarningsCheck.class);
 
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         verify(checkConfig, getPath("annotation" + File.separator
                 + "SuppressWarningsValuePair.java"), expected);

src/test/java/com/puppycrawl/tools/checkstyle/checks/blocks/LeftCurlyCheckTest.java

@@ -256,8 +256,7 @@ public class LeftCurlyCheckTest extends BaseCheckTestSupport {
     public void testIgnoreEnumsOptionTrue() throws Exception {
         checkConfig.addAttribute("option", LeftCurlyOption.EOL.toString());
         checkConfig.addAttribute("ignoreEnums", "true");
-        final String[] expectedWhileTrue = {
-        };
+        final String[] expectedWhileTrue = ArrayUtils.EMPTY_STRING_ARRAY;
         verify(checkConfig, getPath("InputLeftCurlyEnums.java"), expectedWhileTrue);
     }
 

src/test/java/com/puppycrawl/tools/checkstyle/checks/coding/IllegalThrowsCheckTest.java

@@ -23,6 +23,7 @@ import static com.puppycrawl.tools.checkstyle.checks.coding.IllegalThrowsCheck.M
 
 import java.io.File;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.junit.Assert;
 import org.junit.Test;
 
@@ -104,9 +105,7 @@ public class IllegalThrowsCheckTest extends BaseCheckTestSupport {
         DefaultConfiguration checkConfig = createCheckConfig(IllegalThrowsCheck.class);
         checkConfig.addAttribute("ignoreOverriddenMethods", "true");
 
-        String[] expected = {
-
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         verify(checkConfig, getPath("coding" + File.separator
                 + "InputIllegalThrowsCheckIgnoreOverriddenMethods.java"), expected);

src/test/java/com/puppycrawl/tools/checkstyle/checks/coding/PackageDeclarationCheckTest.java

@@ -23,6 +23,7 @@ import static com.puppycrawl.tools.checkstyle.checks.coding.PackageDeclarationCh
 
 import java.io.File;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.junit.Assert;
 import org.junit.Test;
 
@@ -57,8 +58,7 @@ public class PackageDeclarationCheckTest extends BaseCheckTestSupport {
     public void testCorrectFile() throws Exception {
         DefaultConfiguration checkConfig = createCheckConfig(PackageDeclarationCheck.class);
 
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         verify(checkConfig, getPath("coding/InputPackageDeclaration.java"), expected);
     }

src/test/java/com/puppycrawl/tools/checkstyle/checks/imports/CustomImportOrderCheckTest.java

@@ -494,8 +494,7 @@ public class CustomImportOrderCheckTest extends BaseCheckTestSupport {
     public void testDefaultConfiguration() throws Exception {
         final DefaultConfiguration checkConfig =
             createCheckConfig(CustomImportOrderCheck.class);
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         try {
             createChecker(checkConfig);

src/test/java/com/puppycrawl/tools/checkstyle/checks/javadoc/JavadocMethodCheckTest.java

@@ -557,9 +557,7 @@ public class JavadocMethodCheckTest extends BaseCheckTestSupport {
     @Test
     public void testSkipCertainMethods() throws Exception {
         checkConfig.addAttribute("ignoreMethodNamesRegex", "^foo.*$");
-        String[] expected = {
-
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
         verify(checkConfig, getPath("javadoc/InputJavadocMethodIgnoreNameRegex.java"), expected);
     }
 

src/test/java/com/puppycrawl/tools/checkstyle/checks/metrics/BooleanExpressionComplexityCheckTest.java

@@ -23,10 +23,10 @@ import static com.puppycrawl.tools.checkstyle.checks.metrics.BooleanExpressionCo
 
 import java.io.File;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.junit.Test;
 
 import antlr.CommonHiddenStreamToken;
-
 import com.puppycrawl.tools.checkstyle.BaseCheckTestSupport;
 import com.puppycrawl.tools.checkstyle.DefaultConfiguration;
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
@@ -55,8 +55,7 @@ public class BooleanExpressionComplexityCheckTest extends BaseCheckTestSupport {
         checkConfig.addAttribute("max", "5");
         checkConfig.addAttribute("tokens", "BXOR,LAND,LOR");
 
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         verify(checkConfig, getPath("metrics" + File.separator + "BooleanExpressionComplexityCheckTestInput.java"), expected);
     }
@@ -66,8 +65,7 @@ public class BooleanExpressionComplexityCheckTest extends BaseCheckTestSupport {
         DefaultConfiguration checkConfig =
             createCheckConfig(BooleanExpressionComplexityCheck.class);
 
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         verify(checkConfig, getPath("metrics" + File.separator + "InputBooleanExpressionComplexityNPE.java"), expected);
     }

src/test/java/com/puppycrawl/tools/checkstyle/checks/metrics/ClassDataAbstractionCouplingCheckTest.java

@@ -24,10 +24,10 @@ import static org.junit.Assert.fail;
 
 import java.io.File;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.junit.Test;
 
 import antlr.CommonHiddenStreamToken;
-
 import com.puppycrawl.tools.checkstyle.BaseCheckTestSupport;
 import com.puppycrawl.tools.checkstyle.DefaultConfiguration;
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
@@ -57,8 +57,7 @@ public class ClassDataAbstractionCouplingCheckTest extends BaseCheckTestSupport
     public void testDefaultConfiguration() throws Exception {
         DefaultConfiguration checkConfig =
             createCheckConfig(ClassDataAbstractionCouplingCheck.class);
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         try {
             createChecker(checkConfig);

src/test/java/com/puppycrawl/tools/checkstyle/checks/metrics/ClassFanOutComplexityCheckTest.java

@@ -24,6 +24,7 @@ import static org.junit.Assert.fail;
 
 import java.io.File;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.junit.Assert;
 import org.junit.Test;
 
@@ -54,8 +55,7 @@ public class ClassFanOutComplexityCheckTest extends BaseCheckTestSupport {
 
         checkConfig.addAttribute("max", "0");
 
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         verify(checkConfig, getPath("Input15Extensions.java"), expected);
     }
@@ -64,8 +64,7 @@ public class ClassFanOutComplexityCheckTest extends BaseCheckTestSupport {
     public void testDefaultConfiguration() throws Exception {
         DefaultConfiguration checkConfig =
             createCheckConfig(ClassFanOutComplexityCheck.class);
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         try {
             createChecker(checkConfig);

src/test/java/com/puppycrawl/tools/checkstyle/checks/metrics/JavaNCSSCheckTest.java

@@ -26,6 +26,7 @@ import static org.junit.Assert.fail;
 
 import java.io.File;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.junit.Assert;
 import org.junit.Test;
 
@@ -71,8 +72,7 @@ public class JavaNCSSCheckTest extends BaseCheckTestSupport {
     @Test
     public void testDefaultConfiguration() throws Exception {
         DefaultConfiguration checkConfig = createCheckConfig(JavaNCSSCheck.class);
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         try {
             createChecker(checkConfig);

src/test/java/com/puppycrawl/tools/checkstyle/checks/metrics/NPathComplexityCheckTest.java

@@ -22,11 +22,11 @@ package com.puppycrawl.tools.checkstyle.checks.metrics;
 import static com.puppycrawl.tools.checkstyle.checks.metrics.NPathComplexityCheck.MSG_KEY;
 import static org.junit.Assert.fail;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.junit.Assert;
 import org.junit.Test;
 
 import antlr.CommonHiddenStreamToken;
-
 import com.puppycrawl.tools.checkstyle.BaseCheckTestSupport;
 import com.puppycrawl.tools.checkstyle.DefaultConfiguration;
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
@@ -75,8 +75,7 @@ public class NPathComplexityCheckTest extends BaseCheckTestSupport {
     public void testDefaultConfiguration() throws Exception {
         DefaultConfiguration checkConfig =
             createCheckConfig(NPathComplexityCheck.class);
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         try {
             createChecker(checkConfig);

src/test/java/com/puppycrawl/tools/checkstyle/checks/modifier/ModifierOrderCheckTest.java

@@ -73,7 +73,7 @@ public class ModifierOrderCheckTest
         ModifierOrderCheck modifierOrderCheckObj = new ModifierOrderCheck();
         int[] actual = modifierOrderCheckObj.getDefaultTokens();
         int[] expected = {TokenTypes.MODIFIERS};
-        int[] unexpectedEmptyArray = {};
+        int[] unexpectedEmptyArray = ArrayUtils.EMPTY_INT_ARRAY;
         int[] unexpectedArray = {
             TokenTypes.MODIFIERS,
             TokenTypes.OBJBLOCK,
@@ -89,7 +89,7 @@ public class ModifierOrderCheckTest
         ModifierOrderCheck modifierOrderCheckObj = new ModifierOrderCheck();
         int[] actual = modifierOrderCheckObj.getAcceptableTokens();
         int[] expected = {TokenTypes.MODIFIERS};
-        int[] unexpectedEmptyArray = {};
+        int[] unexpectedEmptyArray = ArrayUtils.EMPTY_INT_ARRAY;
         int[] unexpectedArray = {
             TokenTypes.MODIFIERS,
             TokenTypes.OBJBLOCK,

src/test/java/com/puppycrawl/tools/checkstyle/checks/modifier/RedundantModifierTest.java

@@ -123,7 +123,7 @@ public class RedundantModifierTest
     public void testGetRequiredTokens() {
         RedundantModifierCheck redundantModifierCheckObj = new RedundantModifierCheck();
         int[] actual = redundantModifierCheckObj.getRequiredTokens();
-        int[] expected = {};
+        int[] expected = ArrayUtils.EMPTY_INT_ARRAY;
         Assert.assertArrayEquals(expected, actual);
     }
 }

src/test/java/com/puppycrawl/tools/checkstyle/checks/regexp/RegexpCheckTest.java

@@ -200,7 +200,7 @@ public class RegexpCheckTest extends BaseCheckTestSupport {
             createCheckConfig(RegexpCheck.class);
         checkConfigFalse.addAttribute("format", illegalFalse);
         checkConfigFalse.addAttribute("illegalPattern", "true");
-        final String[] expectedFalse = {};
+        final String[] expectedFalse = ArrayUtils.EMPTY_STRING_ARRAY;
         verify(checkConfigFalse, getPath("InputSemantic.java"), expectedFalse);
     }
 

src/test/java/com/puppycrawl/tools/checkstyle/checks/sizes/ExecutableStatementCountCheckTest.java

@@ -22,10 +22,10 @@ package com.puppycrawl.tools.checkstyle.checks.sizes;
 import static com.puppycrawl.tools.checkstyle.checks.sizes.ExecutableStatementCountCheck.MSG_KEY;
 import static org.junit.Assert.fail;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.junit.Test;
 
 import antlr.CommonHiddenStreamToken;
-
 import com.puppycrawl.tools.checkstyle.BaseCheckTestSupport;
 import com.puppycrawl.tools.checkstyle.DefaultConfiguration;
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
@@ -146,8 +146,7 @@ public class ExecutableStatementCountCheckTest
     public void testDefaultConfiguration() throws Exception {
         DefaultConfiguration checkConfig =
             createCheckConfig(ExecutableStatementCountCheck.class);
-        String[] expected = {
-        };
+        String[] expected = ArrayUtils.EMPTY_STRING_ARRAY;
 
         try {
             createChecker(checkConfig);