src/conf/checkstyle.xml

<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
    "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
    "https://checkstyle.org/dtds/configuration_1_3.dtd">
<!-- Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional 
  information regarding copyright ownership. The ASF licenses this file to You 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. -->
<!-- ===================================================================== -->
<!-- Checkstyle configuration that checks the commons-configuration coding conventions -->
<!-- ===================================================================== -->

<module name="Checker">
  <property name="localeLanguage" value="en" />

  <!-- Checks that a package.html file exists for each package. -->
  <!-- See http://checkstyle.sf.net/config_javadoc.html#PackageHtml -->
  <module name="JavadocPackage">
    <property name="allowLegacy" value="true" />
  </module>

  <!-- Checks whether files end with a new line. -->
  <!-- See http://checkstyle.sf.net/config_misc.html#NewlineAtEndOfFile -->
  <module name="NewlineAtEndOfFile" />

  <!-- Checks that property files contain the same keys. -->
  <!-- See http://checkstyle.sf.net/config_misc.html#Translation -->
  <module name="Translation" />

  <!-- Exceptions -->
  <module name="SuppressionFilter">
    <property name="file" value="${basedir}/conf/checkstyle-suppressions.xml" />
  </module>

  <!-- Checks for Headers -->
  <!-- See http://checkstyle.sf.net/config_header.html -->
  <module name="Header">
    <property name="headerFile" value="${basedir}/conf/HEADER.txt" />
    <property name="ignoreLines" value="2" />
  </module>

  <module name="FileTabCharacter" />
  <module name="RegexpSingleline">
    <!-- \s matches whitespace character, $ matches end of line. -->
    <property name="format" value="\s+$" />
    <property name="message" value="Line has trailing spaces." />
  </module>
  <module name="FileLength" />
  <module name="LineLength">
    <property name="max" value="160" />
  </module>

  <module name="TreeWalker">

    <!-- Checks for Javadoc comments. -->
    <!-- See http://checkstyle.sf.net/config_javadoc.html -->
    <module name="JavadocMethod">
      <property name="accessModifiers" value="public" />
    </module>
    <module name="JavadocType">
    </module>
    <module name="JavadocVariable" />

    <!-- Checks for Naming Conventions. -->
    <!-- See http://checkstyle.sf.net/config_naming.html -->
    <module name="ConstantName" />
    <module name="LocalFinalVariableName" />
    <module name="LocalVariableName" />
    <module name="MemberName" />
    <module name="MethodName" />
    <module name="PackageName" />
    <module name="ParameterName" />
    <module name="StaticVariableName" />
    <module name="TypeName" />

    <!-- Following interprets the header file as regular expressions. -->
    <!-- <module name="RegexpHeader"/> -->

    <!-- Checks for imports -->
    <!-- See http://checkstyle.sf.net/config_import.html -->
    <module name="AvoidStarImport" />
    <module name="IllegalImport" /> <!-- defaults to sun.* packages -->
    <module name="RedundantImport" />
    <module name="UnusedImports" />

    <!-- Checks for Size Violations. -->
    <!-- See http://checkstyle.sf.net/config_sizes.html -->
    <module name="MethodLength" />
    <module name="ParameterNumber" />

    <!-- Checks for whitespace -->
    <!-- See http://checkstyle.sf.net/config_whitespace.html -->
    <module name="EmptyForIteratorPad" />
    <module name="NoWhitespaceAfter" />
    <module name="NoWhitespaceBefore" />
    <module name="OperatorWrap" />
    <module name="ParenPad" />
    <module name="WhitespaceAfter" />
    <module name="WhitespaceAround" />

    <!-- Modifier Checks -->
    <!-- See http://checkstyle.sf.net/config_modifiers.html -->
    <module name="ModifierOrder" />
    <module name="RedundantModifier">
      <property name="tokens" value="METHOD_DEF, VARIABLE_DEF, ANNOTATION_FIELD_DEF, INTERFACE_DEF, CLASS_DEF, ENUM_DEF" />
    </module>

    <!-- Checks for blocks. You know, those {}'s -->
    <!-- See http://checkstyle.sf.net/config_blocks.html -->
    <module name="AvoidNestedBlocks" />
    <module name="EmptyBlock" />
    <module name="NeedBraces" />
    <module name="LeftCurly" />
    <module name="RightCurly" />

    <!-- Checks for common coding problems -->
    <!-- See http://checkstyle.sf.net/config_coding.html -->
    <module name="CovariantEquals" />
    <module name="EqualsHashCode" />
    <module name="IllegalInstantiation" />
    <module name="MagicNumber">
      <property name="ignoreNumbers" value="-1,0,1,2,3" />
    </module>
    <module name="SimplifyBooleanExpression" />
    <module name="SimplifyBooleanReturn" />
    <module name="StringLiteralEquality" />
    <module name="SuperClone" />
    <module name="SuperFinalize" />
    <!-- <module name="DeclarationOrder" /> -->
    <module name="ExplicitInitialization" />
    <module name="DefaultComesLast" />
    <module name="FallThrough" />
    <module name="MultipleVariableDeclarations" />

    <!-- Checks for class design -->
    <!-- See http://checkstyle.sf.net/config_design.html -->
    <module name="FinalClass" />
    <module name="HideUtilityClassConstructor" />
    <module name="InterfaceIsType" />
    <module name="VisibilityModifier">
      <property name="protectedAllowed" value="true" />
    </module>

    <!-- Miscellaneous other checks. -->
    <!-- See http://checkstyle.sf.net/config_misc.html -->
    <module name="ArrayTypeStyle" />
    <module name="TodoComment" />
    <module name="UpperEll" />
    <module name="ImportOrder">
      <property name="option" value="top"/>
      <property name="groups" value="java,javax,org"/>
      <property name="ordered" value="true"/>
      <property name="separated" value="true"/>
    </module>
  </module>
  <!-- @author tags are deprecated -->
  <module name="RegexpSingleline">
    <property name="format" value="^\s+\*\s+@author\s" />
    <property name="message" value="Deprecated @author tag" />
    <property name="fileExtensions" value="java" />
    <property name="severity" value="warning" />
  </module>
</module>