Add checkstyle configuration

build.gradle

@@ -12,6 +12,8 @@ plugins {
 
     // Apply the application plugin to add support for building an application
     id 'application'
+
+    id 'checkstyle'
 }
 
 sourceCompatibility = 11
@@ -41,3 +43,8 @@ jar {
         configurations.compile.collect { it.isDirectory() ? it : zipTree(it) }
     }
 }
+
+checkstyleTest{
+    showViolations = true
+    checkstyleTest.enabled = false
+}
\ No newline at end of file

config/checkstyle/checkstyle.xml

+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+          "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
+          "http://checkstyle.sourceforge.net/dtds/configuration_1_3.dtd">
+
+<!--
+
+  Checkstyle configuration that checks the sun coding conventions from:
+
+    - the Java Language Specification at
+      http://java.sun.com/docs/books/jls/second_edition/html/index.html
+
+    - the Sun Code Conventions at http://java.sun.com/docs/codeconv/
+
+    - the Javadoc guidelines at
+      http://java.sun.com/j2se/javadoc/writingdoccomments/index.html
+
+    - the JDK Api documentation http://java.sun.com/j2se/docs/api/index.html
+
+    - some best practices
+
+  Checkstyle is very configurable. Be sure to read the documentation at
+  http://checkstyle.sf.net (or in your downloaded distribution).
+
+  Most Checks are configurable, be sure to consult the documentation.
+
+  To completely disable a check, just comment it out or delete it from the file.
+
+  Finally, it is worth reading the documentation.
+
+-->
+
+<module name="Checker">
+    <!--
+        If you set the basedir property below, then all reported file
+        names will be relative to the specified directory. See
+        http://checkstyle.sourceforge.net/5.x/config.html#Checker
+
+        <property name="basedir" value="${basedir}"/>
+    -->
+
+    <property name="fileExtensions" value="java, properties, xml"/>
+
+    <!-- Checks that a package-info.java file exists for each package.     -->
+    <!-- See http://checkstyle.sf.net/config_javadoc.html#JavadocPackage -->
+    <module name="JavadocPackage"/>
+
+    <!-- Checks whether files end with a new line.                        -->
+    <!-- See http://checkstyle.sf.net/config_misc.html#NewlineAtEndOfFile -->
+    <module name="NewlineAtEndOfFile"/>
+
+    <!-- Checks that property files contain the same keys.         -->
+    <!-- See http://checkstyle.sf.net/config_misc.html#Translation -->
+    <module name="Translation"/>
+
+    <!-- Checks for Size Violations.                    -->
+    <!-- See http://checkstyle.sf.net/config_sizes.html -->
+    <module name="FileLength"/>
+
+    <!-- Checks for whitespace                               -->
+    <!-- See http://checkstyle.sf.net/config_whitespace.html -->
+    <module name="FileTabCharacter"/>
+
+    <!-- Miscellaneous other checks.                   -->
+    <!-- See http://checkstyle.sf.net/config_misc.html -->
+    <module name="RegexpSingleline">
+       <property name="format" value="\s+$"/>
+       <property name="minimum" value="0"/>
+       <property name="maximum" value="0"/>
+       <property name="message" value="Line has trailing spaces."/>
+    </module>
+
+    <!-- Checks for Headers                                -->
+    <!-- See http://checkstyle.sf.net/config_header.html   -->
+    <!-- <module name="Header"> -->
+    <!--   <property name="headerFile" value="${checkstyle.header.file}"/> -->
+    <!--   <property name="fileExtensions" value="java"/> -->
+    <!-- </module> -->
+
+    <module name="TreeWalker">
+
+        <!-- Checks for Javadoc comments.                     -->
+        <!-- See http://checkstyle.sf.net/config_javadoc.html -->
+<!-- We do not need Javadoc for _EVERY_ method / variable. Do on your own discretion.
+        <module name="JavadocMethod"/>
+        <module name="JavadocVariable"/>
+-->
+        <module name="JavadocType"/>
+        <module name="JavadocStyle"/>
+
+        <!-- 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"/>
+
+        <!-- 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">
+            <property name="processJavadoc" value="false"/>
+        </module>
+
+        <!-- Checks for Size Violations.                    -->
+        <!-- See http://checkstyle.sf.net/config_sizes.html -->
+<!-- Disable ancient rules ...
+        <module name="LineLength"/>
+-->
+        <module name="MethodLength"/>
+        <module name="ParameterNumber"/>
+
+        <!-- Checks for whitespace                               -->
+        <!-- See http://checkstyle.sf.net/config_whitespace.html -->
+        <module name="EmptyForIteratorPad"/>
+        <module name="GenericWhitespace"/>
+        <module name="MethodParamPad"/>
+        <module name="NoWhitespaceAfter"/>
+        <module name="NoWhitespaceBefore"/>
+        <module name="OperatorWrap"/>
+        <module name="ParenPad"/>
+        <module name="TypecastParenPad"/>
+        <module name="WhitespaceAfter"/>
+        <module name="WhitespaceAround"/>
+
+        <!-- Modifier Checks                                    -->
+        <!-- See http://checkstyle.sf.net/config_modifiers.html -->
+        <module name="ModifierOrder"/>
+        <module name="RedundantModifier"/>
+
+        <!-- Checks for blocks. You know, those {}'s         -->
+        <!-- See http://checkstyle.sf.net/config_blocks.html -->
+        <module name="AvoidNestedBlocks"/>
+        <module name="EmptyBlock"/>
+        <module name="LeftCurly"/>
+        <module name="NeedBraces"/>
+        <module name="RightCurly"/>
+
+        <!-- Checks for common coding problems               -->
+        <!-- See http://checkstyle.sf.net/config_coding.html -->
+        <module name="AvoidInlineConditionals"/>
+        <module name="EmptyStatement"/>
+        <module name="EqualsHashCode"/>
+        <module name="HiddenField">
+          <property name="ignoreSetter" value="true" />
+          <property name="ignoreConstructorParameter" value="true" />
+        </module>
+        <module name="IllegalInstantiation"/>
+        <module name="InnerAssignment"/>
+        <module name="MagicNumber"/>
+        <module name="MissingSwitchDefault"/>
+        <module name="SimplifyBooleanExpression"/>
+        <module name="SimplifyBooleanReturn"/>
+
+        <!-- Checks for class design                         -->
+        <!-- See http://checkstyle.sf.net/config_design.html -->
+        <module name="DesignForExtension">
+            <!-- Ignore this rule on functions annotated with certain annotations -->
+            <property name="ignoredAnnotations" value="Override, Test"/>
+        </module>
+
+        <module name="FinalClass"/>
+        <module name="HideUtilityClassConstructor"/>
+        <module name="InterfaceIsType"/>
+<!-- This is pretty unneeded when it comes to simple data structures
+        <module name="VisibilityModifier"/>
+-->
+
+        <!-- Miscellaneous other checks.                   -->
+        <!-- See http://checkstyle.sf.net/config_misc.html -->
+        <module name="ArrayTypeStyle"/>
+        <module name="FinalParameters"/>
+<!-- I´d like to use TODO comments, thanks
+        <module name="TodoComment"/>
+-->
+        <module name="UpperEll"/>
+
+    </module>
+
+</module>