add linting

build.gradle

@@ -150,3 +150,14 @@ spotless {
         endWithNewline()
     }
 }
+
+apply plugin: "pmd"
+
+pmd {
+    toolVersion = "7.3.0"
+    consoleOutput = true
+    sourceSets = [project.sourceSets["main"]]
+    reportsDir = file("${project.buildDir}/reports/pmd")
+    ruleSetFiles = files(file("$rootDir/pmd-ruleset.xml"))
+    ruleSets = []
+}

pmd-ruleset.xml

@@ -0,0 +1,127 @@
+<?xml version="1.0"?>
+<ruleset name="WPILibRuleset"
+  xmlns="http://pmd.sourceforge.net/ruleset/2.0.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://pmd.sourceforge.net/ruleset/2.0.0 http://pmd.sourceforge.net/ruleset_2_0_0.xsd">
+
+  <description>PMD Ruleset for WPILib</description>
+
+  <exclude-pattern>.*/*JNI.*</exclude-pattern>
+  <exclude-pattern>.*/*IntegrationTests.*</exclude-pattern>
+  <exclude-pattern>.*/math/proto.*</exclude-pattern>
+
+  <rule ref="category/java/bestpractices.xml">
+    <exclude name="AccessorClassGeneration" />
+    <exclude name="AccessorMethodGeneration" />
+    <exclude name="AvoidPrintStackTrace" />
+    <exclude name="AvoidReassigningCatchVariables" />
+    <exclude name="AvoidReassigningParameters" />
+    <exclude name="AvoidUsingHardCodedIP" />
+    <exclude name="ConstantsInInterface" />
+    <exclude name="JUnitAssertionsShouldIncludeMessage" />
+    <exclude name="JUnitTestContainsTooManyAsserts" />
+    <exclude name="JUnitTestsShouldIncludeAssert" />
+    <exclude name="JUnit4TestShouldUseAfterAnnotation" />
+    <exclude name="JUnit4TestShouldUseBeforeAnnotation" />
+    <exclude name="JUnit4TestShouldUseTestAnnotation" />
+    <exclude name="LooseCoupling" />
+    <exclude name="PreserveStackTrace" />
+    <exclude name="ReplaceHashtableWithMap" />
+    <exclude name="ReplaceVectorWithList" />
+    <exclude name="SwitchStmtsShouldHaveDefault" />
+    <exclude name="SystemPrintln" />
+    <exclude name="UseVarargs" />
+  </rule>
+  <rule ref="category/java/bestpractices.xml/UnusedPrivateMethod">
+    <properties>
+      <property name="violationSuppressRegex"
+        value=".*'.*Arguments\(\)'.*" />
+    </properties>
+  </rule>
+  <rule ref="category/java/bestpractices.xml/UnusedLocalVariable">
+    <properties>
+      <property name="violationSuppressXPath" value="//Resource" />
+    </properties>
+  </rule>
+
+  <rule ref="category/java/design.xml">
+    <exclude name="AvoidThrowingRawExceptionTypes" />
+    <exclude name="AvoidThrowingNewInstanceOfSameException" />
+    <exclude name="CognitiveComplexity" />
+    <exclude name="CouplingBetweenObjects" />
+    <exclude name="CyclomaticComplexity" />
+    <exclude name="DataClass" />
+    <exclude name="ExceptionAsFlowControl" />
+    <exclude name="ExcessiveImports" />
+    <exclude name="ExcessiveParameterList" />
+    <exclude name="ExcessivePublicCount" />
+    <exclude name="GodClass" />
+    <exclude name="LawOfDemeter" />
+    <exclude name="LoosePackageCoupling" />
+    <exclude name="NPathComplexity" />
+    <exclude name="NcssCount" />
+    <exclude name="TooManyFields" />
+    <exclude name="TooManyMethods" />
+  </rule>
+
+  <rule ref="category/java/errorprone.xml">
+    <exclude name="AssignmentToNonFinalStatic" />
+    <exclude name="AvoidCatchingThrowable" />
+    <exclude name="AvoidDuplicateLiterals" />
+    <exclude name="AvoidLiteralsInIfCondition" />
+    <exclude name="CloseResource" />
+    <exclude name="ConstructorCallsOverridableMethod" />
+    <exclude name="DoNotTerminateVM" />
+    <exclude name="EmptyCatchBlock" />
+    <exclude name="FinalizeDoesNotCallSuperFinalize" />
+    <exclude name="JUnitSpelling" />
+    <exclude name="MissingSerialVersionUID" />
+    <exclude name="NonSerializableClass" />
+    <exclude name="NullAssignment" />
+  </rule>
+
+  <rule ref="category/java/multithreading.xml">
+    <exclude name="AvoidSynchronizedAtMethodLevel" />
+    <exclude name="AvoidUsingVolatile" />
+    <exclude name="DoNotUseThreads" />
+    <exclude name="UseConcurrentHashMap" />
+  </rule>
+
+  <rule ref="category/java/performance.xml">
+    <exclude name="AvoidInstantiatingObjectsInLoops" />
+  </rule>
+
+  <rule name="UnnecessaryCastRule" language="java"
+    message="Avoid unnecessary casts"
+    class="net.sourceforge.pmd.lang.java.rule.codestyle.UnnecessaryCastRule"
+    externalInfoUrl="https://github.com/pmd/pmd/blob/master/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/codestyle/UnnecessaryCastRule.java" />
+
+  <!-- Custom Rules -->
+  <rule name="UseRequireNonNull"
+    message="Use Objects.requireNonNull() instead of throwing a NullPointerException yourself."
+    language="java" class="net.sourceforge.pmd.lang.rule.xpath.XPathRule">
+    <description>Use Objects.requireNonNull() instead of throwing a
+      NullPointerException yourself.</description>
+    <properties>
+      <property name="xpath">
+        <value>
+          <![CDATA[
+            //IfStatement[child::Expression//NullLiteral]/Statement//ThrowStatement/Expression/PrimaryExpression/PrimaryPrefix/AllocationExpression/ClassOrInterfaceType[@Image='NullPointerException']
+          ]]>
+        </value>
+      </property>
+    </properties>
+    <priority>3</priority>
+    <example>
+<![CDATA[
+                public class Example {
+                    public Example(Object example) {
+                        if (example == null) {
+                            throw new NullPointerException();
+                        }
+                    }
+                }
+            ]]>
+    </example>
+  </rule>
+</ruleset>

src/main/java/frc/robot/subsystems/vision/VisionSubsystem.java

@@ -27,7 +27,7 @@ public class VisionSubsystem extends SubsystemBase {
    * The pose estimates from the limelights in the following order {shooterLimelight,
    * frontLeftLimelight, frontRightLimelight}
    */
-  private PoseEstimate[] limelightEstimates;
+  private final PoseEstimate[] limelightEstimates;
 
   public VisionSubsystem() {
     limelightEstimates = new PoseEstimate[3];