Add eager null checks in drive base classes (#1447)

All null motors will be specified in the error messages.

wpilibj/src/main/java/edu/wpi/first/wpilibj/drive/DifferentialDrive.java

@@ -7,6 +7,8 @@
 
 package edu.wpi.first.wpilibj.drive;
 
+import java.util.StringJoiner;
+
 import edu.wpi.first.hal.FRCNetComm.tInstances;
 import edu.wpi.first.hal.FRCNetComm.tResourceType;
 import edu.wpi.first.hal.HAL;
@@ -113,6 +115,7 @@ public class DifferentialDrive extends RobotDriveBase {
    * inverted, do so before passing it in.
    */
   public DifferentialDrive(SpeedController leftMotor, SpeedController rightMotor) {
+    verify(leftMotor, rightMotor);
     m_leftMotor = leftMotor;
     m_rightMotor = rightMotor;
     addChild(m_leftMotor);
@@ -121,6 +124,29 @@ public class DifferentialDrive extends RobotDriveBase {
     setName("DifferentialDrive", instances);
   }
 
+  /**
+   * Verifies that all motors are nonnull, throwing a NullPointerException if any of them are.
+   * The exception's error message will specify all null motors, e.g. {@code
+   * NullPointerException("leftMotor, rightMotor")}, to give as much information as possible to
+   * the programmer.
+   *
+   * @throws NullPointerException if any of the given motors are null
+   */
+  @SuppressWarnings("PMD.AvoidThrowingNullPointerException")
+  private void verify(SpeedController leftMotor, SpeedController rightMotor) {
+    if (leftMotor != null && rightMotor != null) {
+      return;
+    }
+    StringJoiner joiner = new StringJoiner(", ");
+    if (leftMotor == null) {
+      joiner.add("leftMotor");
+    }
+    if (rightMotor == null) {
+      joiner.add("rightMotor");
+    }
+    throw new NullPointerException(joiner.toString());
+  }
+
   /**
    * Arcade drive method for differential drive platform.
    * The calculated values will be squared to decrease sensitivity at low speeds.

wpilibj/src/main/java/edu/wpi/first/wpilibj/drive/KilloughDrive.java

@@ -7,6 +7,8 @@
 
 package edu.wpi.first.wpilibj.drive;
 
+import java.util.StringJoiner;
+
 import edu.wpi.first.hal.FRCNetComm.tInstances;
 import edu.wpi.first.hal.FRCNetComm.tResourceType;
 import edu.wpi.first.hal.HAL;
@@ -87,6 +89,7 @@ public class KilloughDrive extends RobotDriveBase {
   public KilloughDrive(SpeedController leftMotor, SpeedController rightMotor,
                        SpeedController backMotor, double leftMotorAngle, double rightMotorAngle,
                        double backMotorAngle) {
+    verify(leftMotor, rightMotor, backMotor);
     m_leftMotor = leftMotor;
     m_rightMotor = rightMotor;
     m_backMotor = backMotor;
@@ -103,6 +106,33 @@ public class KilloughDrive extends RobotDriveBase {
     setName("KilloughDrive", instances);
   }
 
+  /**
+   * Verifies that all motors are nonnull, throwing a NullPointerException if any of them are.
+   * The exception's error message will specify all null motors, e.g. {@code
+   * NullPointerException("leftMotor, rightMotor")}, to give as much information as possible to
+   * the programmer.
+   *
+   * @throws NullPointerException if any of the given motors are null
+   */
+  @SuppressWarnings("PMD.AvoidThrowingNullPointerException")
+  private void verify(SpeedController leftMotor, SpeedController rightMotor,
+                      SpeedController backMotor) {
+    if (leftMotor != null && rightMotor != null && backMotor != null) {
+      return;
+    }
+    StringJoiner joiner = new StringJoiner(", ");
+    if (leftMotor == null) {
+      joiner.add("leftMotor");
+    }
+    if (rightMotor == null) {
+      joiner.add("rightMotor");
+    }
+    if (backMotor == null) {
+      joiner.add("backMotor");
+    }
+    throw new NullPointerException(joiner.toString());
+  }
+
   /**
    * Drive method for Killough platform.
    *

wpilibj/src/main/java/edu/wpi/first/wpilibj/drive/MecanumDrive.java

@@ -7,6 +7,8 @@
 
 package edu.wpi.first.wpilibj.drive;
 
+import java.util.StringJoiner;
+
 import edu.wpi.first.hal.FRCNetComm.tInstances;
 import edu.wpi.first.hal.FRCNetComm.tResourceType;
 import edu.wpi.first.hal.HAL;
@@ -74,6 +76,7 @@ public class MecanumDrive extends RobotDriveBase {
    */
   public MecanumDrive(SpeedController frontLeftMotor, SpeedController rearLeftMotor,
                       SpeedController frontRightMotor, SpeedController rearRightMotor) {
+    verify(frontLeftMotor, rearLeftMotor, frontRightMotor, rearRightMotor);
     m_frontLeftMotor = frontLeftMotor;
     m_rearLeftMotor = rearLeftMotor;
     m_frontRightMotor = frontRightMotor;
@@ -86,6 +89,36 @@ public class MecanumDrive extends RobotDriveBase {
     setName("MecanumDrive", instances);
   }
 
+  /**
+   * Verifies that all motors are nonnull, throwing a NullPointerException if any of them are.
+   * The exception's error message will specify all null motors, e.g. {@code
+   * NullPointerException("frontLeftMotor, rearRightMotor")}, to give as much information as
+   * possible to the programmer.
+   *
+   * @throws NullPointerException if any of the given motors are null
+   */
+  @SuppressWarnings({"PMD.AvoidThrowingNullPointerException", "PMD.CyclomaticComplexity"})
+  private void verify(SpeedController frontLeft, SpeedController rearLeft,
+                      SpeedController frontRight, SpeedController rearRightMotor) {
+    if (frontLeft != null && rearLeft != null && frontRight != null && rearRightMotor != null) {
+      return;
+    }
+    StringJoiner joiner = new StringJoiner(", ");
+    if (frontLeft == null) {
+      joiner.add("frontLeftMotor");
+    }
+    if (rearLeft == null) {
+      joiner.add("rearLeftMotor");
+    }
+    if (frontRight == null) {
+      joiner.add("frontRightMotor");
+    }
+    if (rearRightMotor == null) {
+      joiner.add("rearRightMotor");
+    }
+    throw new NullPointerException(joiner.toString());
+  }
+
   /**
    * Drive method for Mecanum platform.
    *