Issue #3732: fix NPE for Java 8's 'receiver parameter'

src/main/java/com/puppycrawl/tools/checkstyle/checks/javadoc/JavadocMethodCheck.java

@@ -694,7 +694,9 @@ private static List<DetailAST> getParameters(DetailAST ast) {
         while (child != null) {
             if (child.getType() == TokenTypes.PARAMETER_DEF) {
                 final DetailAST ident = child.findFirstToken(TokenTypes.IDENT);
-                returnValue.add(ident);
+                if (ident != null) {
+                    returnValue.add(ident);
+                }
             }
             child = child.getNextSibling();
         }

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

@@ -591,4 +591,10 @@ public void testAllowToSkipOverriden() throws Exception {
         };
         verify(checkConfig, getPath("InputJavadocMethodsNotSkipWritten.java"), expected);
     }
+
+    @Test
+    public void testJava8ReceiverParameter() throws Exception {
+        final String[] expected = CommonUtils.EMPTY_STRING_ARRAY;
+        verify(checkConfig, getPath("InputJavadocReceiverParameter.java"), expected);
+    }
 }

src/test/resources/com/puppycrawl/tools/checkstyle/checks/javadoc/InputJavadocReceiverParameter.java

@@ -0,0 +1,32 @@
+package com.puppycrawl.tools.checkstyle.checks.javadoc;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+import java.nio.ByteBuffer;
+
+public class InputJavadocReceiverParameter {
+
+    @Retention(RetentionPolicy.RUNTIME)
+    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
+    public @interface UnknownInitialization {
+        /**
+         * A dummy annotation to check Java 8's receiver parameter handling.
+         *
+         * @return a class
+         */
+        Class<?> value() default Object.class;
+    }
+
+    /**
+     * Function to check handling of Java 8's receiver parameter.
+     *
+     * @param buffer dummy argument
+     */
+    public void foo(@UnknownInitialization(InputJavadocReceiverParameter.class) InputJavadocReceiverParameter this,
+            final ByteBuffer buffer) {
+        buffer.putInt(1);
+    }
+
+}