fix: A few tweaks

ksql-common/src/main/java/io/confluent/ksql/function/FunctionRegistry.java

@@ -80,7 +80,8 @@ public interface FunctionRegistry {
    * @throws KsqlException on unknown UDAF, or on unsupported {@code argumentType}.
    */
   KsqlAggregateFunction<?, ?, ?> getAggregateFunction(
-      String functionName, Schema argumentType,
+      String functionName,
+      Schema argumentType,
       AggregateFunctionInitArguments initArgs
   );
 

ksql-common/src/main/java/io/confluent/ksql/function/KsqlTableFunction.java

@@ -19,6 +19,10 @@
 import java.util.List;
 import org.apache.kafka.connect.data.Schema;
 
+/**
+ * A wrapper around the actual table function which provides methods to get return type and
+ * description, and allows the function to be invoked.
+ */
 public interface KsqlTableFunction extends FunctionSignature {
 
   Schema getReturnType();

ksql-engine/src/main/java/io/confluent/ksql/function/BaseTableFunction.java

@@ -24,6 +24,9 @@
 import java.util.Objects;
 import org.apache.kafka.connect.data.Schema;
 
+/**
+ * Abstract base class for table functions
+ */
 @Immutable
 public abstract class BaseTableFunction implements KsqlTableFunction {
 

ksql-engine/src/main/java/io/confluent/ksql/function/DynamicFunctionInvoker.java

@@ -21,7 +21,7 @@
 import java.lang.reflect.TypeVariable;
 
 /**
- * An implementation of UdfInvoker which invokes the UDF dynamically using reflection
+ * An implementation of UdfInvoker which invokes the UDF using reflection
  */
 public class DynamicFunctionInvoker implements FunctionInvoker {
 

ksql-engine/src/main/java/io/confluent/ksql/function/InternalFunctionRegistry.java

@@ -244,7 +244,6 @@ private void init() {
       addJsonFunctions();
       addStructFieldFetcher();
       addUdafFunctions();
-      addUdtfFunctions();
     }
 
     private void addStringFunctions() {
@@ -392,10 +391,6 @@ private void addUdafFunctions() {
       functionRegistry.addAggregateFunctionFactory(new TopkDistinctAggFunctionFactory());
     }
 
-    private void addUdtfFunctions() {
-      //functionRegistry.addTableFunctionFactory(new ExplodeFunctionFactory());
-    }
-
     private void addBuiltInFunction(final KsqlFunction ksqlFunction) {
       addBuiltInFunction(ksqlFunction, false);
     }

ksql-engine/src/main/java/io/confluent/ksql/function/UdtfTableFunctionFactory.java

@@ -20,6 +20,9 @@
 import java.util.stream.Collectors;
 import org.apache.kafka.connect.data.Schema;
 
+/**
+ * A table function factory used for creating user defined table functions.
+ */
 public class UdtfTableFunctionFactory extends TableFunctionFactory {
 
   private final UdfIndex<KsqlTableFunction> udtfIndex;

ksql-engine/src/main/java/io/confluent/ksql/function/udtf/array/Explode.java

@@ -22,6 +22,10 @@
 import java.util.Collections;
 import java.util.List;
 
+/**
+ * Implementation of the 'explode' table function. This table function takes an array of values and
+ * explodes it into zero or more rows, one for each value in the array.
+ */
 @UdtfDescription(name = "explode", author = KsqlConstants.CONFLUENT_AUTHOR,
     description =
         "Explodes an array. This function outputs one value for each element of the array.")

ksql-engine/src/test/java/io/confluent/ksql/function/udf/TestUdtf.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 Confluent Inc.
+ * Copyright 2019 Confluent Inc.
  *
  * Licensed under the Confluent Community License (the "License"); you may not use
  * this file except in compliance with the License.  You may obtain a copy of the

ksql-udf/src/main/java/io/confluent/ksql/function/udf/UdfParameter.java

@@ -23,7 +23,7 @@
 @Retention(RetentionPolicy.RUNTIME)
 @Target({ElementType.PARAMETER})
 /*
- * Optionally, applied to @Udf function parameters.
+ * Optionally, applied to @Udf or @Udtf function parameters.
  */
 public @interface UdfParameter {