Fixes #27

This adds methods  and  which effectively perform collect under the hood.  works also, but should be casted to KotlinArray which is not too beautiful and we can't override it (cause Kotlin forbids existing method overloading)

Signed-off-by: Pasha Finkelshteyn <asm0dey@jetbrains.com>

core/src/main/scala/org/jetbrains/spark/extensions/KSparkExtensions.scala

@@ -19,16 +19,23 @@
  */
 package org.jetbrains.spark.extensions
 
+import java.util
+
 import org.apache.spark.SparkContext
 import org.apache.spark.sql._
 
+import scala.collection.JavaConverters
+
 object KSparkExtensions {
   def col(d: Dataset[_], name: String): Column = d.col(name)
 
   def col(name: String): Column = functions.col(name)
 
   def lit(literal: Any): Column = functions.lit(literal)
 
+  def collectAsList[T](ds: Dataset[T]): util.List[T] = JavaConverters.seqAsJavaList(ds.collect())
+
+
   def debugCodegen(df: Dataset[_]): Unit = {
     import org.apache.spark.sql.execution.debug._
     df.debugCodegen()
@@ -39,5 +46,5 @@ object KSparkExtensions {
     df.debug()
   }
 
-  def sparkContext(s:SparkSession): SparkContext = s.sparkContext
+  def sparkContext(s: SparkSession): SparkContext = s.sparkContext
 }

examples/src/main/kotlin/org/jetbrains/spark/api/examples/Collect.kt

@@ -0,0 +1,48 @@
+/*-
+ * =LICENSE=
+ * Kotlin Spark API: Examples
+ * ----------
+ * Copyright (C) 2019 - 2020 JetBrains
+ * ----------
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * =LICENSEEND=
+ */
+package org.jetbrains.spark.api.examples
+
+import org.apache.spark.sql.Row
+import org.jetbrains.spark.api.*
+
+fun main() {
+    withSpark {
+        val sd = dsOf(1, 2, 3)
+        sd.createOrReplaceTempView("ds")
+        spark.sql("select * from ds")
+                .withCached {
+                    println("asList: ${toList<Int>()}")
+                    println("asArray: ${toArray<Int>().contentToString()}")
+                    this
+                }
+                .to<Int>()
+                .withCached {
+                    println("typed collect: " + (collect() as Array<Int>).contentToString())
+                    println("type collectAsList: " + collectAsList())
+                }
+
+        dsOf(1, 2, 3)
+                .map { c(it, it + 1, it + 2) }
+                .to<Row>()
+                .select("_1")
+                .collectAsList()
+                .forEach { println(it) }
+    }
+}

kotlin-spark-api/src/main/kotlin/org/jetbrains/spark/api/ApiV1.kt

@@ -135,6 +135,8 @@ inline fun <reified KEY, reified VALUE> KeyValueGroupedDataset<KEY, VALUE>.reduc
                 .map { t -> t._1 to t._2 }
 
 inline fun <T, reified R> Dataset<T>.downcast(): Dataset<R> = `as`(encoder<R>())
+inline fun <reified R> Dataset<*>.`as`(): Dataset<R> = `as`(encoder<R>())
+inline fun <reified R> Dataset<*>.to(): Dataset<R> = `as`(encoder<R>())
 
 inline fun <reified T> Dataset<T>.forEach(noinline func: (T) -> Unit) = foreach(ForeachFunction(func))
 
@@ -245,6 +247,9 @@ inline fun <reified T, R> Dataset<T>.withCached(blockingUnpersist: Boolean = fal
     return cached.executeOnCached().also { cached.unpersist(blockingUnpersist) }
 }
 
+inline fun <reified T> Dataset<Row>.toList() = KSparkExtensions.collectAsList(to<T>())
+inline fun <reified R> Dataset<*>.toArray(): Array<R> = to<R>().collect() as Array<R>
+
 /**
  * Alternative to [Dataset.show] which returns surce dataset.
  * Useful in debug purposes when you need to view contant of dataset as intermediate operation

kotlin-spark-api/src/main/kotlin/org/jetbrains/spark/api/SparkHelper.kt

@@ -19,7 +19,6 @@
  */
 package org.jetbrains.spark.api
 
-import org.apache.spark.sql.SparkSession
 import org.jetbrains.spark.api.SparkLogLevel.ERROR
 
 /**