[SPARK-15732][SQL] better error message when use java reserved keyword as field name

## What changes were proposed in this pull request?

When users create a case class and use java reserved keyword as field name, spark sql will generate illegal java code and throw exception at runtime.

This PR checks the field names when building the encoder, and if illegal field names are used, throw exception immediately with a good error message.

## How was this patch tested?

new test in DatasetSuite

Author: Wenchen Fan <wenchen@databricks.com>

Closes #13485 from cloud-fan/java.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala

@@ -582,6 +582,11 @@ object ScalaReflection extends ScalaReflection {
         case t if definedByConstructorParams(t) =>
           val params = getConstructorParameters(t)
           val nonNullOutput = CreateNamedStruct(params.flatMap { case (fieldName, fieldType) =>
+            if (javaKeywords.contains(fieldName)) {
+              throw new UnsupportedOperationException(s"`$fieldName` is a reserved keyword and " +
+                "cannot be used as field name\n" + walkedTypePath.mkString("\n"))
+            }
+
             val fieldValue = Invoke(inputObject, fieldName, dataTypeFor(fieldType))
             val clsName = getClassNameFromType(fieldType)
             val newPath = s"""- field (class: "$clsName", name: "$fieldName")""" +: walkedTypePath
@@ -720,6 +725,12 @@ object ScalaReflection extends ScalaReflection {
     tpe <:< localTypeOf[Product] || tpe <:< localTypeOf[DefinedByConstructorParams]
   }
 
+  private val javaKeywords = Set("abstract", "assert", "boolean", "break", "byte", "case", "catch",
+    "char", "class", "const", "continue", "default", "do", "double", "else", "extends", "false",
+    "final", "finally", "float", "for", "goto", "if", "implements", "import", "instanceof", "int",
+    "interface", "long", "native", "new", "null", "package", "private", "protected", "public",
+    "return", "short", "static", "strictfp", "super", "switch", "synchronized", "this", "throw",
+    "throws", "transient", "true", "try", "void", "volatile", "while")
 }
 
 /**

sql/core/src/test/scala/org/apache/spark/sql/DatasetSuite.scala

@@ -786,6 +786,14 @@ class DatasetSuite extends QueryTest with SharedSQLContext {
     val result = joined.collect().toSet
     assert(result == Set(ClassData("a", 1) -> null, ClassData("b", 2) -> ClassData("x", 2)))
   }
+
+  test("better error message when use java reserved keyword as field name") {
+    val e = intercept[UnsupportedOperationException] {
+      Seq(InvalidInJava(1)).toDS()
+    }
+    assert(e.getMessage.contains(
+      "`abstract` is a reserved keyword and cannot be used as field name"))
+  }
 }
 
 case class Generic[T](id: T, value: Double)
@@ -809,6 +817,8 @@ case class ClassNullableData(a: String, b: Integer)
 case class NestedStruct(f: ClassData)
 case class DeepNestedStruct(f: NestedStruct)
 
+case class InvalidInJava(`abstract`: Int)
+
 /**
  * A class used to test serialization using encoders. This class throws exceptions when using
  * Java serialization -- so the only way it can be "serialized" is through our encoders.