Convert scala list to DataFrame or DataSet

Use Seq:

val spark = SparkSession.builder().appName("Spark-SQL").master("local[2]").getOrCreate()

import spark.implicits._

var tom = new TestPerson("Tom Hanks",37,35.5)
var sam = new TestPerson("Sam Smith",40,40.5)

val PersonList = mutable.MutableList[TestPerson]()

//Adding data in list
PersonList += tom
PersonList += sam

//It will be work.
var personDS = Seq(PersonList).toDS()

SQLContext.implicits

case class TestPerson(name: String, age: Long, salary: Double)

val spark = SparkSession.builder().appName("List to Dataset").master("local[*]").getOrCreate()

var tom = new TestPerson("Tom Hanks",37,35.5)
var sam = new TestPerson("Sam Smith",40,40.5)
   
// mutable.MutableList[TestPerson]() is not required , i used below way which was 
// cleaner
val PersonList =  List(tom,sam)

import spark.implicits._
PersonList.toDS().show

Try without Seq:

case class TestPerson(name: String, age: Long, salary: Double)
val tom = TestPerson("Tom Hanks",37,35.5)
val sam = TestPerson("Sam Smith",40,40.5)
val PersonList = mutable.MutableList[TestPerson]()
PersonList += tom
PersonList += sam

val personDS = PersonList.toDS()
println(personDS.getClass)
personDS.show()

val personDF = PersonList.toDF()
println(personDF.getClass)
personDF.show()
personDF.select("name", "age").show()

Output:

class org.apache.spark.sql.Dataset

+---------+---+------+
|     name|age|salary|
+---------+---+------+
|Tom Hanks| 37|  35.5|
|Sam Smith| 40|  40.5|
+---------+---+------+

class org.apache.spark.sql.DataFrame

+---------+---+------+
|     name|age|salary|
+---------+---+------+
|Tom Hanks| 37|  35.5|
|Sam Smith| 40|  40.5|
+---------+---+------+

+---------+---+
|     name|age|
+---------+---+
|Tom Hanks| 37|
|Sam Smith| 40|
+---------+---+

Also, make sure to move the declaration of the case class TestPerson outside the scope of your object.