Class Hierarchy

The Scala class hierarchy has Any at the top, from which all other types derive. It splits into two main branches: AnyVal (value types) and AnyRef (reference types). Additionally, Nothing and Null are special types that serve specific purposes in the hierarchy.

Any
├── AnyVal
│    ├── Unit
│    ├── Boolean
│    ├── Char
│    ├── Byte
│    ├── Short
│    ├── Int
│    ├── Long
│    ├── Float
│    └── Double
├── AnyRef (alias: Object)
│    ├── String
│    ├── List
│    ├── Option
│    │    ├── Some
│    │    └── None
│    ├── Map
│    ├── Set
│    ├── ... (other classes and user-defined classes)
│    └── Null (subtype of every AnyRef type)
└── Nothing (subtype of every other type)

Key Points

Any

• The root of the type hierarchy.
• Defines common methods such as ==, !=, equals, hashCode, and toString.

AnyVal

• The root class of value types, which includes Scala's primitive types.
• Subclasses: Unit, Boolean, Char, Byte, Short, Int, Long, Float, Double.

AnyRef (alias Object)

• The root class of all reference types.
• Subclasses: String, List, Option, Map, Set, and user-defined classes.

Option

• A container for an optional value.
• Subclasses: Some (contains a value), None (represents the absence of a value).

Null

• A subtype of every reference type (AnyRef).
• Can be assigned to any reference type but not to value types.
• The null literal is of type Null.

Nothing

• A subtype of every other type (both AnyVal and AnyRef).
• Represents the type of "no value" or "no normal return".
• Useful for functions that never return normally (e.g., throw an exception).

Examples

Value Types (AnyVal)

val unitValue: Unit = ()
val booleanValue: Boolean = true
val charValue: Char = 'A'
val byteValue: Byte = 1
val shortValue: Short = 1
val intValue: Int = 1
val longValue: Long = 1L
val floatValue: Float = 1.0f
val doubleValue: Double = 1.0

Reference Types (AnyRef)

val stringValue: String = "Hello, Scala!"
val listValue: List[Int] = List(1, 2, 3)
val optionValue: Option[Int] = Some(1)
val noneValue: Option[Int] = None
val mapValue: Map[String, Int] = Map("one" -> 1)
val setValue: Set[Int] = Set(1, 2, 3)

class UserDefinedClass
val userDefinedInstance = new UserDefinedClass

val nullValue: String = null

Special Types

Nothing

def fail(message: String): Nothing = throw new RuntimeException(message)

// This function never returns normally, so it has type Nothing
val failed = fail("This is an error")
// This line is never reached
println("This will never be printed")

None

val maybeValue: Option[Int] = None

maybeValue match
  case Some(value) => println(s"Found value: $value")
  case None => println("No value found")
// Output: No value found