[Scala Tour] 3-Unified Type - TOUR OF SCALA SiFou

In Scala, all values ​​have a type, including numbers and functions. The following figure demonstrates a subset of the type hierarchy.

Scala Type Hierarchy

Any Type is the parent type of all types, also known as the top level type. It defines some common methods such as equals, hashCode and toString. Any has two direct subclasses: AnyVal and AnyRef.

AnyVal represents the value type. There are 9 predefined value types, which are not nullable: Double, Float, Long, Int, Short, Byte, Char, Unit, and Boolean. Unit is a value type without any meaning. Unit Only one instance can be declared like this: (). All functions must return some value, so Unit is a useful return type.

AnyRef represents a reference type. All non-value types are defined as reference types. Every user-defined type in Scala is a subtype of AnyRef. If Scala is used within a Java runtime environment, then AnyRef corresponds to Java.lang.object.

Here is an example that demonstrates that strings, integers, characters, booleans, and functions are objects like other objects:

val list: List[Any] = List(
  "a string",
  732,  // an integer
  'c',  // a character
  true, // a boolean value
  () => "an anonymous function returning a string"
)

list.foreach(element => println(element))

It defines the type List[ Any] variable list. The list is initialized with elements of various types, but they are all instances of scala.Any, so you can add them all to the list.

The following is the output of the program:

a string
732
c
true
<function></function>

Type conversion

Value types can be converted in the following ways:

For example:

val x: Long = 987654321
val y: Float = x  // 9.8765434E8 (note that some precision is lost in this case)

val face: Char = '☺'
val number: Int = face  // 9786

The conversion is one-way. The last statement below will not compile:

val x: Long = 987654321
val y: Float = x  // 9.8765434E8
val z: Long = y  // Does not conform

You can also convert a reference type to a subtype. This will be covered in a later article.

Nothing and Null

Nothing are subtypes of all types, also known as bottom types. Type Nothing has no value. Common uses are to signal non-termination, such as throwing an exception, program exit, or infinite loop (i.e., it is an expression that does not evaluate to a value, or a method that does not return a normal value).

Null is a subtype of all reference types (that is, any subtype of AnyRef). It has a single value identified by the keyword null. Null is primarily used for interoperability with other JVM languages ​​and should almost never be used in Scala code. We'll cover alternatives to null in a later article.

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