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This article will explain the example analysis of Kotlin type system for you in detail. The editor thinks it is very practical, so I share it for you as a reference. I hope you can get something after reading this article.

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In the process of learning Kotlin, I became curious about the type system of Kotlin. Does Kotlin have a common base class similar to Object in Java? Is there a separate branch in Kotlin similar to the Java base type? After some research, the blogger found that the answer was more satisfactory than that of Java,Kotlin, and it was surprisingly simple to follow simple rules to understand the entire type system.

Any

Any is equivalent to the concept of Object in Java, as Any wrote in the comments:

The root of the Kotlin class hierarchy. Every Kotlin class has [Any] as a superclass.

Let's simply verify that Any is the base class of everything.

Class Fruitfun main (args: Array) {println (Fruit () is Any)}

In the above code, we create a new class and then construct an instance of it to see if it is Any. The answer is obviously true.

We are looking at whether the parent class of some basic types in kotlin, that is, Int, Double, Float, Byte, and so on, is also Any.

Println (3.233F is Any) println (2 is Any)

The answer is also true. As an additional explanation, Kotlin does not have the differential treatment of base types and encapsulation types in Java, nor does it unboxing and boxing. The base type is the base type, but they also have Any as the parent class.

Unit

Let's take a look at Unit, a special thing in Kotlin.

/ * The type with only one value: the `Unit` object. * This type corresponds to the `void` type in Java.*/public object Unit {override fun toString () = "kotlin.Unit"}

Every function in kotlin must have a return value.

The concept is explained here and will be mentioned again in subsequent chapters. Kotlin has done a lot of work on the concept that there must be a return value, but the benefits are so obvious that we can look at kotlin functions from a unified perspective.

The concept of Unit means doing nothing, but doing nothing is also a return value. If we don't make any declaration, the return value of the function is Unit, indicating that I returned something that didn't do anything.

Let's verify it, declare an empty function, and then print it. (compiled in Java, but)

Fun justReturn () {} fun main (args: Array) {print (justReturn ())}

As a result, kotlin.Unit is output, which proves that the return value is Unit.

So there is a question, what is the relationship between Unit and Any? Let's look at it through the is keyword.

Fun main (args: Array) {print (justReturn () is Any)}

Well, Unit is also a subclass of Any!

Nothing

Let's continue to extend the concept that every function in kotlin must have a return value. What we saw before is the normal return, so if there is an exception in the program, will there be a return value? Kotlin also continues the concept that there must be a return value in this case. This return value is called-Nothing!

Nothing means unreachable. When the program is actually running, it will not produce any Nothing type objects. What?! How do you understand that. Once kotlin discovers that it has returned Nothing, it ensures that the following code is no longer executed.

So Nothing is often used in cases of abnormal exit such as throw, so that subsequent code will not be executed. Let's look at the example of itself in kotlin.

/ * * Terminates the currently running process.** @ param status serves as a status code; by convention,* a nonzero status code indicates abnormal termination.** @ return This method never returns normally.*/@kotlin.internal.InlineOnlypublic inline fun exitProcess (status: Int): Nothing {System.exit (status) throw RuntimeException ("System.exit returned normally, while it was supposed to halt JVM.")}

Note that let's take a look at the position of Nothing in the type system. Nothing, in contrast to Any, is a subclass of all types! In other words, Nothing is Fruit, School, Money, and Any. Nothing means unreachable state, and each type contains this unreachable state, so this state Nothing is a subclass of these.

Notice the location of Nothing in the figure above.

Nullable

One of the biggest trump cards of kotlin is this nullable type, a type followed by?, and this type can be null. Let's take a look at what the type system looks like after introducing nullable types.

1. First look at the relationship between normal classes and nullable types.

Class Fruitfun main (args: Array) {print (Fruit () is Fruit?)}

The answer is true, which is easy to understand. The difference between the two is whether they can be empty. What can be null is naturally the base class, and what is not nullable is a derivative that can be nullable.

2. Whether Any has a nullable type

The most admirable thing about kotlin is that a concept is carried out to the end. Any also has a nullable type in kotlin. Sensually Any? Is the parent of Any, and Any is the parent of non-nullable types, so Any? Is it also the parent of a non-nullable type? That's the answer. Let's check it out.

Class Fruitfun main (args: Array) {print (Fruit () is Any?)}

3. Whether Unit has a nullable type

Yes, Unit is also available for type Unit? But this is a difficult concept to understand, and it contains two values, Unit and null. In order to continue the unified concept of kotlin, this is rarely used in scenarios, but we need to be clear.

4. Whether Nothing has a nullable type

Of course, Nothing also has a nullable type Nothing?, which itself has only one value, null, that is, null. Nothing itself is unreachable, so there won't be any instance, so it can only be null.

Let's verify it.

Fun main (args: Array) {println (null is Nothing) println (null is Nothing?) println (null is Any) println (null is Any)}

They are false, true, false and true.

Summary

Here to borrow the diagram of natpryce, you take a look at this picture, this is the type system of kotlin.

We only need to understand a few points to fully understand the kotlin type system.

Any and Nothing are the base and subclasses of all objects, respectively.

A nullable type is the parent class of a nullable type.

When we are not clear about the type, we can understand it by comparing the above two concepts.

This is the end of this article on "sample Analysis of Kotlin Type system". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it for more people to see.

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