Data class of kotlin journey

Kotlin And data class [ Data class ]

kotlin The normal way to declare a class class [ Class name ], And java similar

kotlin The declaration of a class can also use data Keyword to mark classes that are only used to hold data , Call it a data class

eg：

data class User constructor(val name: String) {
    

    var nickName: String = ""

}

For example, above user Class is a declared data class

The declaration of data class must meet the following requirements ：

1. The main constructor needs to have at least one parameter

2. All parameters of the main constructor must be marked val perhaps var

3. Data classes cannot be abstract 、 Development 、 Sealed or internal

Why? data class There must be the above constraints ？

Let's take a look at a common statement class class , What is the difference between meeting the above requirements and not meeting the above requirements

```kotlin
class User constructor(name: String) {
    
}
/**  Aforementioned user class , Parameters in the main constructor name Is not to bring val、var *  It means that we should name Just pass in parameters as constructors , This variable can only be used in init Get... In initialization  *  And if necessary name Declared as a class variable , You need to make the following changes  */
class User constructor(name1: String) {
    
    var name: String = ""
    init {
    
        name = name1
    }
}


// The above member variable declaration can be completed on the main constructor , namely 
class User constructor(var name: String) {
    
}

Therefore, the above restrictions require that the data class declare the contained member variables in the main constructor , Then the compiler generates some specific functions for us through these declared member variables .

Official documents also point out that ：

about data class The compiler automatically exports the following members from all properties declared in the main constructor ：

• equals()/hashCode() Yes ;
• toString() The format is "User(name=John, age=42)";
• componentN() function Corresponds to all properties in the declared order ;
• copy() function

That is to say data Generated equals、hashCode、toString Methods are implemented according to the member variables declared in the main constructor .

Here's an example ：

// Declare the following data class 
data class User constructor(var name: String) {
    
    var nickName: String = ""
}


val user1 = User("Lance")
user1.nickName = "ming"
val user2 = User("Lance")
user2.nickName = "ning"


user1==user2  because equals、hashCode Are all based on name, Which is assigned here Lance To compare 
 meanwhile user1、user2 Of toString Method returns User(name=Lance)

 If you modify the above code by 

data class User constructor(var name: String, var nickName: String) {
    
    
}


val user1 = User("Lance", "ming")
val user2 = User("Lance", "ning")

user1!=user2  because equals、hashCode Are all based on name and nickName To compare 
 meanwhile user1、user2 Of toString Method returns User(name=Lance,nickName=ming)、User(name=Lance,nickName=ning)