Androidkotlin comprehensive and detailed class usage grammar learning guide

Catalog

Preface

1. Declaration of a class & Instantiation

2. Constructors

2.1 Primary constructor

2.2 The secondary constructor

3. Attributes of a class

4. Visibility modifier

5. Inherit & rewrite

6. Special class

6.1 Nested class （ Inner class ）

6.2 Interface

6.3 Data class

6.4 Enumeration class

summary

Preface

Kotlin By Google Officially, Android First level programming language developed

today , I will focus on kotlin All the knowledge of the class in , The main contents are as follows ：

1. Declaration of a class & Instantiation // Format class Class name （ Parameter name 1： Parameter type , Parameter name 2： Parameter type ...）{ // to do }// Example class User(userName: String, age: Int){ // to do }// Instantiation // Kotlin No, new keyword , So create an instance of the class directly （ No reference & Ginseng ）：var user = User()var user = User("ABC" , 123)// Additional instructions ：Kotlin Support default parameters // That is, you can call a function without specifying parameters , Then use the default function class User(userName: String = "hjc", age: Int = 26){ // to do }// When instantiating a class, no parameters are passed in ,userName Default = hjc,age Default = 26var user = User()// After setting the default value , If you don't want to use the default value, you can pass in the parameter when you create an instance var user = User("ABC" , 123)// notes ： Named parameter if a default parameter precedes a parameter with no default value , The default value can only be used by calling the function with named parameters class User(userName: String = "hjc", age: Int){ // to do }var user = User(age = 26)

For constructors ,Kotlin A class in can have a primary constructor & Multiple sub constructors , I'll explain in detail .

2. Constructors 2.1 Primary constructor

Part of the class header = Follow the class name , use constructor keyword

Can't contain any code . The initialization code is placed in init Keyword as a prefix in the code block

class Class name constructor（ Parameter name ： Parameter type ）{init { //... }}// Example class User constructor(userName: String) {init { //... }}// notes ： If the main constructor has no comments / Visibility modifier , Omission constructor keyword class Class name （ Parameter name ： Parameter type ）{init { //... }}// Example class User (userName: String) {init { //... }}2.2 The secondary constructor

Must be added constructor keyword

There can be multiple sub constructors in a class , The input parameters are different

// form constructor( Parameter name ： Parameter type ) :{ The body of the function }// Example class User(userName: String) { // Primary constructor init { println(userName) } // The secondary constructor 1： It can be done by this Call the main constructor constructor() : this("hjc") // The secondary constructor 2： It can be done by this Call the main constructor constructor(age: Int) : this("hjc") { println(age) } // The secondary constructor 3： adopt this Call the main constructor constructor(sex: String, age: Int) : this("hjc") { println("$sex$age") }}// Instantiate the class User("hjc") // Call the main constructor User() // Call the secondary constructor 1User(2) // Call the secondary constructor 2User("male",26) // Call the secondary constructor 33. Attributes of a class

Kotlin Can have properties ： keyword var（ Reading and writing ） / keyword val（ read-only ）

class User { var userName: String val sex: String = " male "}// Use attributes = name + quote User().sex // Use this property = Java Of getter Method User().userName = "hjc" // Set this property = Java Of setter Method 4. Visibility modifier

5. Inherit & rewrite

Be similar to Java,Kotlin It's single inheritance = There is only one parent class

difference ：Kotlin Use a colon “ : ” Inherit & Inheritance is not allowed by default （ If you want the class to be inheritable , need open Keywords ）

// use open Keyword indicates that the class is allowed to be inherited open class Food // class Fruits Inheritance class Foodclass Fruits : Food()

Override the method of the parent class for the child class , stay Kotlin in , Methods are also non rewritable by default

If a subclass wants to override a method in a parent class , You need to precede the method of the parent class with open keyword , Then add... Before the method overridden by the subclass override keyword

// Parent class // In the class & Methods are preceded by keywords open, In order to be inherited & Method rewriting open class Food { open fun banana() {}}// Subclass class Fruits : Food(){// Overriding the method of the parent class override fun banana() { super.banana() }}6. Special class

Here are some special classes ：

Nested class （ Inner class ）

Interface

Data class

Enumeration class

6.1 Nested class （ Inner class ）/** * 1. Nested class （ Inner class ） * identification ： keyword inner * Use ： Call nested classes through instances of external classes */ class User { var age: Int = 0 inner class UserName { }}var userName: User.UserName = User().UserName()6.2 Interface /** * 2. Interface * identification ： keyword interface */// Statement interface A{}interface B{}// Method body // Methods in an interface can have a default method body , Methods with a default method body do not need to be overridden // The difference in Java：Java Methods in interfaces are not supported. Methods in interfaces are method bodies .interface UserImpl{ fun getName(): String // No default method body , Must be rewritten fun getAge(): Int{ // There is a default method body , Don't rewrite return 22 }}// Implementation interface UserImpl： Need to rewrite getName() & Don't rewrite getAge()class User :UserImpl{ override fun getName(): String { return "hjc" }}// Implementation interface ： The colon :class Food : A, B {} // Kotlin It's more to achieve class Fruits: Food,A, B {} // Inherit + Implementation interface 6.3 Data class /** * 3. Data class * effect ： Save the data * identification ： keyword data */// Use ： The following methods are automatically created when you create a class ：// 1. getter/setter Method ;// 2. equals() / hashCode() Yes ;// 3. toString() ： Output " Class name ( Parameters + Parameter values )";// 4. copy() function ： Copy an object & Change some of its properties , But the rest remains the same // Example ：// Statement 1 Data classes data class User(var userName: String, var age: Int)// copy Function USES var user = User("hjc",26)var user1 = user.copy(age = 30)// Output user1.toString(), The result is ：User(userName=hjc,age=30)// Particular attention // 1. The main constructor must have at least one parameter , And the parameter must be marked val or var// 2. Data classes cannot be used open、abstract、sealed( Closed class )、inner identification 6.4 Enumeration class /** * 4. Enumeration class * identification ： keyword enum */ // Definition enum class Color { RED, GREEN, BLUE}// Specify a value for the enumeration class enum class Color(rgb: Int) { RED(0xFF0000), GREEN(0x00FF00), BLUE(0x0000FF)}

thus , About kotlin The introduction of class usage in Introductory Grammar is finished .

summary

This paper gives a comprehensive introduction Kotlin Classes in introductory syntax

That's all Android Kotlin Details of the comprehensive and detailed class grammar learning guide , More about Android Kotlin For information about class syntax, please pay attention to other related articles on the software development network ！