Chapter 2 Introduction to Kotlin

The working principle of

Programming languages fall into two broad categories: compiled and interpreted. Java is an interpreted language. Java code is compiled to generate class files that are recognized only by the Java virtual machine. The Java virtual machine acts as an interpreter, interpreting the mutated class file as binary data that the computer can recognize before executing the program.

Kotlin’s compiler compiles Kotlin code into class files. The Java virtual machine doesn’t care if a class file is compiled from Java or Kotlin, it recognizes a class file as long as it meets the specification. That’s how Kotlin works.

Variables and functions

variable

Only two keywords are allowed to be declared before a variable: val and var

Val (short for value) declares an immutable variable, always using val in preference to declaring the variable

Var (variable short) declares a mutable variable
```
val a: Int = 10
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```
Kotlin completely ditches the basic data types in Java and uses all object data types.

Shift + CTRL + P displays variable types

Basic types of

Kotlin unwrapped type (Byte, Integer…)

Byte, int, Float&Long, Char, String

// val c = 12345678910l // compile error.
// L should be capitalized
val c = 12345678910L // ok
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val e: Int = 10
//val f: Long = e // implicitness not allowed
// Explicit conversion, implicit conversion does not compile
val f: Long = e.toLong() // implicitness not allowed
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val k = "Today is a sunny day."
val m = String("Today is a sunny day.".toCharArray())

// Compare addresses
println(k === m) // compare references.
// If the value is equal, Java compares the address. Java compares the content using equals
println(k == m) // compare values.
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Unsigned type

UByte, integer (UShort&UInt&ULong)

val g: UInt = 10u
val h: ULong = 100000000000000000u
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String template

val slogan = "To be No.1"
println("Value of String 'j' is: $slogan") // no need brackets
println("Length of String 'j' is: ${slogan.length}") // need brackets
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val sample = "" "
             
           Hello World   
           
           Hello World 
           This is a demo page. 
          
   """.trimIndent()
println(sample)
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An array of

IntArray: IntArray, Int boxing Array: Array

Integer is a wrapper type for int, which is more wasteful than int. As a good coder, you’ll want to use int instead of Integer if the scope allows.

So let’s look at two approaches
```
fun myList(vararg inner: Int) {
    println(inner.size)
}

fun myList2(vararg inner2: Int?). {
    println(inner2.size)
}
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```
The only difference between these two methods is do you have an Int after it? Number. Let’s look at the important parts of the decompiled code first

Tools->Kotlin->Show Kotlin Bytecode->Decompile
```
public static final void myList(@NotNull int... inner) {
   Intrinsics.checkNotNullParameter(inner."inner");
   int var1 = inner.length;
   boolean var2 = false;
   System.out.println(var1);
}

public static final void myList2(@NotNull Integer... inner2) {
   Intrinsics.checkNotNullParameter(inner2, "inner2");
   int var1 = inner2.length;
   boolean var2 = false;
   System.out.println(var1);
}
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```
Look carefully at the signature of the parameter
- MyList is int… inner
- MyList2 is an Integer… inner2
You can wonder why the Kotlin compiler compiles this way. Right? In fact, the root cause is? , can be empty. When the Kotlin compiler decides that a parameter cannot be null, it is perfectly possible to use unwrapped types to accept parameters, making more efficient use of resources.
```
val intArrayOf = intArrayOf(1.3.4)
myList(*intArrayOf)
myList(1.2.3)

// Notice the null here
val array = arrayOf(1.3.4.null)
myList2(*array)
myList2(1.2.3)
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```
The above code needs to be noted:
- statementIntAn array of type,KotlinTwo methods are provided, one isintArrayOf(), one isarrayOf();
- intArrayOfStress isint, similar toJavaIn theint[], so the parameter cannot benull
- arrayOfsimilarJavaIn theInteger[], so you can assign to itnull
- Arrays in Kotlin cannot be assigned directly to morphable parametersvararg, you need to pass the keyword*
- Reflect on that when we createarrayIf you don’t need tonullAre you usingintArrayBetter?
```
val array = arrayOf(1.3.4.null)
// Prints the contents of the array
println(array.contentToString())
// Get the length
println(array.size)
// reassign
array[1] = 4
/ / value
println(array[0])
/ / traverse
for (i in array) {      
}
// Check if the value is in the array
if (1 in array) {
  println("1 exists in variable 'array'")}Copy the code
```

interval

// [0,10] discrete values
val intRange: IntRange = 1.10.
/ / [0, 10)
val intRange1: IntRange = 1 until 10
/ / [10, 9,..., 1]
val intProgression = 10 downTo 1

/ / step length
val intProgression1 = 1.10. step 2
for (i in intProgression1) {
  // 1 3 5 7 9
  print("$i  ")}// 1, 3, 5, 7, 9
println(intProgression1.joinToString())
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// [1,2] continuous values
val closedFloatingPointRange: ClosedFloatingPointRange<Float> = 1f..2f
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val intArrayOf = intArrayOf(1.2.3.4.5)
for (i in 0 until intArrayOf.size) {
    print("${intArrayOf[i]} ")}// intArrayOf.indices [0,intArrayOf.size)
for (i in intArrayOf.indices) {
    print("${intArrayOf[i]} ")}Copy the code

A collection of

The immutable set is added

Immutable List

, MutableList MutableList

Map…

Set…

// Elements cannot be added or removed
val listOf = listOf(1.2.3)
// Elements can be added or removed
val mutableListOf = mutableListOf(1.2.3)

val mutableMapOf = mutableMapOf("1" to "first"."2" to "Second")
for ((x, y) in mutableMapOf) {
        
}
val pair = "Hello" to "Kotlin"
val pair1 = Pair("Hello"."Kotlin")

val first = pair.first
val second = pair.second
val (x, y) = pair
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/ / Java implementation
List<Integer> integers = Arrays.asList(1.2.3);
// Run an error
integers.add(1);
// The content is mutable
integers.set(1.3);

// The content is immutable
Collections.unmodifiableList(integers)

ArrayList arrayList = new ArrayList();
arrayList.add(1);
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Provides a wealth of easy-to-use methods
Operator-level support to simplify access to the collection framework

function
The function definitions

Fun (Short for function) Defines the key of a function. Fun is followed by the function name. This is followed by a pair of parentheses that contain the parameters. If there is no return value, you can write Unit. Unit is equivalent to the return value of the Java void function.
```
fun functionName(arg1: Int, arg2: Int): Int {
    return 1
}
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```
When a function has only one line of code, Kotlin allows you to write a single line of code at the end of the function definition, with an equal sign in the middle.
```
fun largeNumber(arg1: Int, arg2: Int) = max(arg1, arg2)
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```
Function vs method
- Methods can be thought of as a special type of function
- Formally, functions that have receivers (Java objects, handles) are methods

Type of function

fun foo(a) {}  // The type is () -> Unit
val a: () -> Unit = {}

// Foo is the receiver for the bar method. In Java, this is passed in as the first argument to the function without displaying the call
// Function references are similar to function Pointers in C, used for function passing
// val kFunction: (Foo, String, Long) -> Long = Foo::bar
// val kFunction2: Foo.(String, Long) -> Long = Foo::bar
// val kFunction3: Function3
        
          = Foo::bar
        ,>
class Foo{
    fun bar(p1:String, p2:Long) : Long {
        return p1.length + p2
    }
}

val x: (Foo, String, Long) -> Any = Foo::bar
yy(x)
fun yy(p: (Foo.String.Long) - >Any) {
    p(Foo(), "Hello".3L)
    p.invoke(Foo(), "Hello".3L)}Copy the code

Logical control of a program

Program execution statements are divided into sequential statements, conditional statements and circular statements.
1. Conditional statements
  1. If conditional statement
    
    If conditional statements are no different from Java
    
    The if statement in Kotlin has an additional feature over the Java version in that it can have a return value
```
fun largeNumber(num1: Int, num2: Int): Int {
    val largeInt = if (num1 > num2) {
        num1
    } else {
        num2
    }
    return largeInt
}
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```
  2. When conditional statement
    
    When is similar to switch in Java
    
    Switch bug: Only char, byte, short, int, Character, byte, short, Integer, String, or an enum variables can be passed as conditions. In addition, each case must break; otherwise, execute the following cases after executing the current case.
    
    Basic examples:
```
fun getScore(name: String) = when (name) {
    "Tom" -> 85
    "Jim" -> 77
    else -> 0
}
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```
    Example of type matching:
```
fun checkNumber(num: Number) = when (num) {
    is Int -> "Int"
    is Double -> "Double"
    else -> "not support type"
}
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```
    Is is a type matching keyword, equivalent to the Instanceof keyword in Java.
    
    For the example of when without arguments, Kotlin uses the **== keyword to determine strings or objects, rather than the Equals () ** method used in Java
```
fun getScoreNoParam(name: String) = when {
    name.startsWith("lisa") - >99
    name == "Tom" -> 85
    name == "Jim" -> 77
    else -> 0
}
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```
2. Looping statements
  1. For loop
    
    The For-i loop in Java has been abandoned and the for-each loop in Java has been enhanced to become a for-in loop
    
    Kotlin’s interval is written as follows:
```
val range = 0.10. / / [0, 10]
val range2 = 0 until 10 / / [0, 10)
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```
```
for (i in 0.10.) {
    print("$i  ")
    // Output 0 1 2 3 4 5 6 7 8 9 10
}
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```
    Skipping odd elements using the step keyword is equivalent to the Java I = I + 2 operation
```
for (i in 0 until 10 step 2) {
    print("$i  ")
    // Output 0, 2, 4, 6, 8
}
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```
    DownTo defines the descending range
```
for (i in 10 downTo 0) {
    print("$i  ")
    // Output 10 9 8 7 6 5 4 3 2 10
}
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```
  2. The while loop is no different from the Java while loop in both syntax and usage techniques

Object-oriented programming

class

File is typically used to write Kotlin top-level functions and extension functions

Class is the Class used to write Kotlin

Classes are decorated with public by default

Example:

class Person {
    var name = "Tony"
    var age = 10

    fun introduceSelf(a) {
        println("My name is $name. I am $age years old.")}}Copy the code

Class instantiation:

val person = Person()
person.introduceSelf()
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Inheritance and constructors
1. Define the Student class to inherit from Person:
  1. Make Person inheritable by adding the open keyword to Person. Any non-abstract class in Kotlin cannot be inherited by default, which is equivalent to declaring the final keyword for a class in Java.
```
open class Person {... }Copy the code
```
  2. Make Student inherit from Person
```
class Student : Person() {
  init{... }... }Copy the code
```
    Why parentheses after Person: Subclass constructors must call superclass constructors.
    
    The constructors in Kotlin are: primary constructor and secondary constructor. The main constructor does not have a function body; Kotlin provides an init structure. Which constructor of the parent class is called by the primary constructor of a subclass is specified by parentheses during inheritance.
2. Transform the Person
  
  Person transformation is as follows:
```
open class Person(val name:String, val age:Int) {
    fun introduceSelf(a) {
        println("My name is $name. I am $age years old.")}}Copy the code
```
  Student adjusts accordingly:
```
class Student(val number: Int.val grade : Int, name:String, age:Int) : Person(name, age) {
    init{}}Copy the code
```
  The name and age in the Student constructor cannot be declared as var or val, because declaring val or val would be a field of the class.
3. subconstructor
  
  Classes can have only one primary constructor, but they can have more than one secondary constructor. All secondary constructors must call the primary constructor.
  
  The following is an example:
```
class Student(val number: Int.val grade: Int, name: String, age: Int) : Person(name, age) {
    constructor(name: String, age: Int) : this(0.0, name, age) {
    }

    constructor() : this("Tom".0) {}}Copy the code
```
  The subconstructor is defined by constructor. The instantiation code is as follows:
```
val student1 = Student()
val student2 = Student("Jack".10)
val student3 = Student(1527.2."Jerry".21)
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```
  Class has only secondary constructors, no primary constructors:
```
class Student : Person {

    constructor(number: Int, grade: Int, name: String, age: Int) : super(name, age) {

    }

    constructor(name: String, age: Int) : this(0.0, name, age) {

    }

    constructor() : this("Tom".0) {}}Copy the code
```
  Arguments cannot be declared by var or val in a subconstructor, because the arguments declared by var or val become fields of the class. The secondary constructor does not have to be called, so it is not initialized because it is not called, so it is not allowed to use var or val declarations.

An abstract class

abstract class AbsClass {
    abstract fun absMethod(a)
    // Add open before overwriting
    open fun overridable(a){}
    fun nonOverridable(a){}}Copy the code

Method before the open keyword, can be copied

Add the final keyword if the inherited parent class has a method that can be overridden, but does not want the method to be overridden by a subclass

open class SimpleClass(var x: Int.val y: String)
    : AbsClass() {

    override fun absMethod(a) {}

    // Word classes that inherit SimpleClass cannot override this method
    final override fun overridable(a){}}Copy the code

interface

The interfaces in Kotlin are almost identical to those in Java.

Interface use:
1. Creating the Study Interface
```
interface Study {
    fun readBooks(a)
    fun doHomework(a)
}
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```
2. Implementing an interface
```
class Student(name: String, age: Int) : Person(name, age), Study {
    override fun readBooks(a){}override fun doHomework(a){}}Copy the code
```
  Those of you familiar with Java know that the extends keyword is inherited, implements is implemented, and Kotlin uses colons separated by commas.
3. Example:
```
fun main(a) {
    val student = Student("Jack".10)
    doStudy(student)
}

fun doStudy(study: Study) {
    study.doHomework()
    study.readBooks()
}
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```
  Because Student implements the Study interface, the Student class can be passed to the doStudy function and then call the methods of the Study interface. This is called interface oriented programming, or polymorphism.
The default implementation
```
interface Study {
    fun readBooks(a)
    fun doHomework(a) {
        println("doHomework default implementation")}}Copy the code
```
Functions implemented by default are optional to implement or not implement. No implementation will use the default implementation logic.

Visibility modifier

The modifier	Java	Kotlin
public	All classes visible	All classes visible (default)
private	Current class visible	Current class visible
protected	Visible to current class, subclass, and class under the same package	The current class and subclass are visible
default	Classes visible under the same package (default)	There is no
internal	There is no	Classes visible in the same module

property(Kotlin) vs field(Java)

Kotlin property = Java (field + get + set)

If val is qualified, only get

Get + set if var modifier

class Person(age: Int, name: String) {
    var age: Int = age //property
        get() {
            return field
        }
        set(value) {
            println("setAge: $value")
            field = value
        }
    var name: String = name
        get() {
            return field // backing field
        }
        set(value) {

        }
}
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Attribute references

// No receiver is bound
val ageRef = Person::age
val person = Person(18."Bennyhuo")
/ / bind the receiver
val nameRef = person::name
ageRef.set(person, 20)
nameRef.set("Andyhuo")
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Data classes and singleton classes
1. Data classes
  
  Data classes typically need to override equals(), hashCode(), and toString() methods
  
  Equals () determines whether data is equal
  
  HashCode () HashMap, HashSet and other hash-related classes work properly
  
  ToString () clear log output
  
  Example:
```
data class Cellphone(val brand: String, val price: Double)
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```
  Declaring data before a class indicates that the class is a data class. Kotlin automatically implements equals(), hashCode(), toString() and other fixed methods that make no real logical sense.
2. Singleton class
  
  There is only one instance globally
```
object Singleton {
    fun singletonTest(a) {
        println("SingletonTest is called")}}Copy the code
```
  use
```
Singleton.singletonTest()
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```

Lambda programming
1. Collection creation and traversal
  1. Create a List
```
listOf<String>("Hello"."world"."this"."is"."kotlin")
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```
  2. Traverse the List
```
for (s in listOf) {
    println(s)
}
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```
  3. The listOf() function creates an immutable collection, while the mutableListOf() function is a mutable collection
```
val mutableList = mutableListOf<String>("Hello"."world"."this"."is"."kotlin")
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```
  4. Create a Map
```
var fruitAndPrice :HashMap<String, Int> = HashMap()
fruitAndPrice["Apple"] = 3

val fruitAndIndex = mapOf("Apple" to 1."Banana" to 2."Orange" to 3."pear" to 4."Grape" to 5)
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```
  5. Traverse Map
```
for ((fruit, number) in fruitAndIndex) {
    println("fruit is $fruit, index is $number")}Copy the code
```
2. The functional API for collections
  1. Take the word with the longest length
```
val wordList = listOf<String>("Hello"."world"."this"."is"."kotlin")
// When there are no elements in the set, the result is empty
val maxByOrNull = wordList.maxByOrNull { it.length }
// Empty string is used when the result is empty
valresult: String = maxByOrNull ? :""
println(result)
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```
  2. Lambda is a piece of code that can be passed as an argument, Lambda syntax structure
```
{parameter name1: Parameter type, parameter name2: Parameter type -> function body}Copy the code
```
  3. Simplified derivation
    
    The maxByOrNull above is just a simple function whose internal implementation iterates through the collection and calls a lambda expression (the collection element is passed into the expression) to get the length of the elements and compare, eventually returning the longest collection element.
    1. Lambda expressions as variables
```
val wordList = listOf<String>("Hello"."world"."this"."is"."kotlin")
val function = { fruit: String -> fruit.length }
val maxByOrNull = wordList.maxByOrNull(function)
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```
    2. Pass it directly as a parameter
```
val maxByOrNull = wordList.maxByOrNull({ fruit: String -> fruit.length })
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```
    3. When lambda is the last argument to a function, you can move the lambda expression out of parentheses
```
val maxByOrNull = wordList.maxByOrNull() { fruit: String -> fruit.length }
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```
    4. If lambda is the only argument to a function, you can omit the parentheses
```
val maxByOrNull = wordList.maxByOrNull { fruit: String -> fruit.length }
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```
    5. Kotlin has an excellent type derivation mechanism, and the argument lists in Lambda expressions don’t actually have to declare parameter types in most cases
```
val maxByOrNull = wordList.maxByOrNull { fruit -> fruit.length }
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```
    6. When there is only one argument in the argument list of a Lambda expression, there is no need to declare the argument name, but the IT keyword can be used instead
```
val maxByOrNull = wordList.maxByOrNull { it.length }
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```
  4. Java functional API usage
    
    Limitation: The functional API can be used when a Java method is called in Kotlin code and the method receives a Single Abstract Java method interface argument. A Java single abstract method interface is one in which there is only one method to implement. If there are multiple methods to implement in an interface, functional apis cannot be used.
    
    Take Runnable for example:
    1. Java implementation:
```
new Thread(new Runnable() {
    @Override
    public void run(a) {
        TODO("Not yet implemented")}});Copy the code
```
    2. Kotlin implementation:
```
Thread(object : Runnable {
    override fun run(a) {
        TODO("Not yet implemented")}})Copy the code
```
      Kotlin discarded the new keyword in favor of the object keyword
      
      Again, this is simplified because Runnable has only one method to implement, without explicitly overwriting the run() method
```
Thread(Runnable {
    TODO("Not yet implemented")})Copy the code
```
      We can also omit the interface name if there is only one Java single abstract method interface parameter in the argument list of a Java method
```
Thread({
    TODO("Not yet implemented")})Copy the code
```
      If the Lambda expression is still the only argument to the method, you can also omit the parentheses of the method
```
Thread {
    TODO("Not yet implemented")}Copy the code
```
Null pointer check
Nullable types
```
fun doStudy(study: Study) {
    study.doHomework()
    study.readBooks()
}
Copy the code
```
There is no null pointer risk in this code, because Kotlin defaults to non-null parameters and variables, so the Study parameter passed here must not be null either.

Kotlin has moved null-pointer exception checks up to compile time, so if our program is at risk of null-pointer exceptions, it will report an error at compile time.

Allow null example:
```
fun doStudy(study: Study?). {
    if(study ! =null) {
        study.doHomework()
        study.readBooks()
    }
}
Copy the code
```
Void detection AIDS
1. With the aid of? The. Operator removes the if statement
```
fun doStudy(study: Study?).{ study? .doHomework() study? .readBooks() }Copy the code
```
2. ? The: operator, which receives an expression on both sides and returns the result of the left expression if the left expression is not empty, or the right expression otherwise
  
  General writing:
```
val a:String = "a"
val c = if (a == null) a else ""
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```
  ? : operator:
```
valc = a ? :""
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```
  The example gets the length of the string
```
fun getWordLength(word : String) : Int {
    if (word == null) {
        return 0
    } else {
        return word.length
    }
}
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```
  Simplified:
```
fun getWordLength(word : String?). : Int {
    returnword? .length ? :0
}
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```
3. *Non-empty assertion tool, not recommendedThis is a risky way to write it
```
fun getWordLength(word : String?). : Int {
    returnword!! .length }Copy the code
```
4. Auxiliary tool — let
```
fun doStudy(study: Study?).{ study? .let { stu -> stu.readBooks() stu.doHomework() } }Copy the code
```
  The let function can handle global nulling, whereas the if statement cannot.
  
  We changed the argument in the doStudy() function to a global variable, which still works fine using the let function, but will prompt an error using the if statement.
  
  The reason for this error is that the value of the global variable can be changed by another thread at any time, and even if nulled, there is no guarantee that the study variable in the if statement is free from null pointer risk.
```
var student: Student? = Student("Lucy".19)

fun study(a) {
    if(student ! =null) {
        student.doHomework()
        student.readBooks()
    }
}
Copy the code
```
Kotlin small magic

String embedded expression

class Student(name: String, age: Int) : Person(name, age), Study {
    override fun readBooks(a) {
        println("My name is $name, I'm ${age} years old. I'm reading!")}}Copy the code

Function default arguments

fun sing(songName:String = "Pretty Girl", artist:String){}Copy the code

Call:

sing("hello"."jams")
// Assign via key-value pairs
sing(artist = "lucy")
Copy the code

Add default values to main constructors to avoid multiple constructors

class Student(val number: Int = 0, val grade: Int = 0, name: String="Tom", age: Int=10) : Person(name, age), Study {

}
Copy the code

Chapter 2 Introduction to Kotlin

Chapter 2 Introduction to Kotlin

The working principle of

Variables and functions

variable

Basic types of

Unsigned type

String template

Hello World

An array of

interval

A collection of

function

The function definitions

Logical control of a program

Conditional statements

If conditional statement

When conditional statement

Looping statements

For loop

The while loop is no different from the Java while loop in both syntax and usage techniques

Object-oriented programming

class

Inheritance and constructors

Define the Student class to inherit from Person:

Transform the Person

subconstructor

An abstract class

interface

Interface use:

The default implementation

Visibility modifier

property(Kotlin) vs field(Java)

Data classes and singleton classes

Data classes

Singleton class

Lambda programming

Collection creation and traversal

Create a List

Traverse the List

The listOf() function creates an immutable collection, while the mutableListOf() function is a mutable collection

Create a Map

Traverse Map

The functional API for collections

Take the word with the longest length

Lambda is a piece of code that can be passed as an argument, Lambda syntax structure

Simplified derivation

Java functional API usage

Null pointer check

Nullable types

Void detection AIDS

With the aid of? The. Operator removes the if statement

? The: operator, which receives an expression on both sides and returns the result of the left expression if the left expression is not empty, or the right expression otherwise

*Non-empty assertion tool, not recommendedThis is a risky way to write it

Auxiliary tool — let

Kotlin small magic

String embedded expression

Function default arguments

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