原作者： amitshekhariitbhu
链接： https://github.com/amitshekhariitbhu/from-java-to-kotlin
来源：Github
著作权归原作者所有。商业转载请联系原作者获得授权，非商业转载请注明出处。

打印日志(Print to Console) {#打印日志-Print-to-Console}

• Java

|-------------|-----------------------------------------------------------------------------------| | 1 2 | System.out.print( "Amit Shekhar" ); System.out.println( "Amit Shekhar" ); |

• Kotlin

|-------------|-----------------------------------------------------------| | 1 2 | print( "Amit Shekhar" ) println( "Amit Shekhar" ) |

常量与变量(Constants and Variables) {#常量与变量-Constants-and-Variables}

• Java

|-------------|-----------------------------------------------------------------------------| | 1 2 | String name = "Amit Shekhar" ; final String name = "Amit Shekhar" ; |

• Kotlin

|-------------|-------------------------------------------------------------| | 1 2 | var name = "Amit Shekhar" val name = "Amit Shekhar" |

null声明(Assigning the null value) {#null声明-Assigning-the-null-value}

• Java

|-------------|----------------------------------------------| | 1 2 | String otherName; otherName = null ; |

• Kotlin

|-------------|--------------------------------------------------| | 1 2 | var otherName : String? otherName = null |

空判断(Verify if value is null) {#空判断-Verify-if-value-is-null}

• Java

|---------------|------------------------------------------------------------| | 1 2 3 | if (text != null ) { int length = text.length(); } |

• Kotlin

|-------------------|---------------------------------------------------------------------------------------| | 1 2 3 4 5 | text?.let { val length = text.length } // or simply val length = text?.length |

非空判断(Verify if value is NotNull OR NotEmpty) {#非空判断-Verify-if-value-is-NotNull-OR-NotEmpty}

• Java

|-----------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 | String sampleString = "Shekhar" ; if (!sampleString.isEmpty()) { myTextView.setText(sampleString); } if (sampleString!= null && !sampleString.isEmpty()){ myTextView.setText(sampleString); } |

• Kotlin

|-----------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 | var sampleString = "Shekhar" if (sampleString.isNotEmpty()){ //the feature of kotlin extension function myTextView.text=sampleString } if (!sampleString.isNullOrEmpty()){ myTextView.text=sampleString } |

字符串构造器(Concatenation of strings) {#字符串构造器-Concatenation-of-strings}

• Java

|---------------|---------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 | String firstName = "Amit" ; String lastName = "Shekhar" ; String message = "My name is: " + firstName + " " + lastName; |

• Kotlin

|---------------|-----------------------------------------------------------------------------------------------------------| | 1 2 3 | var firstName = "Amit" var lastName = "Shekhar" var message = "My name is: $firstName $lastName " |

字符串拼接(Substring) {#字符串拼接-Substring}

• Java

|---------------|---------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 | String firstName = "Amit" ; String lastName = "Shekhar" ; String message = "My name is: " + firstName + " " + lastName; |

• Kotlin

|---------------|-----------------------------------------------------------------------------------------------------------| | 1 2 3 | val firstName = "Amit" val lastName = "Shekhar" val message = "My name is: $firstName $lastName " |

字符串换行(New line in string) {#字符串换行-New-line-in-string}

• Java

|---------------|-------------------------------------------------------------------------| | 1 2 3 | String text = "First Line\n" + "Second Line\n" + "Third Line" ; |

• Kotlin

|-------------------|-------------------------------------------------------------------------------| | 1 2 3 4 5 | val text = """ |First Line |Second Line |Third Line """ .trimMargin() |

三元表达式(Ternary Operations) {#三元表达式-Ternary-Operations}

• Java

|-----------|----------------------------------------------------| | 1 | String text = x > 5 ? "x > 5" : "x <= 5" ; |

• Kotlin

|---------------|------------------------------------------------------| | 1 2 3 | val text = if (x > 5 ) "x > 5" else "x <= 5" |

比特操作符(Bitwise Operators) {#比特操作符-Bitwise-Operators}

• java

|---------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 | final int andResult = a & b; final int orResult = a | b; final int xorResult = a ^ b; final int rightShift = a >> 2 ; final int leftShift = a << 2 ; final int unsignedRightShift = a >>> 2 ; |

• Kotlin

|---------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 | val andResult = a and b val orResult = a or b val xorResult = a xor b val rightShift = a shr 2 val leftShift = a shl 2 val unsignedRightShift = a ushr 2 |

类型判断和转换-声明式(Check the type and casting) {#类型判断和转换-声明式-Check-the-type-and-casting}

• Java

|---------------|----------------------------------------------------------------| | 1 2 3 | if (object instanceof Car) { } Car car = (Car) object; |

• Kotlin

|---------------|---------------------------------------------------------| | 1 2 3 | if ( object is Car) { } var car = object as Car |

类型判断和转换-隐式(Check the type and casting, implicit) {#类型判断和转换-隐式-Check-the-type-and-casting-implicit}

• Java

|---------------|----------------------------------------------------------------| | 1 2 3 | if (object instanceof Car) { Car car = (Car) object; } |

• Kotlin

|---------------|-----------------------------------------------------------| | 1 2 3 | if ( object is Car) { var car = object // 聪明的转换 } |

更灵活的if语句(Multiple conditions) {#更灵活的if语句-Multiple-conditions}

• Java

|-----------|----------------------------------------------| | 1 | if (score >= 0 && score <= 300 ) { } |

• Kotlin

|-----------|------------------------------------| | 1 | if (score in 0. .300 ) { } |

更灵活的case语句(Multiple Conditions, Switch case) {#更灵活的case语句-Multiple-Conditions-Switch-case}

• Java

|------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 | int score = // some score; String grade; switch (score) { case 10 : case 9 : grade = "Excellent" ; break ; case 8 : case 7 : case 6 : grade = "Good" ; break ; case 5 : case 4 : grade = "OK" ; break ; case 3 : case 2 : case 1 : grade = "Fail" ; break ; default : grade = "Fail" ; } |

• Kotlin

|-------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 | var score = // some score var grade = when (score) { 9 , 10 -> "Excellent" in 6. .8 -> "Good" 4 , 5 -> "OK" in 1. .3 -> "Fail" else -> "Fail" } |

for循环(For-loops) {#for循环-For-loops}

• Java

|---------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 | for ( int i = 1 ; i <= 10 ; i++) { } for ( int i = 1 ; i < 10 ; i++) { } for ( int i = 10 ; i >= 0 ; i--) { } for ( int i = 1 ; i <= 10 ; i+= 2 ) { } for ( int i = 10 ; i >= 0 ; i-= 2 ) { } for (String item : collection) { } for (Map.Entry<String, String> entry: map.entrySet()) { } |

• Kotlin

|---------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 | for (i in 1. .10 ) { } for (i in 1 until 10 ) { } for (i in 10 downTo 0 ) { } for (i in 1. .10 step 2 ) { } for (i in 10 downTo 0 step 2 ) { } for (item in collection) { } for ((key, value) in map) { } |

更方便的集合操作(Collections) {#更方便的集合操作-Collections}

• Java

|---------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 | final List<Integer> listOfNumber = Arrays.asList( 1 , 2 , 3 , 4 ); final Map<Integer, String> keyValue = new HashMap <Integer, String>(); map.put( 1 , "Amit" ); map.put( 2 , "Ali" ); map.put( 3 , "Mindorks" ); // Java 9 final List<Integer> listOfNumber = List.of( 1 , 2 , 3 , 4 ); final Map<Integer, String> keyValue = Map.of( 1 , "Amit" , 2 , "Ali" , 3 , "Mindorks" ); |

• Kotlin

|-----------------|-----------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 | val listOfNumber = listOf( 1 , 2 , 3 , 4 ) val keyValue = mapOf( 1 to "Amit" , 2 to "Ali" , 3 to "Mindorks" ) |

for-each遍历(for each) {#for-each遍历-for-each}

• Java

|------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | // Java 7 and below for (Car car : cars) { System.out.println(car.speed); } // Java 8+ cars.forEach(car -> System.out.println(car.speed)); // Java 7 and below for (Car car : cars) { if (car.speed > 100 ) { System.out.println(car.speed); } } // Java 8+ cars.stream().filter(car -> car.speed > 100 ).forEach(car -> System.out.println(car.speed)); cars.parallelStream().filter(car -> car.speed > 100 ).forEach(car -> System.out.println(car.speed)); |

• Kotlin

|------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 | cars.forEach { println(it.speed) } cars.filter { it.speed > 100 } .forEach { println(it.speed)} // kotlin 1.1+ cars.stream().filter { it.speed > 100 }.forEach { println(it.speed)} cars.parallelStream().filter { it.speed > 100 }.forEach { println(it.speed)} |

方法定义(Defining methods) {#方法定义-Defining-methods}

• Java

|-----------------------|-----------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 | void doSomething () { // logic here } void doSomething ( int ... numbers) { // logic here } |

• Kotlin

|-----------------------|--------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 | fun doSomething () { // logic here } fun doSomething ( vararg numbers: Int ) { // logic here } |

分割数组(Splitting arrays) {#分割数组-Splitting-arrays}

• Java

|---------------|--------------------------------------------------------------------------------------------------------------| | 1 2 3 | String[] splits = "param=car" .split( "=" ); String param = splits[ 0 ]; String value = splits[ 1 ]; |

• Kotlin

|-----------|--------------------------------------------------------| | 1 | val (param, value) = "param=car" .split( "=" ) |

方法定义(Defining methods) {#方法定义-Defining-methods-v2}

• Java

|---------------|-----------------------------------------------| | 1 2 3 | void doSomething () { // logic here } |

• Kotlin

|---------------|----------------------------------------------| | 1 2 3 | fun doSomething () { // logic here } |

方法中参数的默认值(Default values for method parameters) {#方法中参数的默认值-Default-values-for-method-parameters}

• Java

|-------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 | double calculateCost ( int quantity, double pricePerItem) { return pricePerItem * quantity; } double calculateCost ( int quantity) { // default price is 20.5 return 20.5 * quantity; } |

• Kotlin

|-----------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 | fun calculateCost (quantity: Int , pricePerItem: Double = 20.5 ) = quantity * pricePerItem calculateCost( 10 , 25.0 ) // 250 calculateCost( 10 ) // 205 |

可变参数(Variable number of arguments) {#可变参数-Variable-number-of-arguments}

• Java

|---------------|---------------------------------------------------------------| | 1 2 3 | void doSomething ( int ... numbers) { // logic here } |

• Kotlin

|---------------|-------------------------------------------------------------------| | 1 2 3 | fun doSomething ( vararg numbers: Int ) { // logic here } |

无参数，带返回值的方法(Defining methods with return) {#无参数，带返回值的方法-Defining-methods-with-return}

• Java

|-----------------|---------------------------------------------------------| | 1 2 3 4 | int getScore () { // logic here return score; } |

• Kotlin

|------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 | fun getScore () : Int { // logic here return score } // as a single-expression function fun getScore () : Int = score // even simpler (type will be determined automatically) fun getScore () = score // return-type is Int |

有参数，带返回值的方法(Returning result of an operation) {#有参数，带返回值的方法-Returning-result-of-an-operation}

• Java

|-----------------|-----------------------------------------------------------------------| | 1 2 3 4 | int getScore ( int value) { // logic here return 2 * value; } |

• Kotlin

|---------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 | fun getScore (value: Int ) : Int { // logic here return 2 * value } // as a single-expression function fun getScore (value: Int ) : Int = 2 * value // even simpler (type will be determined automatically) fun getScore (value: Int ) = 2 * value // return-type is int |

构造器(Constructors) {#构造器-Constructors}

• Java

|---------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 | public class Utils { private Utils () { // This utility class is not publicly instantiable } public static int getScore ( int value) { return 2 * value; } } |

• Kotlin

|------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | class Utils private constructor () { companion object { fun getScore (value: Int ) : Int { return 2 * value } } } // another way object Utils { fun getScore (value: Int ) : Int { return 2 * value } } |

Getter和Setter(Getters and Setters) {#Getter和Setter-Getters-and-Setters}

• Java

|---------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 | public class Developer { private String name; private int age; public Developer (String name, int age) { this .name = name; this .age = age; } public String getName () { return name; } public void setName (String name) { this .name = name; } public int getAge () { return age; } public void setAge ( int age) { this .age = age; } @Override public boolean equals (Object o) { if ( this == o) return true ; if (o == null || getClass() != o.getClass()) return false ; Developer developer = (Developer) o; if (age != developer.age) return false ; return name != null ? name.equals(developer.name) : developer.name == null ; } @Override public int hashCode () { int result = name != null ? name.hashCode() : 0 ; result = 31 * result + age; return result; } @Override public String toString () { return "Developer{" + "name='" + name + '\'' + ", age=" + age + '}' ; } } |

• Kotlin

|-----------|-----------------------------------------------------------------| | 1 | data class Developer ( val name: String, val age: Int ) |

对象克隆或复制(Cloning or copying) {#对象克隆或复制-Cloning-or-copying}

• Java

|------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 | public class Developer implements Cloneable { private String name; private int age; public Developer (String name, int age) { this .name = name; this .age = age; } @Override protected Object clone () throws CloneNotSupportedException { return (Developer) super .clone(); } } // cloning or copying Developer dev = new Developer ( "Messi" , 30 ); try { Developer dev2 = (Developer) dev.clone(); } catch (CloneNotSupportedException e) { // handle exception } |

• Kotlin

|-----------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 | data class Developer ( var name: String, var age: Int ) // cloning or copying val dev = Developer( "Messi" , 30 ) val dev2 = dev.copy() // in case you only want to copy selected properties val dev2 = dev.copy(age = 25 ) |

类方法(Class methods) {#类方法-Class-methods}

• Java

|---------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 | public class Utils { private Utils () { // This utility class is not publicly instantiable } public static int triple ( int value) { return 3 * value; } } int result = Utils.triple( 3 ); |

• Kotlin

|-------------------|--------------------------------------------------------------------------------| | 1 2 3 4 5 | fun Int . triple () : Int { return this * 3 } var result = 3. triple() |

泛型(Generics) {#泛型-Generics}

• Java

|------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 | // Example #1 interface SomeInterface <T> { void doSomething (T data) ; } class SomeClass implements SomeInterface <String> { @Override public void doSomething (String data) { // some logic } } // Example #2 interface SomeInterface <T extends Collection <?>> { void doSomething (T data) ; } class SomeClass implements SomeInterface <List<String>> { @Override public void doSomething (List<String> data) { // some logic } } |

|---------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | interface SomeInterface <T> { fun doSomething (data: T) } class SomeClass : SomeInterface<String> { override fun doSomething (data: String) { // some logic } } interface SomeInterface <T: Collection<*>> { fun doSomething (data: T) } class SomeClass : SomeInterface<List<String>> { override fun doSomething (data: List<String>) { // some logic } } |

• Kotlin

|-------------------|--------------------------------------------------------------------------------| | 1 2 3 4 5 | fun Int . triple () : Int { return this * 3 } var result = 3. triple() |

定义未初始化的对象(Defining uninitialized objects) {#定义未初始化的对象-Defining-uninitialized-objects}

• Java

|-----------|------------------------| | 1 | Person person; |

• Kotlin

|-----------|----------------------------------------------| | 1 | internal lateinit var person: Person |

枚举(enum) {#枚举-enum}

• Java

|------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | public enum Direction { NORTH( 1 ), SOUTH( 2 ), WEST( 3 ), EAST( 4 ); int direction; Direction( int direction) { this .direction = direction; } public int getDirection () { return direction; } } |

• Kotlin

|---------------------|-------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 | enum class Direction ( val direction: Int ) { NORTH( 1 ), SOUTH( 2 ), WEST( 3 ), EAST( 4 ); } |

List自定义排序(Sorting List) {#List自定义排序-Sorting-List}

• Java

|---------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 | List<Profile> profiles = loadProfiles(context); Collections.sort(profiles, new Comparator <Profile>() { @Override public int compare (Profile profile1, Profile profile2) { if (profile1.getAge() > profile2.getAge()) return 1 ; if (profile1.getAge() < profile2.getAge()) return - 1 ; return 0 ; } }); |

• Kotlin

|---------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 | val profile = loadProfiles(context) profile.sortedWith(Comparator({ profile1, profile2 -> if (profile1.age > profile2.age) return @Comparator 1 if (profile1.age < profile2.age) return @Comparator - 1 return @Comparator 0 })) |

匿名类(Anonymous Class) {#匿名类-Anonymous-Class}

• Java

|---------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 | AsyncTask<Void, Void, Profile> task = new AsyncTask <Void, Void, Profile>() { @Override protected Profile doInBackground (Void... voids) { // fetch profile from API or DB return null ; } @Override protected void onPreExecute () { super .onPreExecute(); // do something } }; |

• Kotlin

|---------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 | val task = object : AsyncTask< Void , Void , Profile>() { override fun doInBackground ( vararg voids: Void ) : Profile? { // fetch profile from API or DB return null } override fun onPreExecute () { super .onPreExecute() // do something } } |

非static（非静态）代码块（初始化块）(Initialization block) {#非static（非静态）代码块（初始化块）-Initialization-block}

• Java

|---------------------|----------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 | public class User { { //Initialization block System.out.println( "Init block" ); } } |

• Kotlin

|-------------------|---------------------------------------------------------------------------------| | 1 2 3 4 5 | class User { init { // Initialization block println( "Init block" ) } } |

在 Kotlin 中需要了解的重要事项 {#在-Kotlin-中需要了解的重要事项}

• Kotlin 中的 Java 静态方法等价物是什么？

• "const"和"val"有什么区别？

• 学习 Kotlin - lateinit vs lazy

• 学习 Kotlin - apply vs with

• 学习 Kotlin - 数据类

• 学习 Kotlin - 解构声明

• 学习 Kotlin - 扩展函数

• 学习 Kotlin - 密封类

• 了解 Kotlin 中的高阶函数和 Lambda

• 了解 Kotlin 中的 inline、noinline 和 crossinline

• 在 Android 中掌握 Kotlin 协程 - 分步指南

• 在 Kotlin 中使用作用域函数 - let、run、with、also、apply

• Kotlin 中的具体类型是什么？