java与kotlin 写法区别

原文链接：https://gitcode.net/mirrors/mindorksopensource/from-java-to-kotlin?utm_source=csdn_github_accelerator#assigning-the-null-value

Print to Console 打印到控制台

Java

System.out.print("Amit Shekhar");
System.out.println("Amit Shekhar");

Kotlin

print("Amit Shekhar")
println("Amit Shekhar")

---

Constants and Variables 常量和变量

Java

String name = "Amit Shekhar";
final String name = "Amit Shekhar";

Kotlin

var name = "Amit Shekhar"
val name = "Amit Shekhar"

---

Assigning the null value 分配空值

Java

String otherName;
otherName = null;

Kotlin

var otherName : String?
otherName = null

---

Verify if value is null 验证值是否为空

Java

if (text != null) {int length = text.length();
}

Kotlin

text?.let {val length = text.length
}
// or simply
val length = text?.length

---

Verify if value is NotNull OR NotEmpty 验证值是否为 NotNull 或 NotEmpty

Java

String sampleString = "Shekhar";
if (!sampleString.isEmpty()) {myTextView.setText(sampleString);
}
if(sampleString!=null && !sampleString.isEmpty()){myTextView.setText(sampleString); 
}

Kotlin

var sampleString ="Shekhar"
if(sampleString.isNotEmpty()){  //the feature of kotlin extension functionmyTextView.text=sampleString
}
if(!sampleString.isNullOrEmpty()){myTextView.text=sampleString 
}

---

Concatenation of strings 字符串的连接

Java

String firstName = "Amit";
String lastName = "Shekhar";
String message = "My name is: " + firstName + " " + lastName;

Kotlin

var firstName = "Amit"
var lastName = "Shekhar"
var message = "My name is: $firstName $lastName"

---

New line in string 字符串中的新行

Java

String text = "First Line\n" +"Second Line\n" +"Third Line";

Kotlin

val text = """|First Line|Second Line|Third Line""".trimMargin()

---

Substring

Java

String str = "Java to Kotlin Guide";
String substr = "";//print java
substr = str.substring(0, 4);
System.out.println("substring = " + substr);//print kotlin
substr = str.substring(8, 14);
System.out.println("substring = " + substr);

Kotlin

var str = "Java to Kotlin Guide"
var substr = ""//print java
substr = str.substring(0..3) //
println("substring $substr")//print kotlin
substr = str.substring(8..13)
println("substring $substr")

---

Ternary Operations 三元运算

Java

String text = x > 5 ? "x > 5" : "x <= 5";String message = null;
log(message != null ? message : "");

Kotlin

val text = if (x > 5) "x > 5" else "x <= 5"val message: String? = null
log(message ?: "")

---

Bitwise Operators

Java

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

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

---

Check the type and casting 检查类型和铸件

Java

if (object instanceof Car) {Car car = (Car) object;
}

Kotlin

if (object is Car) {
var car = object as Car
}// if object is null
var car = object as? Car // var car = object as Car?

---

Check the type and casting (implicit) 检查类型和铸造（隐式）

Java

if (object instanceof Car) {Car car = (Car) object;
}

Kotlin

if (object is Car) {var car = object // smart casting
}// if object is null
if (object is Car?) {var car = object // smart casting, car will be null
}

---

Multiple conditions 多个条件

Java

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

Kotlin

if (score in 0..300) { }

---

Multiple Conditions (Switch case) 多个条件（切换案例）

Java

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

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

---

For-loops

Java

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

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) { }

---

Collections

Java

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, "Anand");
map.put(3, "Messi");// Java 9
final List<Integer> listOfNumber = List.of(1, 2, 3, 4);final Map<Integer, String> keyValue = Map.of(1, "Amit",2, "Anand",3, "Messi");

Kotlin

val listOfNumber = listOf(1, 2, 3, 4)
val keyValue = mapOf(1 to "Amit",2 to "Anand",3 to "Messi")

---

for each

Java

// 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

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)}

---

Splitting arrays

java

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

kotlin

val (param, value) = "param=car".split("=")

---

Defining methods

Java

void doSomething() {// logic here
}

Kotlin

fun doSomething() {// logic here
}

Default values for method parameters 方法参数的默认值

Java

double calculateCost(int quantity, double pricePerItem) {return pricePerItem * quantity;
}double calculateCost(int quantity) {// default price is 20.5return 20.5 * quantity;
}

Kotlin

fun calculateCost(quantity: Int, pricePerItem: Double = 20.5) = quantity * pricePerItemcalculateCost(10, 25.0) // 250
calculateCost(10) // 205

---

Variable number of arguments 可变数量的参数

Java

void doSomething(int... numbers) {// logic here
}

Kotlin

fun doSomething(vararg numbers: Int) {// logic here
}

---

Defining methods with return 用 return 定义方法

Java

int getScore() {// logic herereturn score;
}

Kotlin

fun getScore(): Int {// logic herereturn score
}// as a single-expression functionfun getScore(): Int = score// even simpler (type will be determined automatically)fun getScore() = score // return-type is Int

---

Returning result of an operation 返回操作结果

Java

int getScore(int value) {// logic herereturn 2 * value;
}

Kotlin

fun getScore(value: Int): Int {// logic herereturn 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

---

Constructors

Java

public class Utils {private Utils() {// This utility class is not publicly instantiable}public static int getScore(int value) {return 2 * value;}}

Kotlin

class Utils private constructor() {companion object {fun getScore(value: Int): Int {return 2 * value}}
}// another wayobject Utils {fun getScore(value: Int): Int {return 2 * value}}

---

Getters and Setters getter 和 setter

Java

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;}@Overridepublic 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;}@Overridepublic int hashCode() {int result = name != null ? name.hashCode() : 0;result = 31 * result + age;return result;}@Overridepublic String toString() {return "Developer{" +"name='" + name + '\'' +", age=" + age +'}';}
}

Kotlin

data class Developer(var name: String, var age: Int)

---

Cloning or copying 克隆或复制

Java

public class Developer implements Cloneable {private String name;private int age;public Developer(String name, int age) {this.name = name;this.age = age;}@Overrideprotected 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

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)

---

Class methods

Java

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);

---

Generics

Java

// Example #1
interface SomeInterface<T> {void doSomething(T data);
}class SomeClass implements SomeInterface<String> {@Overridepublic void doSomething(String data) {// some logic}
}// Example #2
interface SomeInterface<T extends Collection<?>> {void doSomething(T data);
}class SomeClass implements SomeInterface<List<String>> {@Overridepublic void doSomething(List<String> data) {// some logic}
}
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

fun Int.triple(): Int {return this * 3
}var result = 3.triple()

---

Defining uninitialized objects 定义未初始化的对象

Java

Person person;

Kotlin

internal lateinit var person: Person

---

enum

Java

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

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

---

Sorting List

Java

List<Profile> profiles = loadProfiles(context);
Collections.sort(profiles, new Comparator<Profile>() {@Overridepublic int compare(Profile profile1, Profile profile2) {if (profile1.getAge() > profile2.getAge()) return 1;if (profile1.getAge() < profile2.getAge()) return -1;return 0;}
});

Kotlin

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

---

Anonymous Class

Java

 AsyncTask<Void, Void, Profile> task = new AsyncTask<Void, Void, Profile>() {@Overrideprotected Profile doInBackground(Void... voids) {// fetch profile from API or DBreturn null;}@Overrideprotected void onPreExecute() {super.onPreExecute();// do something}
};

Kotlin

val task = object : AsyncTask<Void, Void, Profile>() {override fun doInBackground(vararg voids: Void): Profile? {// fetch profile from API or DBreturn null}override fun onPreExecute() {super.onPreExecute()// do something}
}

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Initialization block 初始化块

Java

public class User {{  //Initialization blockSystem.out.println("Init block");}
}

Kotlin

   class User {init { // Initialization blockprintln("Init block")}}

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