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MindorksOpenSource/from-java-to-kotlin: From Java To Kotlin - Your Cheat Sheet F ...

原作者: [db:作者] 来自: 网络 收藏 邀请

开源软件名称：

MindorksOpenSource/from-java-to-kotlin

开源软件地址：

https://github.com/MindorksOpenSource/from-java-to-kotlin

开源编程语言：

Java 93.6%

开源软件介绍：

FromJavaToKotlin

From Java To Kotlin

Mindorks Mindorks Community Mindorks Android Store

From Java To Kotlin - Your Cheat Sheet For Java To Kotlin

中文支持

Português

Español

Print to Console

Java

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

Kotlin

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

Constants and Variables

Java

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

Kotlin

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

Assigning the null value

Java

String otherName;
otherName = null;

Kotlin

var otherName : String?
otherName = null

Verify if value is null

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

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 function
    myTextView.text=sampleString
}
if(!sampleString.isNullOrEmpty()){
   myTextView.text=sampleString 
}

Concatenation of strings

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

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

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

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)

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

Java

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

Kotlin

if (score in 0..300) { }

Multiple Conditions (Switch case)

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, "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

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

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

Java

double calculateCost(int quantity, double pricePerItem) {
    return pricePerItem * quantity;
}

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

Kotlin

fun calculateCost(quantity: Int, pricePerItem: Double = 20.5) = quantity * pricePerItem

calculateCost(10, 25.0) // 250
calculateCost(10) // 205

Variable number of arguments

Java

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

Kotlin

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

Defining methods with return

Java

int getScore() {
   // logic here
   return score;
}

Kotlin

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

Returning result of an operation

Java

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

Kotlin

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

Constructors

Java

public class Utils {

    private Utils() {
      // This utility class is not publicly instantiable
    }

    public static int getScore(int value

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