From Java To Kotlin

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打印日志

• Java

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

• Kotlin

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

---

常量与变量

• Java

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

• Kotlin

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

---

null声明

• Java

String otherName;
otherName = null;

• Kotlin

var otherName : String?
otherName = null

---

空判断

• Java

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

• Kotlin

text?.let {
    val length = text.length
}

---

字符串拼接

• Java

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

• Kotlin

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

---

换行

• Java

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

• Kotlin

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

---

三元表达式

• Java

String text = x > 5 ? "x > 5" : "x <= 5";

• Kotlin

val text = if (x > 5)
              "x > 5"
           else "x <= 5"

---

类型判断和转换 (声明式)

• Java

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

• Kotlin

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

---

类型判断和转换 (隐式)

• Java

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

• Kotlin

if (object is Car) {
   var car = object // smart casting
}

---

多重条件

• Java

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

• Kotlin

if (score in 0..300) { }

---

更灵活的case语句

• 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"
    in 1..3 -> "Fail"
    else -> "Fail"
}

---

for循环

• 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 1 step 2) { }

for (item in collection) { }

for ((key, value) in map) { }

---

更方便的集合操作

• 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")

---

遍历

• 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));

• Kotlin

cars.forEach {
    println(it.speed)
}

cars.filter { it.speed > 100 }
      .forEach { println(it.speed)}

---

方法定义

• Java

void doSomething() {
   // logic here
}

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

• Kotlin

fun doSomething() {
   // logic here
}

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

---

带返回值的方法

• Java

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

• Kotlin

fun getScore(): Int {
   // logic here
   return score
}

// as a single-expression function

fun getScore(): Int = score

---

无结束符号

• 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

---

constructor 构造器

• 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
        }
        
    }
}

// other way is also there

object Utils {

    fun getScore(value: Int): Int {
        return 2 * value
    }

}

---

Get Set 构造器

• 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;
    }

    @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

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

• 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);

• Kotlin

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

var result = 3.triple()

---

你需要知道一些比较重要的事

• Learn Kotlin - lateinit vs lazy
• Learn Kotlin - apply vs with
• Learn Kotlin - 定义实体对象
• [Learn Kotlin - 声明式解构(https://mindorks.com/blog/learn-kotlin-destructuring-declarations)
• [Learn Kotlin - Sealed Classes](https://mindorks.com/blog/learn-kotlin-sealed-classes

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License

   Copyright (C) 2017 MINDORKS NEXTGEN PRIVATE LIMITED

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.