《From Java To Kotlin》-Kotlin与Java的简单对比

前言：

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

本文也只是简单比较kotlin与java的不同之处，实际学习kotlin还是去官网学习比较好。我个人学习的一个网站贴出来：Kotlin中文网

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

final String name = null;
String otherName;
otherName = null;

• Kotlin

val name : String? = null
var otherName : String?
otherName = null

空判断

• Java

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

• Kotlin

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+=2) { }

for (String item : collection) { }

for (Map.Entry entry: map.entrySet()) { }

• Kotlin

for (i in 1..10) { }

for (i in 1..10 step 2) {}

for (item in collection) {}

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

更方便的集合操作

• Java

final List listOfNumber = Arrays.asList(1, 2, 3, 4);

final Map keyValue = new HashMap();
map.put(1, "Amit");
map.put(2, "Ali");
map.put(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

for (Car car : cars) {
  System.out.println(car.speed);
}

for (Car car : cars) {
  if(cars.speed > 100) {
    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
}

• Kotlin

fun doSomething() {
   // logic here
}

带返回值的方法

• Java

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

• Kotlin

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

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)

扩展函数

• 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()

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.