本期教程给大家分享Kotlin与Java上的主要异同，希望大家利用现有的Java知识能快速上手Kotlin开发。

打印日志

Java

System.
out.print(
"hello world");

System.
out.println(
"hello world");

Kotlin

print(
"hello world")


println(
"hello world")

定义变量与常量常

Java

String name = 
"hello world";


final 
String name = 
"hello world";

Kotlin

var name = 
"hello world"


val name = 
"hello world"

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

}


// or simply


val length = text?.length

字符串拼接

Java

String firstName = 
"Android";


String 
lastName = 
"Architect";


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

Kotlin

val firstName = 
"Android"


val lastName = 
"Architect"


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"

操作符

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

类型判断和转换 (声明式)

Java

Car car = (Car) 
object;

Kotlin

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 
// 自动识别


}

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 
0 
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, 
"Android");


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, "Android",


                                             2, "Ali",


                                             3, "Mindorks");

Kotlin

val listOfNumber = listOf(
1, 
2, 
3, 
4)


val keyValue = mapOf(
1 to 
"Android",

                     
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

        }



    }

}




// another way




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;

    }

}

Kotlin

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