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>

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 listOfNumber = Arrays.asList(

1,

2,

3,

4);

final Map keyValue =

new HashMap();

map.put(

1,

"Android");

map.put(

2,

"Ali");

map.put(

3,

"Mindorks");

// Java 9

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

final Map 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)