from java to kotlin
1.0.0
От Java к Kotlin — ваша шпаргалка по переходу с Java на Kotlin
Привет, я Амит Шекхар, соучредитель @ Outcome School • IIT 2010–14 • Я обучал и наставлял многих разработчиков, и их усилия обеспечили им высокооплачиваемую работу в сфере технологий, помогли многим технологическим компаниям в решении их уникальных проблем и создали многие библиотеки с открытым исходным кодом используются ведущими компаниями. Я с энтузиазмом делюсь знаниями через открытый исходный код, блоги и видео.
Вы можете связаться со мной по:
Ява
System . out . print ( "Amit Shekhar" );
System . out . println ( "Amit Shekhar" );Котлин
print ( " Amit Shekhar " )
println ( " Amit Shekhar " )Ява
String name = "Amit Shekhar" ;
final String name = "Amit Shekhar" ;Котлин
var name = " Amit Shekhar "
val name = " Amit Shekhar " Ява
String otherName ;
otherName = null ;Котлин
var otherName : String?
otherName = null Ява
if ( text != null ) {
int length = text . length ();
}Котлин
text?. let {
val length = text.length
}
// or simply
val length = text?.lengthЯва
String sampleString = "Shekhar" ;
if (! sampleString . isEmpty ()) {
myTextView . setText ( sampleString );
}
if ( sampleString != null && ! sampleString . isEmpty ()){
myTextView . setText ( sampleString );
}Котлин
var sampleString = " Shekhar "
if (sampleString.isNotEmpty()){ // the feature of kotlin extension function
myTextView.text = sampleString
}
if ( ! sampleString.isNullOrEmpty()){
myTextView.text = sampleString
}Ява
String firstName = "Amit" ;
String lastName = "Shekhar" ;
String message = "My name is: " + firstName + " " + lastName ;Котлин
var firstName = " Amit "
var lastName = " Shekhar "
var message = " My name is: $firstName $lastName " Ява
String text = "First Line n " +
"Second Line n " +
"Third Line" ;Котлин
val text = """
|First Line
|Second Line
|Third Line
""" .trimMargin()Ява
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 );Котлин
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 " )Ява
String text = x > 5 ? "x > 5" : "x <= 5" ;
String message = null ;
log ( message != null ? message : "" );Котлин
val text = if (x > 5 ) " x > 5 " else " x <= 5 "
val message : String? = null
log(message ? : " " )Ява
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 ;Котлин
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 Ява
if ( object instanceof Car ) {
Car car = ( Car ) object ;
}Котлин
if ( object is Car) {
var car = object as Car
}
// if object is null
var car = object as ? Car // var car = object as Car?Ява
if ( object instanceof Car ) {
Car car = ( Car ) object ;
}Котлин
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
}Ява
if ( score >= 0 && score <= 300 ) { }Котлин
if (score in 0 .. 300 ) { }Ява
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" ;
}Котлин
var score = // some score
var grade = when (score) {
9 , 10 -> " Excellent "
in 6 .. 8 -> " Good "
4 , 5 -> " OK "
else -> " Fail "
}Ява
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 ()) { }Котлин
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) { }Ява
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 , "Anand" );
map . put ( 3 , "Messi" );
// Java 9
final List < Integer > listOfNumber = List . of ( 1 , 2 , 3 , 4 );
final Map < Integer , String > keyValue = Map . of ( 1 , "Amit" ,
2 , "Anand" ,
3 , "Messi" );Котлин
val listOfNumber = listOf ( 1 , 2 , 3 , 4 )
val keyValue = mapOf ( 1 to " Amit " ,
2 to " Anand " ,
3 to " Messi " )Ява
// 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 ));Котлин
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)}Ява
String [] splits = "param=car" . split ( "=" );
String param = splits [ 0 ];
String value = splits [ 1 ];котлин
val (param, value) = " param=car " .split( " = " )Ява
void doSomething () {
// logic here
}Котлин
fun doSomething () {
// logic here
}Ява
double calculateCost ( int quantity , double pricePerItem ) {
return pricePerItem * quantity ;
}
double calculateCost ( int quantity ) {
// default price is 20.5
return 20.5 * quantity ;
}Котлин
fun calculateCost ( quantity : Int , pricePerItem : Double = 20.5) = quantity * pricePerItem
calculateCost( 10 , 25.0 ) // 250
calculateCost( 10 ) // 205
Ява
void doSomething ( int ... numbers ) {
// logic here
}Котлин
fun doSomething ( vararg numbers : Int ) {
// logic here
}Ява
int getScore () {
// logic here
return score ;
}Котлин
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 Ява
int getScore ( int value ) {
// logic here
return 2 * value ;
}Котлин
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 Ява
public class Utils {
private Utils () {
// This utility class is not publicly instantiable
}
public static int getScore ( int value ) {
return 2 * value ;
}
}Котлин
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
}
}Ява
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 +
'}' ;
}
}Котлин
data class Developer ( var name : String , var age : Int )
Ява
public class Developer implements Cloneable {
private String name ;
private int age ;
public Developer ( String name , int age ) {
this . name = name ;
this . age = age ;
}
@ Override
protected Object clone () throws CloneNotSupportedException {
return ( Developer ) super . clone ();
}
}
// cloning or copying
Developer dev = new Developer ( "Messi" , 30 );
try {
Developer dev2 = ( Developer ) dev . clone ();
} catch ( CloneNotSupportedException e ) {
// handle exception
}Котлин
data class Developer ( var name : String , var age : Int )
// cloning or copying
val dev = Developer ( " Messi " , 30 )
val dev2 = dev.copy()
// in case you only want to copy selected properties
val dev2 = dev.copy(age = 25 )
Ява
// Example #1
interface SomeInterface < T > {
void doSomething ( T data );
}
class SomeClass implements SomeInterface < String > {
@ Override
public void doSomething ( String data ) {
// some logic
}
}
// Example #2
interface SomeInterface < T extends Collection <?>> {
void doSomething ( T data );
}
class SomeClass implements SomeInterface < List < String >> {
@ Override
public void doSomething ( List < String > data ) {
// some logic
}
}Котлин
interface SomeInterface < T > {
fun doSomething ( data : T )
}
class SomeClass : SomeInterface < String > {
override fun doSomething ( data : String ) {
// some logic
}
}
interface SomeInterface < T : Collection < * >> {
fun doSomething ( data : T )
}
class SomeClass : SomeInterface < List < String >> {
override fun doSomething ( data : List < String >) {
// some logic
}
}Ява
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 );Котлин
fun Int. triple (): Int {
return this * 3
}
var result = 3 .triple()Ява
Person person ;Котлин
internal lateinit var person : Person Ява
public enum Direction {
NORTH ( 1 ),
SOUTH ( 2 ),
WEST ( 3 ),
EAST ( 4 );
int direction ;
Direction ( int direction ) {
this . direction = direction ;
}
public int getDirection () {
return direction ;
}
}Котлин
enum class Direction ( val direction : Int ) {
NORTH ( 1 ),
SOUTH ( 2 ),
WEST ( 3 ),
EAST ( 4 );
}Ява
List < Profile > profiles = loadProfiles ( context );
Collections . sort ( profiles , new Comparator < Profile >() {
@ Override
public int compare ( Profile profile1 , Profile profile2 ) {
if ( profile1 . getAge () > profile2 . getAge ()) return 1 ;
if ( profile1 . getAge () < profile2 . getAge ()) return - 1 ;
return 0 ;
}
});Котлин
val profile = loadProfiles(context)
profile.sortedWith( Comparator ({ profile1, profile2 ->
if (profile1.age > profile2.age) return @Comparator 1
if (profile1.age < profile2.age) return @Comparator - 1
return @Comparator 0
}))Ява
AsyncTask < Void , Void , Profile > task = new AsyncTask < Void , Void , Profile >() {
@ Override
protected Profile doInBackground ( Void ... voids ) {
// fetch profile from API or DB
return null ;
}
@ Override
protected void onPreExecute () {
super . onPreExecute ();
// do something
}
};Котлин
val task = object : AsyncTask < Void , Void , Profile >() {
override fun doInBackground ( vararg voids : Void ): Profile ? {
// fetch profile from API or DB
return null
}
override fun onPreExecute () {
super .onPreExecute()
// do something
}
}Ява
public class User {
{ //Initialization block
System . out . println ( "Init block" );
}
}Котлин
class User {
init { // Initialization block
println ( " Init block " )
}
} Copyright (C) 2024 Amit Shekhar
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.
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