From Java To Kotlin

About me

Live session on "Android Developer Interview Preparation for Product-based Companies"

Print to Console

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

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

Constants and Variables

Stringname ="Amit Shekhar";finalStringname ="Amit Shekhar";

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

Assigning the null value

StringotherName;otherName =null;

var otherName:String?otherName=null

Verify if value is null

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

text?.let {val length= text.length}// or simplyval length= text?.length

Verify if value is NotNull OR NotEmpty

StringsampleString ="Shekhar";if (!sampleString.isEmpty()) {myTextView.setText(sampleString);}if(sampleString!=null && !sampleString.isEmpty()){myTextView.setText(sampleString); }

var sampleString="Shekhar"if(sampleString.isNotEmpty()){//the feature of kotlin extension function    myTextView.text=sampleString}if(!sampleString.isNullOrEmpty()){   myTextView.text=sampleString }

Concatenation of strings

StringfirstName ="Amit";StringlastName ="Shekhar";Stringmessage ="My name is: " +firstName +" " +lastName;

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

New line in string

Stringtext ="First Line\n" +"Second Line\n" +"Third Line";

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

Substring

Stringstr ="Java to Kotlin Guide";Stringsubstr ="";//print javasubstr =str.substring(0,4);System.out.println("substring = " +substr);//print kotlinsubstr =str.substring(8,14);System.out.println("substring = " +substr);

var str="Java to Kotlin Guide"var substr=""//print javasubstr= str.substring(0..3)//println("substring$substr")//print kotlinsubstr= str.substring(8..13)println("substring$substr")

Ternary Operations

Stringtext =x >5 ?"x > 5" :"x <= 5";Stringmessage =null;log(message !=null ?message :"");

val text=if (x>5)"x > 5"else"x <= 5"val message:String?=nulllog(message?:"")

Bitwise Operators

finalintandResult  =a &b;finalintorResult   =a |b;finalintxorResult  =a ^b;finalintrightShift =a >>2;finalintleftShift  =a <<2;finalintunsignedRightShift =a >>>2;

val andResult= aand bval orResult= aor bval xorResult= a xor bval rightShift= a shr2val leftShift= a shl2val unsignedRightShift= a ushr2

Check the type and casting

if (objectinstanceofCar) {Carcar = (Car)object;}

if (objectis Car) {var car=objectas Car}// if object is nullvar car=objectas? Car //var car =objectas Car?

Check the type and casting (implicit)

if (objectinstanceofCar) {Carcar = (Car)object;}

if (objectis Car) {var car=object// smart casting}// if object is nullif (objectis Car?) {var car=object// smart casting, car will be null}

Multiple conditions

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

if (scorein0..300) { }

Multiple Conditions (Switch case)

intscore =// some score;Stringgrade;switch (score) {case10:case9:grade ="Excellent";break;case8:case7:case6:grade ="Good";break;case5:case4:grade ="OK";break;case3:case2:case1:grade ="Fail";break;default:grade ="Fail";       }

var score=// some scorevar grade=when (score) {9,10->"Excellent"in6..8->"Good"4,5->"OK"else->"Fail"}

For-loops

for (inti =1;i <=10 ;i++) { }for (inti =1;i <10 ;i++) { }for (inti =10;i >=0 ;i--) { }for (inti =1;i <=10 ;i+=2) { }for (inti =10;i >=0 ;i-=2) { }for (Stringitem :collection) { }for (Map.Entry<String,String>entry:map.entrySet()) { }

for (iin1..10) { }for (iin1 until10) { }for (iin10 downTo0) { }for (iin1..10 step2) { }for (iin10 downTo0 step2) { }for (itemin collection) { }for ((key, value)in map) { }

Collections

finalList<Integer>listOfNumber =Arrays.asList(1,2,3,4);finalMap<Integer,String>keyValue =newHashMap<Integer,String>();map.put(1,"Amit");map.put(2,"Anand");map.put(3,"Messi");// Java 9finalList<Integer>listOfNumber =List.of(1,2,3,4);finalMap<Integer,String>keyValue =Map.of(1,"Amit",2,"Anand",3,"Messi");

val listOfNumber=listOf(1,2,3,4)val keyValue=mapOf(1 to"Amit",2 to"Anand",3 to"Messi")

for each

// Java 7 and belowfor (Carcar :cars) {System.out.println(car.speed);}// Java 8+cars.forEach(car ->System.out.println(car.speed));// Java 7 and belowfor (Carcar :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));

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)}

Splitting arrays

String[]splits ="param=car".split("=");Stringparam =splits[0];Stringvalue =splits[1];

val (param, value)="param=car".split("=")

Defining methods

voiddoSomething() {// logic here}

fundoSomething() {// logic here}

Default values for method parameters

doublecalculateCost(intquantity,doublepricePerItem) {returnpricePerItem *quantity;}doublecalculateCost(intquantity) {// default price is 20.5return20.5 *quantity;}

funcalculateCost(quantity:Int,pricePerItem:Double = 20.5)= quantity* pricePerItemcalculateCost(10,25.0)// 250calculateCost(10)// 205

Variable number of arguments

voiddoSomething(int...numbers) {// logic here}

fundoSomething(varargnumbers:Int) {// logic here}

Defining methods with return

intgetScore() {// logic herereturnscore;}

fungetScore():Int {// logic herereturn score}// as a single-expression functionfungetScore():Int= score// even simpler (type will be determined automatically)fungetScore()= score// return-type is Int

Returning result of an operation

intgetScore(intvalue) {// logic herereturn2 *value;}

fungetScore(value:Int):Int {// logic herereturn2* value}// as a single-expression functionfungetScore(value:Int):Int=2* value// even simpler (type will be determined automatically)fungetScore(value:Int)=2* value// return-type is int

Constructors

publicclassUtils {privateUtils() {// This utility class is not publicly instantiable    }publicstaticintgetScore(intvalue) {return2 *value;    }}

classUtils private constructor() {companionobject {fungetScore(value:Int):Int {return2* value        }    }}// another wayobject Utils {fungetScore(value:Int):Int {return2* value    }}

Getters and Setters

publicclassDeveloper {privateStringname;privateintage;publicDeveloper(Stringname,intage) {this.name =name;this.age =age;    }publicStringgetName() {returnname;    }publicvoidsetName(Stringname) {this.name =name;    }publicintgetAge() {returnage;    }publicvoidsetAge(intage) {this.age =age;    }@Overridepublicbooleanequals(Objecto) {if (this ==o)returntrue;if (o ==null ||getClass() !=o.getClass())returnfalse;Developerdeveloper = (Developer)o;if (age !=developer.age)returnfalse;returnname !=null ?name.equals(developer.name) :developer.name ==null;    }@OverridepublicinthashCode() {intresult =name !=null ?name.hashCode() :0;result =31 *result +age;returnresult;    }@OverridepublicStringtoString() {return"Developer{" +"name='" +name +'\'' +", age=" +age +'}';    }}

data classDeveloper(varname:String,varage:Int)

Cloning or copying

publicclassDeveloperimplementsCloneable {    privateStringname;privateintage;publicDeveloper(Stringname,intage) {this.name =name;this.age =age;    }@OverrideprotectedObjectclone()throwsCloneNotSupportedException {return (Developer)super.clone();    }}// cloning or copyingDeveloperdev =newDeveloper("Messi",30);try {Developerdev2 = (Developer)dev.clone();}catch (CloneNotSupportedExceptione) {// handle exception}

data classDeveloper(varname:String,varage:Int)// cloning or copyingval dev=Developer("Messi",30)val dev2= dev.copy()// in case you only want to copy selected propertiesval dev2= dev.copy(age=25)

Generics

// Example #1interfaceSomeInterface<T> {voiddoSomething(Tdata);}classSomeClassimplementsSomeInterface<String> {@OverridepublicvoiddoSomething(Stringdata) {// some logic    }}// Example #2interfaceSomeInterface<TextendsCollection<?>> {voiddoSomething(Tdata);}classSomeClassimplementsSomeInterface<List<String>> {@OverridepublicvoiddoSomething(List<String>data) {// some logic    }}

interfaceSomeInterface<T> {fundoSomething(data:T)}classSomeClass:SomeInterface<String> {overridefundoSomething(data:String) {// some logic    }}interfaceSomeInterface<T:Collection<*>> {fundoSomething(data:T)}classSomeClass:SomeInterface<List<String>> {overridefundoSomething(data:List<String>) {// some logic    }}

Extension function

publicclassUtils {privateUtils() {// This utility class is not publicly instantiable    }publicstaticinttriple(intvalue) {return3 *value;    }}intresult =Utils.triple(3);

fun Int.triple():Int {returnthis*3}var result=3.triple()

Defining uninitialized objects

Personperson;

internallateinitvar person:Person

enum

publicenumDirection {NORTH(1),SOUTH(2),WEST(3),EAST(4);intdirection;Direction(intdirection) {this.direction =direction;        }publicintgetDirection() {returndirection;        }    }

enumclassDirection(valdirection:Int) {NORTH(1),SOUTH(2),WEST(3),EAST(4);}

Sorting List

List<Profile>profiles =loadProfiles(context);Collections.sort(profiles,newComparator<Profile>() {@Overridepublicintcompare(Profileprofile1,Profileprofile2) {if (profile1.getAge() >profile2.getAge())return1;if (profile1.getAge() <profile2.getAge())return -1;return0;    }});

val profile= loadProfiles(context)profile.sortedWith(Comparator({ profile1, profile2->if (profile1.age> profile2.age)return@Comparator1if (profile1.age< profile2.age)return@Comparator-1return@Comparator0}))

Anonymous Class

AsyncTask<Void,Void,Profile>task =newAsyncTask<Void,Void,Profile>() {@OverrideprotectedProfiledoInBackground(Void...voids) {// fetch profile from API or DBreturnnull;    }@OverrideprotectedvoidonPreExecute() {super.onPreExecute();// do something    }};

val task=object:AsyncTask<Void,Void,Profile>() {overridefundoInBackground(varargvoids:Void):Profile? {// fetch profile from API or DBreturnnull    }overridefunonPreExecute() {super.onPreExecute()// do something    }}

Initialization block

publicclassUser {    {//Initialization blockSystem.out.println("Init block");    }}

classUser {init {// Initialization blockprintln("Init block")        }    }

Important things to know in Kotlin

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License

   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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