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Help otlin Help You @calren24

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An ‘early’ taste of Kotlin: from Kotlin in Action

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Data classes Nullability Scope functions Coroutines Dangers of synthetic binding

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Data classes

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public class User { private String name; @Nullable private String emailAddress; public User(String name) { … } public User(String name, String emailAddress) { … } public String getName() { … } public void setName(String name) { … } @Nullable public String getEmailAddress() { … } public void setEmailAddress(@Nullable String emailAddress) { … } @Override public String toString() { … } @Override public boolean equals(Object o) { … } @Override public int hashCode() { … } }

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public class User { private String name; @Nullable private String emailAddress; public User(String name) { … } public User(String name, String emailAddress) { … } public String getName() { … } public void setName(String name) { … } @Nullable public String getEmailAddress() { … } public void setEmailAddress(@Nullable String emailAddress) { … } @Override public String toString() { … } @Override public boolean equals(Object o) { … } @Override public int hashCode() { … } }

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• Maintaining data classes can be a hassle

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• Maintaining data classes can be a hassle • Creating objects can be confusing if builder pattern doesn’t exist
 
 new Book("Jane Eyre", "Charlotte Brontë”);

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• Maintaining data classes can be a hassle • Creating objects can be confusing if builder pattern doesn’t exist
 
 new Book("Jane Eyre", "Charlotte Brontë”); ??? ???

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No content

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public class User { private String name; @Nullable private String emailAddress; public User(String name) { … } public User(String name, String emailAddress) { … } public String getName() { … } public void setName(String name) { … } @Nullable public String getEmailAddress() { … } public void setEmailAddress(@Nullable String emailAddress) { … } @Override public String toString() { … } @Override public boolean equals(Object o) { … } @Override public int hashCode() { … } }

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data class User (val name: String, var emailAddress: String? = null)

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data class User (val name: String, var emailAddress: String? = null)

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• Automatically creates equals() , hashCode(), toString()
 data class User (val name: String, var emailAddress: String? = null)

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• Automatically creates equals() , hashCode(), toString() • val vs var
 data class User (val name: String, var emailAddress: String? = null)

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• Automatically creates equals() , hashCode(), toString() • val vs var • denotes nullable fields
 
 val userDroid = User(name = "Droid")
 data class User (val name: String, var emailAddress: String? = null)

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• Automatically creates equals() , hashCode(), toString() • val vs var • denotes nullable fields
 
 val userDroid = User(name = "Droid") • default values
 data class User( val name: String, var emailAddress: String? = null, var optIntoEmails: Boolean = true )

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Nullability

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!! usually signals a code smell and is not as safe as you think

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!! usually signals a code smell and is not as safe as you think be wary of silent failures using ? 


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!! usually signals a code smell and is not as safe as you think be wary of silent failures using ? 
 user?.paymentMethod?.executeTransaction()

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!! usually signals a code smell and is not as safe as you think be wary of silent failures using ? 
 user?.paymentMethod?.executeTransaction() ?: Log.i("TAG", “No payment method stored”)

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// Java-like if (userDroid.emailAddress != null && userDroid.optIntoEmails) { sendMarketingEmail(userDroid.emailAddress!!) }

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// Java-like if (userDroid.emailAddress != null && userDroid.optIntoEmails) { sendMarketingEmail(userDroid.emailAddress!!) } // Kotlin-esque userDroid.emailAddress?. takeIf { userDroid.optIntoEmails }?. let { sendMarketingEmail(it) }

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Scope Functions

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Functions that allow for changing ‘scope’ of variables Scope Functions

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Functions that allow for changing ‘scope’ of variables user?.fullName?.let { fullName -> val formattedName = format(fullName) displayGreeting(formattedName) } Scope Functions

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Functions that allow for changing ‘scope’ of variables also { } apply { } let { } run { } with("an argument") { doSomething(this) } Similar in functionality, differences in appropriate use case Scope Functions

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Scope Functions From Medium article: Kotlin Demystified - Scope Functions

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let { }

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let { } Useful for null check and self-contained variables

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if (user != null && user.emailAddress != null) { sendMarketingEmail(user.emailAddress!!) } let { }

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if (user != null && user.emailAddress != null) { sendMarketingEmail(user.emailAddress!!) } user?.emailAddress?.let { sendMarketingEmail(it) } let { }

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if (user != null && user.emailAddress != null) { sendMarketingEmail(user.emailAddress!!) } user?.emailAddress?.let { sendMarketingEmail(it) } user?.emailAddress?.let { emailAddress -> sendMarketingEmail(emailAddress) } let { }

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if (user != null && user.emailAddress != null) { sendMarketingEmail(user.emailAddress!!) } user?.emailAddress?.let { sendMarketingEmail(it) } user?.emailAddress?.let { emailAddress -> sendMarketingEmail(emailAddress) } Database.getConnection().let { } // connection doesn't exist outside let { }

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apply { }

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apply { } Useful for object configuration

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val view = View(this).apply { setBackgroundColor() setOnLongClickListener { } setOnClickListener { } } apply { }

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val view = View(this).apply { setBackgroundColor() setOnLongClickListener { } setOnClickListener { } } val view2 = View(this) view2.setBackgroundColor() view2.setOnLongClickListener { } view2.setOnLongClickListener { } apply { }

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also { } Useful for calls that need a contextual object, but doesn’t alter the object itself

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also { } val paint = Paint() .apply { color = Color.BLUE pathEffect = PathEffect() } .also { print("Starting paint with ${it.color}") }

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run { }

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run { } Combination of with and let

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run { } Combination of with and let Useful for initialization

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run { } val randomWelcomeGreeting = GreetingGenerator.obtain().run { day = getDayOfWeek() mood = Mood.Cheerful voice = Voice.OPRAH generate() }

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Scope Functions let Executing code with null check 
 Introducing local variables apply Object configuration
 run Top level initialization also Additional effects with Grouping function calls

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Kotlin Standard Functions Cheat Sheet

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Coroutines

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Simplify managing background threads without managing callbacks

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Simplify managing background threads without managing callbacks Sequential code is a lot easier to read and manage

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Simplify managing background threads without managing callbacks Sequential code is a lot easier to read and manage
 
 // Async callbacks networkRequest { result -> // Successful network request databaseSave(result) { rows -> // Result saved } }

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Simplify managing background threads without managing callbacks Sequential code is a lot easier to read and manage
 
 // Async callbacks networkRequest { result -> // Successful network request databaseSave(result) { rows -> // Result saved } } // The same code with coroutines val result = networkRequest() // Successful network request databaseSave(result) // Result saved vs

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Synthetic Binding

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import kotlinx.android.synthetic.main. activity_main.*
 
 button.text = "Click me" Activity.kt activity.xml

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import kotlinx.android.synthetic.main. activity_main.*
 
 button.text = "Click me" Activity.kt activity.xml Makes available ALL ids defined in the project

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import kotlinx.android.synthetic.main. activity_main.*
 
 button.text = "Click me" Activity.kt activity.xml Makes available ALL ids defined in the project Views unrelated to inflated layout are accessible

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import kotlinx.android.synthetic.main. activity_main.*
 
 button.text = "Click me" Activity.kt activity.xml Makes available ALL ids defined in the project Views unrelated to inflated layout are accessible Doesn’t expose nullability well

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import kotlinx.android.synthetic.main. activity_main.*
 
 button.text = "Click me" Activity.kt activity.xml Makes available ALL ids defined in the project Views unrelated to inflated layout are accessible Doesn’t expose nullability well Other options: Data Binding or Butter Knife

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Codelabs

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Refactoring to Kotlin Using Kotlin Coroutines in your Android app Android Data Binding Room with a View