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Property and Backing Field @MoyuruAizawa

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Ѫᖒ๖ (Moyuru Aizawa) - Kotlin engineer at CyberAgent, Inc. - FRESH! lvla0805 MoyuruAizawa

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Property?

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class Person { val firstName: String val lastName: String … } Property

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class Person { val firstName: String val lastName: String … } Property

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Kotlin compiler generates Backing Fields and Accessors

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public final class Person { @NotNull private final String firstName; @NotNull private final String lastName; … @NotNull public final String getFirstName() { return this.firstName; } @NotNull public final String getLastName() { return this.lastName; } } Backing Fields and Accessors

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public final class Person { @NotNull private final String firstName; @NotNull private final String lastName; … @NotNull public final String getFirstName() { return this.firstName; } @NotNull public final String getLastName() { return this.lastName; } } Backing Fields

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public final class Person { @NotNull private final String firstName; @NotNull private final String lastName; … @NotNull public final String getFirstName() { return this.firstName; } @NotNull public final String getLastName() { return this.lastName; } } Accessors

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public final class Person { @NotNull private final String firstName; @NotNull private final String lastName; … @NotNull public final String getFirstName() { return this.firstName; } @NotNull public final String getLastName() { return this.lastName; } } Backing Fields

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Declare Custom Accessors

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" … } Declare custom accessors

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" … } Declare custom accessors

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" … } Declare custom accessors

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" … } person.fullName Declare custom accessors

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public final class Person { @NotNull private final String firstName; @NotNull private final String lastName; … @NotNull public final String getFirstName() { return this.firstName; } @NotNull public final String getLastName() { return this.lastName; } @NotNull public final String getFullName() { return this.firstName + " " + this.lastName; } } Declare custom accessors

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" var age: Int set(value) { require(value >= 0) field = value } … } Declare custom accessors

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" var age: Int set(value) { require(value >= 0) field = value } … } Declare custom accessors

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" var age: Int set(value) { require(value >= 0) field = value } … } Declare custom accessors

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" var age: Int set(value) { require(value >= 0) field = value } … } Declare custom accessors

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" var age: Int set(value) { require(value >= 0) age = value } … } Declare custom accessors Stack Overflow

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class Person { val firstName: String val lastName: String val fullName get() = "$firstName $lastName" var age: Int set(value) { require(value >= 0) field = value } … } person.age = 26 Declare custom accessors

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public final class Person { … private int age; … public final int getAge() { return this.age; } public final void setAge(int value) { required(value) this.age = value; } } Declare custom accessors

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public final class Person { … private int age; … public final int getAge() { return this.age; } public final void setAge(int value) { required(value) this.age = value; } } Declare custom accessors

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public final class Person { … private int age; … public final int getAge() { return this.age; } public final void setAge(int value) { required(value) this.age = value; } } Declare custom accessors

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var [: ] [= ] [] [] Declare custom accessors

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Properties without Backing Fields

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class NoBackingField { val property get() = "property" } No Backing Fields

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public final class NoBackingField { @NotNull public final String getProperty() { return "property"; } } No Backing Fields

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Property without Backing Field ≒ Function

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Interface can have Properties without Backing Fields

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interface User { val id: Long val name: String } data class Follower(override val id: Long, override val name: String): User data class Me(override val id: Long, override val name: String, val email: String): User Interface can have Properties without Backing Fields

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interface User { val id: Long val name: String } data class Follower(override val id: Long, override val name: String): User data class Me(override val id: Long, override val name: String, val email: String): User Interface can have Properties without Backing Fields

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interface User { val id: Long val name: String } data class Follower(override val id: Long, override val name: String): User data class Me(override val id: Long, override val name: String, val email: String): User Interface can have Properties without Backing Fields

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Extend a class with new Properties without Backing Fields

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val View.isVisible get() = visibility == View.VISIBLE val Activity.applicationComponent get() = (application as CustomApplication).component Extend a class with new Properties without Backing Fields

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val View.isVisible get() = visibility == View.VISIBLE val Activity.applicationComponent get() = (application as CustomApplication).component Extend a class with new Properties without Backing Fields

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val View.isVisible get() = visibility == View.VISIBLE val Activity.applicationComponent get() = (application as CustomApplication).component Extend a class with new Properties without Backing Fields

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‣ Kotiln generates Backing Fields and Accessors ‣ You can declare custom Accessors ‣ Interface can have properties without Backing Fields ‣ You can extend a class with new Properties without Backing Fields

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Thank you