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Integer x = 300; Integer y = 300; if(x == y) { println("Same Int"); } else { println("Diff Int”); }A

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Integer x = 300; Integer y = 300; if(x == y) { println("Same Int"); } else { println("Diff Int”); }A prints Diff Int

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Integer x = 300; Integer y = 300; if(x.equals(y)) { println("Same Int"); } else { println("Diff Int”); }A prints Diff Int

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Integer x = 300; Integer y = 300; if(x.equals(y)) { println("Same Int"); } else { println("Diff Int”); }A prints Same Int

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(ᵒ‒Д‒)ᵒኯᵲᴸᵲ

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ᵣᴷᵣ ϊ( ʄ-ʄϊ)

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Effective Java To Effective Kotlin Israel Ferrer Camacho @rallat

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Item 3: Enforce the singleton property with a private constructor or an enum type

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public enum EffectiveSingleton { INSTANCE }A

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object EffectiveSingleton public enum EffectiveSingleton { INSTANCE }A

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object EffectiveSingleton public final class EffectiveSingleton { public static final EffectiveSingleton INSTANCE; static { EffectiveSingleton var0 = new EffectiveSingleton(); INSTANCE = var0; } }

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Item 2: Consider a builder when faced with many constructor parameters

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public class Item { public final String id; public final String name; public final String description; }A

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public class Item { public final String id; public final String name; public final String description; }A public static class Builder { private String name = ""; private String description = ""; public Item create(String id) { return new Item(id, description, name); }B public Builder name(String name) { this.name = name; return this; }C public Builder description(String description) { this.description = description; return this; }D }E

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public static class Builder { private String name = ""; private String description = ""; public Item create(String id) { return new Item(id, description, name); }B public Builder name(String name) { this.name = name; return this; }C public Builder description(String description) { this.description = description; return this; }D }E

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class Item( val id: String, val name: String = "", val description: String = "" ) public static class Builder { private String name = ""; private String description = ""; public Item create(String id) { return new Item(id, description, name); }B public Builder name(String name) { this.name = name; return this; }C public Builder description(String description) { this.description = description; return this; }D }E

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class Item @JvmOverloads constructor( val id: String, val name: String = "", val description: String = "" ) public static class Builder { private String name = ""; private String description = ""; public Item create(String id) { return new Item(id, description, name); }B public Builder name(String name) { this.name = name; return this; }C public Builder description(String description) { this.description = description; return this; }D }E

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class Item @JvmOverloads constructor( val id: String, val name: String = "", val description: String = "" )

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class Item @JvmOverloads constructor( val id: String, val name: String = "", val description: String = "" ) Item("1")

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class Item @JvmOverloads constructor( val id: String, val name: String = "", val description: String = "" ) Item("1") Item(name= "the name",id = "1")

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Item 40: Consistently use the Override annotation

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interface Header{ public void inflate(); } class AdHeader implements Header { public void inflate() { } }

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interface Header{ public void inflate(); } class AdHeader implements Header { public void inflate() { } }

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Item 17: Design for inheritance or else prohibit it

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class NewMainActivity

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class NewMainActivity

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open class NewMainActivity class ClassicActivity: NewMainActivity()

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Item 18: Favor composition over inheritance

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interface Pump { void startMotor(); } class RotaryVane implements Pump interface Boiler { void heat(); } class DoubleBoiler implements Boiler

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public class EspressoCoffeeMachine implements Boiler, Pump { private final DoubleBoiler boiler; private final RotaryVane pump; public EspressoCoffeeMachine(DoubleBoiler boiler, RotaryVane pump) { this.boiler = boiler; this.pump = pump; } @Override public void heat() { boiler.heat(); } @Override public void startMotor() { pump.startMotor(); } }

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class EspressoCoffeeMachine : Boiler by DoubleBoiler, Pump by RotaryVane public class EspressoCoffeeMachine implements Boiler, Pump { private final DoubleBoiler boiler; private final RotaryVane pump; public EspressoCoffeeMachine(DoubleBoiler boiler, RotaryVane pump) { this.boiler = boiler; this.pump = pump; } @Override public void heat() { boiler.heat(); } @Override public void startMotor() { pump.startMotor(); } }

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class EspressoCoffeeMachine : Boiler by DoubleBoiler, Pump by RotaryVane

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public final class EspressoCoffeeMachine implements Boiler, Pump { private final DoubleBoiler $$delegate_0; private final RotaryVane $$delegate_1; public EspressoCoffeeMachine() { this.$$delegate_0 = DoubleBoiler.INSTANCE; this.$$delegate_1 = RotaryVane.INSTANCE; } } class EspressoCoffeeMachine : Boiler by DoubleBoiler, Pump by RotaryVane

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Item 77: Don't ignore exceptions

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public void close(Closeable c) { try { c.close(); } catch (IOException ignored) { } }

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public int getCastValueToInt(String str) { int num = -1; try { num = Integer.parseInt(str); } catch (NumberFormatException ignored) {} return num; }

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public int getCastValueToInt(String str) { int num = -1; try { num = Integer.parseInt(str); } catch (NumberFormatException ignored) {} return num; } fun getCastValueToInt(string: String) = try { Integer.parseInt(string); } catch (e: NumberFormatException) { -1 }

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Item 15 Minimize mutability

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Item 8 Obey contract when overriding equals

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Item 9: Always override hashCode when you override Equals

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public class ItemJava { @NotNull public final String id; @NotNull public final String name; @NotNull public final String description; } public ItemJava(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, " this.id = id; this.name = name; this.description = description;

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public class ItemJava { @NotNull public final String id; @NotNull public final String name; @NotNull public final String description; public ItemJava(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); this.id = id; this.name = name; this.description = description; }A } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof ItemJava) { ItemJava var2 = (ItemJava) var1; return Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) &&

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public class ItemJava { @NotNull public final String id; @NotNull public final String name; @NotNull public final String description; public ItemJava(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); this.id = id; this.name = name; this.description = description; }A public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof ItemJava) { ItemJava var2 = (ItemJava) var1; return Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description); }B return false; } else { return true; }C }D } public int hashCode() {

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public class ItemJava { @NotNull public final String id; @NotNull public final String name; @NotNull public final String description; public ItemJava(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); this.id = id; this.name = name; this.description = description; }A public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof ItemJava) { ItemJava var2 = (ItemJava) var1; return Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description); }B return false; } else { return true; }C }D public int hashCode() { return (this.id.hashCode() * 31 + this.name.hashCode()) * 31 + this.description.hashCode(); }E }F

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public class ItemJava { @NotNull public final String id; @NotNull public final String name; @NotNull public final String description; public ItemJava(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); this.id = id; this.name = name; this.description = description; }A public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof ItemJava) { ItemJava var2 = (ItemJava) var1; return Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description); }B return false; } else { return true; }C }D public int hashCode() { return (this.id.hashCode() * 31 + this.name.hashCode()) * 31 + this.description.hashCode(); }E }F

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public class ItemJava { @NotNull public final String id; @NotNull public final String name; @NotNull public final String description; public ItemJava(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); this.id = id; this.name = name; this.description = description; }A public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof ItemJava) { ItemJava var2 = (ItemJava) var1; return Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description); }B return false; } else { return true; }C }D public int hashCode() { return (this.id.hashCode() * 31 + this.name.hashCode()) * 31 + this.description.hashCode(); }E }F data class Item(val id: String, val name: String, val description: String)

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data class Item(val id: String, val name: String, val description: String) public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true;

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data class Item(val id: String, val name: String, val description: String) public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; }

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data class Item(val id: String, val name: String, val description: String) public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true;

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data class Item(val id: String, val name: String, val description: String) @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; }

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data class Item(val id: String, val name: String, val description: String) public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true;

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data class Item(val id: String, val name: String, val description: String) public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true; } }

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data class Item(val id: String, val name: String, val description: String) public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true;

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data class Item(val id: String, val name: String, val description: String) public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); }

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data class Item(val id: String, val name: String, val description: String) public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true;

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data class Item(val id: String, val name: String, val description: String) @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; }

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data class Item(val id: String, val name: String, val description: String) public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true;

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data class Item(val id: String, val name: String, val description: String) @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; }

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data class Item(val id: String, val name: String, val description: String) @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } val(id, name, description) = fooItem() fun fooItem(): Item { return Item("id", "name", "description") }

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data class Item(val id: String, val name: String, val description: String) : Parcelable public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true; val(id, name, description) = fooItem() fun fooItem(): Item { return Item("id", "name", "description") }

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data class Item(val id: String, val name: String, val description: String) : Parcelable public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true;

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@Parcelize data class Item(val id: String, val name: String, val description: String) : Parcelable public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true;

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public void writeToParcel(@NotNull Parcel parcel, int flags) { Intrinsics.checkParameterIsNotNull(parcel, "parcel"); parcel.writeString(this.id); parcel.writeString(this.name); parcel.writeString(this.description); } @Metadata( mv = {1, 1, 13}, bv = {1, 0, 3}, k = 3 ) public static class Creator implements Parcelable.Creator { @NotNull public final Object[] newArray(int size) { return new Item[size]; } @NotNull public final Object createFromParcel(@NotNull Parcel in) { Intrinsics.checkParameterIsNotNull(in, "in"); return new Item(in.readString(), in.readString(), in.readString()); } } @Parcelize data class Item(val id: String, val name: String, val description: String) : Parcelable

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public final class Item { @NotNull private final String id; @NotNull private final String name; @NotNull private final String description; @NotNull public final String getId() { return this.id; } @NotNull public final String getName() { return this.name; } @NotNull public final String getDescription() { return this.description; } public Item(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); super(); this.id = id; this.name = name; this.description = description; } @NotNull public final String component1() { return this.id; } @NotNull public final String component2() { return this.name; } @NotNull public final String component3() { return this.description; } @NotNull public final Item copy(@NotNull String id, @NotNull String name, @NotNull String description) { Intrinsics.checkParameterIsNotNull(id, "id"); Intrinsics.checkParameterIsNotNull(name, "name"); Intrinsics.checkParameterIsNotNull(description, "description"); return new Item(id, name, description); } // $FF: synthetic method // $FF: bridge method @NotNull public static Item copy$default(Item var0, String var1, String var2, String var3, int var4, Object var5) { if ((var4 & 1) != 0) { var1 = var0.id; } if ((var4 & 2) != 0) { var2 = var0.name; } if ((var4 & 4) != 0) { var3 = var0.description; } return var0.copy(var1, var2, var3); } public String toString() { return "Item(id=" + this.id + ", name=" + this.name + ", description=" + this.description + ")"; } public int hashCode() { return ((this.id != null ? this.id.hashCode() : 0) * 31 + (this.name != null ? this.name.hashCode() : 0)) * 31 + (this.description != null ? this.description.hashCode() : 0); } public boolean equals(Object var1) { if (this != var1) { if (var1 instanceof Item) { Item var2 = (Item)var1; if (Intrinsics.areEqual(this.id, var2.id) && Intrinsics.areEqual(this.name, var2.name) && Intrinsics.areEqual(this.description, var2.description)) { return true; } } return false; } else { return true; @Parcelize data class Item(val id: String, val name: String, val description: String) : Parcelable

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Item 43: Always return an empty collection instead of null

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Handling Null references

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public List getItem(String itemId)

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@NotNull public List getItem(@NotNull String itemId)

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@NotNull public List getItem(@NotNull String itemId)

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@NotNull public List getItem(@NotNull String itemId)

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@NotNull public List getItem(@NotNull String itemId) fun getItem(itemId: String) : List

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fun getItem(itemId: String) : List

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fun getItem(itemId: String) : List @NotNull public final List getItem(@NotNull String itemId)

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fun getItem(itemId: String) : List

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fun getItem(itemId: String) : List

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fun getItem(itemId: String) : List

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fun getItem(itemId: String) : List

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fun getItem(itemId: String) : List

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Chapter 6: Enums and annotations

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Sealed classes

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sealed class RemoteData

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sealed class RemoteData { class NotInitialized : RemoteData() }A

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sealed class RemoteData { object NotInitialized : RemoteData() }A

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sealed class RemoteData { object NotInitialized : RemoteData() class Loading : RemoteData() }A

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sealed class RemoteData { object NotInitialized : RemoteData() data class Loading(val progress: Progress) : RemoteData() }A

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sealed class RemoteData { object NotInitialized : RemoteData() data class Loading(val progress: Progress) : RemoteData() class Success : RemoteData() }A

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sealed class RemoteData { object NotInitialized : RemoteData() data class Loading(val progress: Progress) : RemoteData() class Success : RemoteData() class Failure : RemoteData() }A

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sealed class RemoteData { object NotInitialized : RemoteData() data class Loading(val progress: Progress) : RemoteData() class Success(result: Response) : RemoteData() class Failure : RemoteData() }A

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sealed class RemoteData { object NotInitialized : RemoteData() data class Loading(val progress: Progress) : RemoteData() class Success(result: Response) : RemoteData() class Failure(exception: Exception) : RemoteData() }A

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fun newState(request: RemoteData) = when (request) { RemoteData.NotInitialized -> request }A

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fun newState(request: RemoteData) = when (request) { RemoteData.NotInitialized -> request is RemoteData.Loading -> { viewModel.loading(request.progress) }B }A

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fun newState(request: RemoteData) = when (request) { RemoteData.NotInitialized -> request is RemoteData.Loading -> { viewModel.loading(request.progress) }B is RemoteData.Success -> { viewModel.transform(request.result) }C }A

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fun newState(request: RemoteData) = when (request) { RemoteData.NotInitialized -> request is RemoteData.Loading -> { viewModel.loading(request.progress) }B is RemoteData.Success -> { viewModel.transform(request.result) }C is RemoteData.Failure -> { viewModel.showError(request.exception) }D }A

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Generics

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Item 27: Favor generic methods

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Reified

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inline fun Context.start() = startActivity(Intent(this, T::class.java))

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inline fun Context.start() = startActivity(Intent(this, T::class.java))

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inline fun Context.start() = startActivity(Intent(this, T::class.java))

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inline fun Context.start() = startActivity(Intent(this, T::class.java)) start()

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Item 28: Prefer lists to arrays

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Java arrays are covariant

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public final class Integer extends Number

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Integer[] integers = new Integer[1]; Number[] numbers = integers; numbers[0] = 1L; public final class Integer extends Number

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Integer[] integers = new Integer[1]; Number[] numbers = integers; numbers[0] = 1L; java.lang.ArrayStoreException: Long cannot be stored in an array of type Integer[] public final class Integer extends Number

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Java List with generics are invariant

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List enforces type safety at compile time.

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Arrays in Kotlin are invariant and allow generics

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Integer[] integers = new Integer[1]; Number[] numbers = integers; numbers[0] = 1L; java.lang.ArrayStoreException: Long cannot be stored in an array of type Integer[]

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Integer[] integers = new Integer[1]; Number[] numbers = integers; numbers[0] = 1L; java.lang.ArrayStoreException: Long cannot be stored in an array of type Integer[]

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Item 31: Use bounded wildcards to increase API flexibility

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class Stack { public void pushAll(Iterable src) { for (E e : src) push(e); }A }B

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class Stack { public void pushAll(Iterable src) { for (E e : src) push(e); }A }B

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?

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extends T>

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extends T> super T>

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PECS stands for producer- extends, consumer-super.

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class Stack { public void pushAll(Iterable src) { for (E e : src) push(e); } }

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class Stack { public void pushAll(Iterable extends E> src) { for (E e : src) push(e); } }

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class Stack { public void pushAll(Iterable extends E> src) { for (E e : src) push(e); }A }B Stack stack = new Stack<>(); List integers = new ArrayList<>(); stack.pushAll(integers);

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class Stack { public void pushAll(Iterable extends E> src) { for (E e : src) push(e); } public void popAll(Collection super E> dst) { while (!isEmpty()) dst.add(pop()); } }

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Use-site variance

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class Stack { public void pushAll(Iterable extends E> src) { for (E e : src) push(e); } public void popAll(Collection super E> dst) { while (!isEmpty()) dst.add(pop()); } }

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class Stack { public void pushAll(Iterable extends E> src) { for (E e : src) push(e); } public void popAll(Collection super E> dst) { while (!isEmpty()) dst.add(pop()); } } class Stack { fun pushAll(src: Iterable) { for (e in src) push(e) } fun popAll(dst: MutableList) { while (!isEmpty()) dst.add(pop()) } }

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Consumer in, Producer out!

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Declaration-site variance

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interface Source { fun next(): T } interface Consumer { fun consume(t: T) }

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interface Source { fun next(): T } interface Consumer { fun consume(t: T) }

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Generics

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Kotlin solves many of the Java language issues

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Kotlin brings fluency

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itemArray .asSequence() .filter { specialItem.contains(it.id) } .map { it.price } .average()

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Less Boilerplate More Features

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IntelliJ

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IntelliJ Lambda expressions

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IntelliJ Lambda expressions Smart casts

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IntelliJ Lambda expressions Smart casts String templates

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IntelliJ Lambda expressions Smart casts String templates Type inference

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IntelliJ Lambda expressions Smart casts String templates Type inference Typealias

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IntelliJ Lambda expressions Smart casts String templates Type inference Typealias Range expressions

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IntelliJ Lambda expressions Smart casts String templates Type inference Typealias Range expressions Operator overloading

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IntelliJ Lambda expressions Smart casts String templates Type inference Typealias Range expressions Operator overloading Companion objects

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IntelliJ Lambda expressions Smart casts String templates Type inference Typealias Range expressions Operator overloading Companion objects Coroutines

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IntelliJ Lambda expressions Smart casts String templates Type inference Typealias Range expressions Operator overloading Companion objects Coroutines Contracts

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IntelliJ Lambda expressions Smart casts String templates Type inference Typealias Range expressions Operator overloading Companion objects Coroutines Contracts Inline classes

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Why are we still using Java?

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Effective Java To Effective Kotlin Israel Ferrer Camacho @rallat