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kotlin-inline-classes.md

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Inline classes in Kotlin

Kotlin language has concept of inline class. It's safe to say that it's enhanced alias as inline class can have only one member:

inline class PostalCode(val value: String)

This class will be inlined to plain String in runtime.

inline class vs. alias

inline class is a safer choice in terms of methods' signstures. Take a look at this snippet:

inline class PostalCode(val value: String)

typealias PostalAlias = String

fun acceptPostalCode(postalCode: PostalCode) {}
fun acceptPostalAlias(postalAlias: PostalAlias) {}

fun main() {

    val foobarCode: String = "123456"

    acceptPostalAlias(foobarCode) // OK. As PostalAlias is String, compiler will accept foobarCode even if it's distinctly String
    acceptPostalCode(foobarCode) // Not OK. The method accepts only members of class PostalCode, foobarCode is not one
}

Recent updates on inline class

Kotlin 1.4.30 introduced some improvements. The one that I like is that init blocks are available now:

inline class PostalCode(val value: String) {
    init {
        require(value.length == 6)
    }
}

That can be nice and usefull addition.

Java compatibility

It's not possible to call methods that accepts inline classes in signature in Java.

It's not possible to create inline class object in Java

The reason is that Kotlin will handle it in generated code as class with private constructor with public static methods, which will invoke the contructor. The problem is that those classes will be named with - character which is not supported in Java. Take a look at sample generated code:

import kotlin.Metadata;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;

@Metadata(
   mv = {1, 4, 0},
   bv = {1, 0, 3},
   k = 1,
   d1 = {"\u0000\"\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0000\n\u0002\u0010\u000e\n\u0002\b\u0005\n\u0002\u0010\u000b\n\u0002\b\u0002\n\u0002\u0010\b\n\u0002\b\u0002\b\u0086@\u0018\u00002\u00020\u0001B\u0012\u0012\u0006\u0010\u0002\u001a\u00020\u0003ø\u0001\u0000¢\u0006\u0004\b\u0004\u0010\u0005J\u0013\u0010\b\u001a\u00020\t2\b\u0010\n\u001a\u0004\u0018\u00010\u0001HÖ\u0003J\t\u0010\u000b\u001a\u00020\fHÖ\u0001J\t\u0010\r\u001a\u00020\u0003HÖ\u0001R\u0011\u0010\u0002\u001a\u00020\u0003¢\u0006\b\n\u0000\u001a\u0004\b\u0006\u0010\u0007ø\u0001\u0000\u0082\u0002\u0004\n\u0002\b\u0019¨\u0006\u000e"},
   d2 = {"LPostalCode;", "", "value", "", "constructor-impl", "(Ljava/lang/String;)Ljava/lang/String;", "getValue", "()Ljava/lang/String;", "equals", "", "other", "hashCode", "", "toString", "InlineClassesPlayground"}
)
public final class PostalCode {
   @NotNull
   private final String value;

   @NotNull
   public final String getValue() {
      return this.value;
   }

   // $FF: synthetic method
   private PostalCode(@NotNull String value) {
      Intrinsics.checkNotNullParameter(value, "value");
      super();
      this.value = value;
   }

   @NotNull
   public static String constructor_impl/* $FF was: constructor-impl*/(@NotNull String value) {
      Intrinsics.checkNotNullParameter(value, "value");
      return value;
   }

   // $FF: synthetic method
   @NotNull
   public static final PostalCode box_impl/* $FF was: box-impl*/(@NotNull String v) {
      Intrinsics.checkNotNullParameter(v, "v");
      return new PostalCode(v);
   }

   @NotNull
   public static String toString_impl/* $FF was: toString-impl*/(String var0) {
      return "PostalCode(value=" + var0 + ")";
   }

   public static int hashCode_impl/* $FF was: hashCode-impl*/(String var0) {
      return var0 != null ? var0.hashCode() : 0;
   }

   public static boolean equals_impl/* $FF was: equals-impl*/(String var0, @Nullable Object var1) {
      if (var1 instanceof PostalCode) {
         String var2 = ((PostalCode)var1).unbox-impl();
         if (Intrinsics.areEqual(var0, var2)) {
            return true;
         }
      }

      return false;
   }

   public static final boolean equals_impl0/* $FF was: equals-impl0*/(@NotNull String p1, @NotNull String p2) {
      return Intrinsics.areEqual(p1, p2);
   }

   // $FF: synthetic method
   @NotNull
   public final String unbox_impl/* $FF was: unbox-impl*/() {
      return this.value;
   }

   public String toString() {
      return toString-impl(this.value);
   }

   public int hashCode() {
      return hashCode-impl(this.value);
   }

   public boolean equals(Object var1) {
      return equals-impl(this.value, var1);
   }
}

so none of those static methods can be invoked from Java class.

It's not possible to invoke method wich accepts inline class in signature from Java

Assume having Kotlin file:

inline class PostalCode(val value: String)

fun formatPostalCode(postalCode: PostalCode): String {
    return "#${postalCode.value}"
}

Because of managling, this method will look like this in Java generated code:

public final class WrapperKt {
   @NotNull
   public static final String formatPostalCode_Egzjoak/* $FF was: formatPostalCode-Egzjoak*/(@NotNull String postalCode) {
      Intrinsics.checkNotNullParameter(postalCode, "postalCode");
      return '#' + postalCode;
   }
}

as you can see here again the original name of the method contained - symbol which is prohibited in Java methods signatures. This means that this method is not available in Java. More about mangling: https://kotlinlang.org/docs/reference/inline-classes.html#mangling

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