Kotlin default constructor

Question

The docs say:

On the JVM, if all of the parameters of the primary constructor have default values, the compiler will generate an additional parameterless constructor which will use the default values. This makes it easier to use Kotlin with libraries such as Jackson or JPA that create class instances through parameterless constructors.

But this does not appear to be the case:

Welcome to Kotlin version 1.2.71 (JRE 10.0.2+13-Ubuntu-1ubuntu0.18.04.2)
Type :help for help, :quit for quit
>>> class A(val x: Int = 1, val y: Int = 2)
>>> for (c in A::class.java.constructors) println(c)
public Line_0$A(int,int,int,kotlin.jvm.internal.DefaultConstructorMarker)
public Line_0$A(int,int)
>>>

What am I missing?

Where are you running that code? Decompiling to java bytecode or printing the constructors in code does indeed show the parameterless constructor. — user
– user, Commented Oct 2, 2018 at 4:43
@Luksprog The regular kotlinc REPL, version 1.2.71 and JRE 10.0.2, as shown above. — Tobia
– Tobia, Commented Oct 2, 2018 at 5:07

Ricky Mo · Accepted Answer · 2018-10-02 05:34:58Z

6

I think the REPL runs the kotlin code as script, which does not compile completely.

When running test.kt :

class A(val x: Int = 1, val y: Int = 2)
fun main(args: Array<String>) {
    for (c in A::class.java.constructors) println(c)
}

with

kotlinc test.kt -include-runtime -d test.jar
kotlin test.jar

it correctly prints

public A(int,int,int,kotlin.jvm.internal.DefaultConstructorMarker)
public A()
public A(int,int)

When running test.kts:

class A(val x: Int = 1, val y: Int = 2)
for (c in A::class.java.constructors) println(c)

with

kotlinc -script test.kts

it prints

public Test$A(int,int,int,kotlin.jvm.internal.DefaultConstructorMarker)
public Test$A(int,int)

same as REPL.

So it should be safe to say that it does compile with the parameterless constructor.

answered Oct 2, 2018 at 5:34

Ricky Mo

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