Question

I am having issues with the following code in Scala

import org.junit.Test
import org.junit.Assert._

class BoxingTest {

    val holder: Holder[Integer] = new Holder[Integer]();

    @Test
    def Holder_Eq_Primitive(){        
        assertEquals(holder, holder eq 1);
    }

    @Test
    def Holder_Eq_Boxed(){        
        assertEquals(holder, holder eq 1.asInstanceOf[Integer]);
    }

}

class Holder[T] {

    def eq(other: T): Holder[_] = this;
}

I get the following error in compilation

/BoxingTest.scala:12: error: type mismatch;
[INFO]  found   : Int
[INFO]  required: AnyRef
[INFO] Note: primitive types are not implicitly converted to AnyRef.
[INFO] You can safely force boxing by casting x.asInstanceOf[AnyRef].
[INFO]         assertEquals(holder, holder eq 1);
[INFO]                                     ^
[ERROR] one error found
[INFO] -------------------------

Why doesn’t the implicit conversion from Int to Integer handle the issue?

I could easily fix the code by not using eq, but this just doesn’t seem right. IMHO the available implicit conversions should be applied here.

UPDATE

I got the issue fixed by using the signature like this

import org.junit.Test
import org.junit.Assert._

class BoxingTest {

    @Test
    def Holder_Eq_Primitive(){
        val holder: Holder[Int] = new Holder[Int]();
        assertEquals(holder, holder eq 1);
    }

    @Test
    def Holder_Eq_Boxed(){
        val holder: Holder[Integer] = new Holder[Integer]();
        assertEquals(holder, holder eq 1.asInstanceOf[Integer]);
    }

}

class Holder[T] {

    def eq(other: T): Holder[_] = ...;
}

Still, it would be good to use the wrapper types instead.

Answer 1

I tried comparing an Int value against integer literal and got some interesting note from the compiler. This may shed some light about the causes of this behaviour.

scala> val a = 1
scala> a eq 1
<console>:6: error: type mismatch;
 found   : Int
 required: ?{val eq: ?}
Note that implicit conversions are not applicable because they are ambiguous:
 both method int2Integer in object Predef of type (Int)java.lang.Integer
 and method intWrapper in object Predef of type (Int)scala.runtime.RichInt
 are possible conversion functions from Int to ?{val eq: ?}
       a eq 1
       ^