// USING COMPANION OBJECT FOR DEPENDENCY INJECTION

interface SomeComponent {

    fun foo(): String

}



class SomeComponentImpl : SomeComponent {

    override fun foo() = "foo"

}



abstract class SomeModule {

    val _someComponentSingleton = SomeComponentImpl()

    val _someComponentNonSingleton = SomeComponentImpl::class.java

}



// "injected" unless overridden

class SomeService(val someComponentSingleton: SomeComponent = _someComponentSingleton,

                  val someComponentNonSingleton: SomeComponent = _someComponentNonSingleton.newInstance()) {

    companion object : SomeModule()

}





class SomeServiceTest {

    @Test

    fun singleton() {

        val bean1 = SomeService()

        val bean2 = SomeService()

        assertSame(bean1.someComponentSingleton, bean2.someComponentSingleton)

        assertNotSame(bean1.someComponentNonSingleton, bean2.someComponentNonSingleton)

    }

}