How to mock a system class for type casting

I have the following code to get the current system memory:

val memClass = (context.getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager).memoryClass

My initial goal is to return an Int value for it in testing. Something like this:

whenever((context.getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager).memoryClass)
.thenReturn(500)

Since its a jUnit test and involves using the Context, I ended up mocking everything:

val context: Context = mock()

val activityService: Service = mock()

whenever(context.getSystemService(Context.ACTIVITY_SERVICE))
            .thenReturn(activityService)

whenever((context.getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager).memoryClass)
            .thenReturn(500)

Now the problem is Mockito can't create a typecast for ActivityManager and throwing this error:

java.lang.ClassCastException: android.app.Service$MockitoMock$594525704 cannot be cast to android.app.ActivityManager

I also tried mocking the ActivityManager but it can't be used as a typecast:

I don't have a specific requirement to stick with the current solution. I will appreciate a better approach to my initial goal.

Maybe we have a misunderstanding. What I am talking about was this:

val context: Context = mock()

val activityService: ActivityManager = mock()

whenever(context.getSystemService(Context.ACTIVITY_SERVICE))
            .thenReturn(activityService)

whenever((context.getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager).memoryClass)
            .thenReturn(500)

As far as I understand: context.getSystemService( ... ) returns an Object.

There is no relation between the classes android.app.ActivityManager and android.app.Service.

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Edit:

The last line might need to be replaced with the equivalent of
(the given code is probably not be the correct kotlin syntax)

    when(activityService.memoryClass).thenReturn(500);