Scala: Mocking an enclosed object - scala

Question

I'm trying to write a unit test for object A, which uses some methods from another object B. How do I override B's methods?
A.scala
import somePackage.B
object A { // This is to be tested
def fun1(str: String) = {
val str2 = processStr(str) //Say str2 = "sahil"
B.doSomething(str2) // This is to be overridden
}
B.scala
object B {
private def doSomething(str: String) = {
// do something
"abc"
}
}
ATest.scala
class ATest extends FlatSpec {
it "should return sahil instead of abc" in {
// Do something to override B.doSomething
A.fun1("someString")
}
}

Answer 1

This is a design failure at architectural level and a typical XY problem. You should not use singleton without a (really) compelling reason.
Instead for both testability and modularity, consider injecting your dependency explicitly (right now you are declaring the dependency between A and B implicitly through singleton).
class A(b: B) {
def fun1(str: String) = b.doSomething(str)
}
class B {
def doSomething(str: String) = "abc"
}
class ATest extends FlatSpec {
it "" in {
val b = new B { override def doSomething(str: String) = "testing..." }
val a = new A(b)
a.fun1("someString") must beEqual "testing..."
}
}

Answer 2

There is no easy way to do this. Without using DI the only thing that comes to my mind is to overload fun1 with package-private modifier:
package somePackage
import somePackage.B
object A {
def fun1(str: String) = fun1(B.doSomething)
private[somePackage] def fun1(str: String, f: (String) => String) = {
val str2 = processStr(str)
f(str2)
}
}
Then you can pass any mock you would like into fun1