MainActivityTest.kt

package com.theapache64.mockitosample

import androidx.test.core.app.ActivityScenario
import androidx.test.ext.junit.runners.AndroidJUnit4
import org.junit.Assert.assertEquals
import org.junit.Before
import org.junit.Test
import org.junit.runner.RunWith
import org.mockito.Mock
import org.mockito.MockitoAnnotations

class Calculator {
    fun add(x: Int, y: Int) = x + y
}

@RunWith(AndroidJUnit4::class)
class MainActivityTest {

    @Mock
    lateinit var calculator: Calculator

    @Before
    fun before() {
        MockitoAnnotations.initMocks(this)
    }

    @Test
    fun test() {
        assertEquals(calculator.add(10, 10), 20)
    }
}