diff --git a/src/test/kotlin/org/springframework/samples/petclinic/model/ValidatorTests.kt b/src/test/kotlin/org/springframework/samples/petclinic/model/ValidatorTests.kt
index 3f648d9..9d1836a 100644
--- a/src/test/kotlin/org/springframework/samples/petclinic/model/ValidatorTests.kt
+++ b/src/test/kotlin/org/springframework/samples/petclinic/model/ValidatorTests.kt
@@ -32,7 +32,7 @@ class ValidatorTests {
         val validator = createValidator()
         val constraintViolations = validator.validate(person)
 
-        assertThat(constraintViolations.size).isEqualTo(1)
+        assertThat(constraintViolations).hasSize(1)
         val violation = constraintViolations.iterator().next()
         assertThat(violation.propertyPath.toString()).isEqualTo("firstName")
         assertThat(violation.message).isEqualTo("must not be empty")
diff --git a/src/test/kotlin/org/springframework/samples/petclinic/owner/OwnerControllerTest.kt b/src/test/kotlin/org/springframework/samples/petclinic/owner/OwnerControllerTest.kt
index 3ab9ce1..c4df90d 100644
--- a/src/test/kotlin/org/springframework/samples/petclinic/owner/OwnerControllerTest.kt
+++ b/src/test/kotlin/org/springframework/samples/petclinic/owner/OwnerControllerTest.kt
@@ -193,8 +193,8 @@ class OwnerControllerTest {
 
                     @Suppress("UNCHECKED_CAST")
                     override fun matches(item: Any?): Boolean {
-                        val pets : List<Pet> = item as List<Pet>
-                        val pet = pets.get(0)
+                        val pets : Set<Pet> = item as Set<Pet>
+                        val pet = pets.iterator().next()
                         return !pet.getVisits().isEmpty()
                     } })))
                 .andExpect(view().name("owners/ownerDetails"))
diff --git a/src/test/kotlin/org/springframework/samples/petclinic/owner/PetTypeFormatterTest.kt b/src/test/kotlin/org/springframework/samples/petclinic/owner/PetTypeFormatterTest.kt
index 9944548..b868b43 100644
--- a/src/test/kotlin/org/springframework/samples/petclinic/owner/PetTypeFormatterTest.kt
+++ b/src/test/kotlin/org/springframework/samples/petclinic/owner/PetTypeFormatterTest.kt
@@ -1,13 +1,13 @@
 package org.springframework.samples.petclinic.owner
 
 
-import org.junit.jupiter.api.Assertions.assertEquals
+import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Assertions.assertThrows
 import org.junit.jupiter.api.BeforeEach
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.extension.ExtendWith
 import org.mockito.Mock
-import org.mockito.Mockito
+import org.mockito.BDDMockito.given
 import org.springframework.test.context.junit.jupiter.SpringExtension
 import java.text.ParseException
 import java.util.*
@@ -35,21 +35,21 @@ class PetTypeFormatterTest {
         val petType = PetType()
         petType.name = "Hamster"
         val petTypeName = this.petTypeFormatter.print(petType, Locale.ENGLISH)
-        assertEquals("Hamster", petTypeName)
+        assertThat("Hamster").isEqualTo(petTypeName)
     }
 
     @Test
     @Throws(ParseException::class)
     fun shouldParse() {
-        Mockito.`when`(this.pets.findPetTypes()).thenReturn(makePetTypes())
+        given(this.pets.findPetTypes()).willReturn(makePetTypes())
         val petType = petTypeFormatter.parse("Bird", Locale.ENGLISH)
-        assertEquals("Bird", petType.name)
+        assertThat("Bird").isEqualTo(petType.name)
     }
 
     @Test
     @Throws(ParseException::class)
     fun shouldThrowParseException() {
-        Mockito.`when`(this.pets.findPetTypes()).thenReturn(makePetTypes())
+        given(this.pets.findPetTypes()).willReturn(makePetTypes())
         assertThrows(ParseException::class.java, { petTypeFormatter.parse("Fish", Locale.ENGLISH) })
     }