Image Credit: Mockito

Install in Gradle

dependencies {
    // Use JUnit Jupiter API for testing.
    testImplementation ..., 'org.mockito:mockito-inline:3.8.0',
    'org.mockito:mockito-junit-jupiter:3.8.0'

    ...
}

Add Mockito to Test Class

import static org.junit.jupiter.api.Assertions.assertTrue;
import static org.mockito.Mockito.when;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
public class UnitTestClass {
    // tests here
}

Use Strict Stubs

import static org.junit.jupiter.api.Assertions.assertTrue;
import static org.mockito.Mockito.when;

import org.junit.jupiter.api.Test;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoSettings;
import org.mockito.quality.Strictness;

@MockitoSettings(strictness = Strictness.STRICT_STUBS)
public class UnitTestClass {
    // tests here
}

Creating Fake Objects

@MockitoSettings(strictness = Strictness.STRICT_STUBS)
public class UnitTestClass {

    @Mock
    Person mockPerson;
    @Mock
    Teacher mockTeacher;

    public void testClassroomHasTeacher() {
        Classroom classroom = new Classroom()
        assertTrue(classroom.hasTeacher() == false);

        classroom.addTeacher(mockTeacher);
        assertTrue(classroom.hasTeacher() == true);
    }
}

Adding a Stub

@MockitoSettings(strictness = Strictness.STRICT_STUBS)
public class ClassroomTest {

    @Mock
    Person mockPerson;
    @Mock
    Teacher mockTeacher;

    @Test
    public void testClassroomGetTeacherName() {
        // create a method stub for `getName`
        when(mockTeacher.getName()).thenReturn("Teacher Person");

        Classroom classroom = new Classroom();
        classroom.addTeacher(mockTeacher);

        // assert that the classroom returns the teacher's name
        assertTrue(classroom.getTeacherName().equals("Teacher Person"));
    }
}

Static Classes

@Test
public void testTeacherFailsMinimumAgeRequirement() {
    // Create mock static class
    try (MockedStatic mockTeacherRules = Mockito.mockStatic(TeacherRules.class)) {

        // Create method stub for static class
        mockTeacherRules.when(() -> TeacherRules.getMinAge()).thenReturn(16);

        // Create method stub for fake Teacher
        when(mockTeacher.getAge()).thenReturn(15);

        // Test functionality
        Classroom classroom = new Classroom();
        assertThrows(IllegalArgumentException.class, () -> classroom.addTeacher(mockTeacher));
    }
}