SomeTest.java

import static org.junit.Assert.assertEquals;
import static org.mockito.ArgumentMatchers.anyList;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

import org.junit.Before;
import org.junit.Test;

public class SomeTest {

    static class Employee {}

    static class EmployeeService {
        public List<Employee> getEntities(List<String> manager){
            //some processing, make db call and return entities
            return new ArrayList<>();
        }
    }

    private EmployeeService employeeService = mock(EmployeeService.class);

    @Before
    public void setup(){
        Employee employee = new Employee();
        List<Employee> employees = new ArrayList<>(Arrays.asList(employee));
        when(employeeService.getEntities(anyList())).thenReturn(employees);
    }

    @Test
    public void test() {
        assertEquals(1, employeeService.getEntities(Collections.emptyList()).size());
    }
}