Map computeIfAbsent方法的单元测试

Question

如何为Map computeIfAbsent方法编写Junit。我有以下代码-

CompletableFuture<Employee>[] employeeDetailFutures =
        empIds.stream().map(empId ->
        employeeCache.computeIfAbsent(empId, k -> employeeService.employeeDetails(Integer.valueOf(empId))))
        .filter(Objects::nonNull)
        .toArray(CompletableFuture[]::new);

这里employeeCache是Map，到目前为止，我想为employeeCache.computeIfAbsent(...)写一个Junit，但每次尝试新地图都在创建并获取空值

Map<String, CompletableFuture<Employee>> mockMap;
@Before
public void setUp() {
    this.mockMap = Mockito.mock(Map.class);
} 

when(mockMap.computeIfAbsent(any(), any()).thenReturn(CompletableFuture.completedFuture(Employee Object);

每次我得到computeIfAbsent的空值。我在这里想念的。