嘲笑一个集合的方法

Question

我有调用MongoDB集合以获取列表的方法。我想嘲笑列表的结果，但我在其中得到了NPE。

public String convertId(String Id, int counter) {
    String id2 = "test";
    List<Object> objectList = new ArrayList<>();
    collection.find(eq(id,id2)).into(objectList);
    Object obj = objectList.get(0);
    return obj.getId();
}

我在Collection.find方法上获得了零指针。撰写测试案例是：

@Test
public void convertIdTest() {
    List<Object> objectList = new ArrayList<>();
    Document doc1 = new Document();
    Mockito.when(collection.find(Mockito.any(BasicDBObject.class))).thenReturn(iterable);
    Mockito.when(iterable.iterator()).thenReturn(cursor);
    Mockito.when(cursor.hasNext()).thenReturn(true).thenReturn(false);
    Mockito.when(cursor.next).thenReturn(doc1);
    assertNotNull(request.converId("1233",1);
}

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FindIterable<Object> iterable = mock(FindIterable.class)
MongoCursor cursor = mock(MongoCursor.class)