Java Hashtable操作演示:实现跨平台特性与面向对象设计

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在Java实用教程中,章节标题“操作Hashtable对象的输出结果”可能涉及到对Java集合框架中的Hashtable类的使用和实例演示。Hashtable是一种古老的散列映射(Hash Map)实现,在Java中主要用于存储键值对,它提供了无序且快速查找的功能,因为它是基于哈希表实现的。 在讲解这部分内容时,可能会涉及以下知识点: 1. **Hashtable简介**:Hashtable是Java早期的线程安全的键值对存储容器,它在Java 1.4之后被HashMap所取代,但出于教学目的或历史研究,仍可能作为示例出现。Hashtable的主要特点是键(Key)和值(Value)的映射关系,以及同步机制,确保在多线程环境下的数据一致性。 2. **创建和使用Hashtable**:作者可能展示如何实例化一个Hashtable对象,设置键值对,并演示如何通过键来检索和修改对应的值。这通常包括如何添加(put())、获取(get())、删除(remove())元素,以及遍历Hashtable的遍历器(Enumeration或Iterator)。 3. **输出结果的分析**:输出的结果可能是Hashtable中存储的数据,或者在操作过程中产生的信息,例如查看键值对的数量、验证并发访问的正确性,以及演示不同方法的性能差异。 4. **线程安全与性能**:由于Hashtable是同步的,可能会讨论同步机制对性能的影响,以及为何推荐在现代Java中使用HashMap(非同步,但在大部分场景下性能更好)。 5. **Java集合框架的发展**:这部分内容可能提及Java集合框架的变迁,从Hashtable到HashMap、HashSet、TreeMap等其他高级容器的选择,以及它们各自的特点和适用场景。 6. **练习与示例代码**:教程中会提供实际的代码示例,学生可以通过实践操作来加深对Hashtable的理解,通过编译和运行这些代码,理解输出结果背后的逻辑。 总结来说,这部分内容将深入剖析Hashtable的内部工作机制,并通过具体的实例操作演示其在Java编程中的应用场景,帮助读者掌握如何有效地管理和操作键值对数据结构。

Traceback (most recent call last): File "D:\anaconda\envs\mytest\lib\site-packages\pandas\core\indexes\base.py", line 3081, in get_loc return self._engine.get_loc(casted_key) File "pandas\_libs\index.pyx", line 70, in pandas._libs.index.IndexEngine.get_loc File "pandas\_libs\index.pyx", line 101, in pandas._libs.index.IndexEngine.get_loc File "pandas\_libs\hashtable_class_helper.pxi", line 1625, in pandas._libs.hashtable.Int64HashTable.get_item File "pandas\_libs\hashtable_class_helper.pxi", line 1632, in pandas._libs.hashtable.Int64HashTable.get_item KeyError: 0 The above exception was the direct cause of the following exception: Traceback (most recent call last): File "D:\PyCharm\learning\1\BPmain.py", line 28, in <module> if test_output.loc[i, 0] == y_pred[i, 0]: File "D:\anaconda\envs\mytest\lib\site-packages\pandas\core\indexing.py", line 889, in __getitem__ return self._getitem_tuple(key) File "D:\anaconda\envs\mytest\lib\site-packages\pandas\core\indexing.py", line 1060, in _getitem_tuple return self._getitem_lowerdim(tup) File "D:\anaconda\envs\mytest\lib\site-packages\pandas\core\indexing.py", line 831, in _getitem_lowerdim return getattr(section, self.name)[new_key] File "D:\anaconda\envs\mytest\lib\site-packages\pandas\core\indexing.py", line 895, in __getitem__ return self._getitem_axis(maybe_callable, axis=axis) File "D:\anaconda\envs\mytest\lib\site-packages\pandas\core\indexing.py", line 1124, in _getitem_axis return self._get_label(key, axis=axis) File "D:\anaconda\envs\mytest\lib\site-packages\pandas\core\indexing.py", line 1073, in _get_label return self.obj.xs(label, axis=axis) File "D:\anaconda\envs\mytest\lib\site-packages\pandas\core\generic.py", line 3739, in xs loc = index.get_loc(key) File "D:\anaconda\envs\mytest\lib\site-packages\pandas\core\indexes\base.py", line 3083, in get_loc raise KeyError(key) from err KeyError: 0

2023-07-16 上传

KeyError Traceback (most recent call last) D:\anaconda3\lib\site-packages\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance) 2894 try: -> 2895 return self._engine.get_loc(casted_key) 2896 except KeyError as err: pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_loc() pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_loc() pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item() pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item() KeyError: '累计参会时长' The above exception was the direct cause of the following exception: KeyError Traceback (most recent call last) <ipython-input-120-194e4b87e045> in <module> 12 13 # 将时间列中的所有时间字符串转换为分钟数 ---> 14 df3['累计参会时长'] = df3['累计参会时长'].apply(convert_to_minutes) 15 16 # 输出转换后的DataFrame D:\anaconda3\lib\site-packages\pandas\core\frame.py in __getitem__(self, key) 2900 if self.columns.nlevels > 1: 2901 return self._getitem_multilevel(key) -> 2902 indexer = self.columns.get_loc(key) 2903 if is_integer(indexer): 2904 indexer = [indexer] D:\anaconda3\lib\site-packages\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance) 2895 return self._engine.get_loc(casted_key) 2896 except KeyError as err: -> 2897 raise KeyError(key) from err 2898 2899 if tolerance is not None: KeyError: '累计参会时长' 以上代码有此报错 应该怎么改

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Traceback (most recent call last): File "D:\Anaconda\lib\site-packages\pandas\core\indexes\base.py", line 3802, in get_loc return self._engine.get_loc(casted_key) File "pandas\_libs\index.pyx", line 138, in pandas._libs.index.IndexEngine.get_loc File "pandas\_libs\index.pyx", line 165, in pandas._libs.index.IndexEngine.get_loc File "pandas\_libs\hashtable_class_helper.pxi", line 5745, in pandas._libs.hashtable.PyObjectHashTable.get_item File "pandas\_libs\hashtable_class_helper.pxi", line 5753, in pandas._libs.hashtable.PyObjectHashTable.get_item KeyError: 'A' The above exception was the direct cause of the following exception: Traceback (most recent call last): File "e:\Mydata\py\提取指定范围excel数据.py", line 7, in <module> data = df.loc[0:9, 'A':'B'] # 例如提取第1行到第10行,第1列到第3列的数据 File "D:\Anaconda\lib\site-packages\pandas\core\indexing.py", line 1067, in __getitem__ return self._getitem_tuple(key) File "D:\Anaconda\lib\site-packages\pandas\core\indexing.py", line 1256, in _getitem_tuple return self._getitem_tuple_same_dim(tup) File "D:\Anaconda\lib\site-packages\pandas\core\indexing.py", line 924, in _getitem_tuple_same_dim retval = getattr(retval, self.name)._getitem_axis(key, axis=i) File "D:\Anaconda\lib\site-packages\pandas\core\indexing.py", line 1290, in _getitem_axis return self._get_slice_axis(key, axis=axis) File "D:\Anaconda\lib\site-packages\pandas\core\indexing.py", line 1324, in _get_slice_axis indexer = labels.slice_indexer(slice_obj.start, slice_obj.stop, slice_obj.step) File "D:\Anaconda\lib\site-packages\pandas\core\indexes\base.py", line 6559, in slice_indexer start_slice, end_slice = self.slice_locs(start, end, step=step) File "D:\Anaconda\lib\site-packages\pandas\core\indexes\base.py", line 6767, in slice_locs start_slice = self.get_slice_bound(start, "left") File "D:\Anaconda\lib\site-packages\pandas\core\indexes\base.py", line 6686, in get_slice_bound raise err File "D:\Anaconda\lib\site-packages\pandas\core\indexes\base.py", line 6680, in get_slice_bound slc = self.get_loc(label) File "D:\Anaconda\lib\site-packages\pandas\core\indexes\base.py", line 3804, in get_loc raise KeyError(key) from err KeyError: 'A'

2023-07-16 上传

帮我解释一下错误:KeyError Traceback (most recent call last) File ~\anaconda3\lib\site-packages\pandas\core\indexes\base.py:3802, in Index.get_loc(self, key, method, tolerance) 3801 try: -> 3802 return self._engine.get_loc(casted_key) 3803 except KeyError as err: File ~\anaconda3\lib\site-packages\pandas\_libs\index.pyx:138, in pandas._libs.index.IndexEngine.get_loc() File ~\anaconda3\lib\site-packages\pandas\_libs\index.pyx:165, in pandas._libs.index.IndexEngine.get_loc() File pandas\_libs\hashtable_class_helper.pxi:5745, in pandas._libs.hashtable.PyObjectHashTable.get_item() File pandas\_libs\hashtable_class_helper.pxi:5753, in pandas._libs.hashtable.PyObjectHashTable.get_item() KeyError: 'is_acc' The above exception was the direct cause of the following exception: KeyError Traceback (most recent call last) Cell In[2], line 2 1 import statsmodels.api as sm ----> 2 y = data['is_acc'] 3 X = data[['ST_MP', 'Length', 'NLane', 'LaneWidth', 'LShoulderWidth', 'RShoulderWidth', 'AADT']] 4 X = sm.add_constant(X) File ~\anaconda3\lib\site-packages\pandas\core\frame.py:3807, in DataFrame.__getitem__(self, key) 3805 if self.columns.nlevels > 1: 3806 return self._getitem_multilevel(key) -> 3807 indexer = self.columns.get_loc(key) 3808 if is_integer(indexer): 3809 indexer = [indexer] File ~\anaconda3\lib\site-packages\pandas\core\indexes\base.py:3804, in Index.get_loc(self, key, method, tolerance) 3802 return self._engine.get_loc(casted_key) 3803 except KeyError as err: -> 3804 raise KeyError(key) from err 3805 except TypeError: 3806 # If we have a listlike key, _check_indexing_error will raise 3807 # InvalidIndexError. Otherwise we fall through and re-raise 3808 # the TypeError. 3809 self._check_indexing_error(key) KeyError: 'is_acc'In [ ]: ​

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