如何在Java中将Spark dataframe中的列值转换为小写/大写？
例如，下面是输入 Dataframe ：

name | country | src        | city       | debit
---------------------------------------------
"foo"| "NZ"    | salary     | "Auckland" | 15.0
"bar"| "Aus"   | investment | "Melbourne"| 12.5

我需要将“city”列转换为小写

name | country | src        | city       | debit
------------------------------------------------
"foo"| "NZ"    | salary     | "auckland" | 15.0
"bar"| "Aus"   | investment | "melbourne"| 12.5

我在Scala和Python中找到了解决方案，但在Java中没有，如下所示
How to change case of whole column to lowercase?
在java中有一个转换列名的解决方案，但不转换它的数据。
How to lower the case of column names of a data frame but not its values?
如何将列值转换为小写？