java - 如何在java spark中形成地图列表

Question

root
 |-- code: string (nullable = true)
 |-- mnemonic: string (nullable = true)
 |-- key: long (nullable = true)
 |-- country: string (nullable = true)
 |-- crossID: struct (nullable = false)
 |    |-- codeKey: long (nullable = true)
 |    |-- codeGroupCode: string (nullable = true)
 |    |-- description: string (nullable = true)
 |    |-- externalCode: string (nullable = true)
 |-- ITEM: struct (nullable = false)
 |    |-- type: string (nullable = true)
 |    |-- indicator: string (nullable = true)
 |    |-- imageIndicator: string (nullable = true)
 |    |-- col20: struct (nullable = false)
 |    |    |-- cCode: string (nullable = true)
 |    |    |-- vCode: string (nullable = true)

以上是我的数据框的架构。在这里，我想将 crossID 及其元素分组为地图列表。请帮助我如何在java spark中做到这一点。