java8集合类聚合GroupingBy的妙用

    public void arrayToMapTest() {
    String tableStr = "db01.dbo.table_00,db02.dbo.table_00,db02.dbo.table_01,db02.dbo2.table_01,db02.dbo2.table_02";
    String[] sqlServerArr = tableStr.split(",");
    System.out.println(Arrays.toString(sqlServerArr));
    Map<String, List<String>> sqlServerMap =
        Arrays.stream(sqlServerArr)
              .collect(Collectors.groupingBy(dbTable ->
                                                 StringUtils.substring(dbTable, 0, dbTable.indexOf("."))));
    Map<String, List<String>> sqlServerMap2 =
        Arrays.stream(sqlServerArr)
              .collect(Collectors.groupingBy(dbTable -> StringUtils.substring(dbTable, 0, dbTable.lastIndexOf(".")),
                                             Collectors.mapping(k -> StringUtils.substring(k, k.lastIndexOf(".") + 1, k.length()),
                                                                Collectors.toList())));
    System.out.println(sqlServerMap.toString());
    System.out.println(sqlServerMap2.toString());
  }

输出如下：
[db01.dbo.table_00, db02.dbo.table_00, db02.dbo.table_01, db02.dbo2.table_01, db02.dbo2.table_02]
{db01=[db01.dbo.table_00], db02=[db02.dbo.table_00, db02.dbo.table_01, db02.dbo2.table_01, db02.dbo2.table_02]}
{db02.dbo=[table_00, table_01], db01.dbo=[table_00], db02.dbo2=[table_01, table_02]}