val df = sc.parallelize(Seq(
     |   (0,"cat26","cat26"),
     |   (1,"cat67","cat26"),
     |   (2,"cat56","cat26"),
     |   (3,"cat8","cat26"))).toDF("Hour", "Category", "Value")

scala> df.na.replace("*", Map[Any, Any](
     |      "cat26" -> "cat23"
     |    )).show()
+----+--------+-----+
|Hour|Category|Value|
+----+--------+-----+
|   0|   cat23|cat23|
|   1|   cat67|cat23|
|   2|   cat56|cat23|
|   3|    cat8|cat23|
+----+--------+-----+

spark官方源码示例:org/apache/spark/sql/DataFrameNaFunctionsSuite.scala
name是列名

df.na.replace("name", Map(
        "Bob" -> "Bravo",
        "Alice" -> null
df.na.replace("*", Map[Any, Any](
     false -> null
替换hour列中的0为9
import com.google.common.collect.ImmutableMap;
scala> df.na.replace("hour", ImmutableMap.of(0, 9)).show()
+----+--------+-----+
|Hour|Category|Value|
+----+--------+-----+
|   9|   cat26|cat26|
|   1|   cat67|cat26|
|   2|   cat56|cat26|
|   3|    cat8|cat26|
+----+--------+-----+
替换所有列中"cat26"为"cat222"
scala> df.na.replace("*", ImmutableMap.of("cat26", "cat222")).show()
+----+--------+------+
|Hour|Category| Value|
+----+--------+------+
|   0|  cat222|cat222|
|   1|   cat67|cat222|
|   2|   cat56|cat222|
|   3|    cat8|cat222|
+----+--------+------+
spark官方源码示例:
org/apache/spark/sql/DataFrameNaFunctions.scala
* {{{
*   import com.google.common.collect.ImmutableMap;
*   // Replaces all occurrences of 1.0 with 2.0 in column "height".
*   df.na.replace("height", ImmutableMap.of(1.0, 2.0));
*   // Replaces all occurrences of "UNKNOWN" with "unnamed" in column "name".
*   df.na.replace("name", ImmutableMap.of("UNKNOWN", "unnamed"));
*   // Replaces all occurrences of "UNKNOWN" with "unnamed" in all string columns.
*   df.na.replace("*", ImmutableMap.of("UNKNOWN", "unnamed"));
* }}}