I have 2 data frames
df1
+--------------------+---+--------------------+--------------------+
|                 ID |B  |C                   |            D       |
+--------------------+---+--------------------+--------------------+
|                   1|1.0|                 1.0|                 1.0|
|                   2|2.0|                 2.0|                 2.0|
|                   3|3.0|                 3.0|                 3.0|
|                   4|4.0|                 4.0|                 4.0|
+--------------------+---+--------------------+--------------------+

df2
+--------------------+---+--------------------+--------------------+
|                 ID |B  |C                   |            D       |
+--------------------+---+--------------------+--------------------+
|                   1|100|                 1.0|                 100|
+--------------------+---+--------------------+--------------------+

If ID in df2 matches an ID in df1, I want to replace the row in df1 with the updated values in df2. So the new df1 looks like:
df1
+--------------------+---+--------------------+--------------------+
|                 ID |B  |C                   |            D       |
+--------------------+---+--------------------+--------------------+
|                   1|100|                 1.0|                 100|
|                   2|2.0|                 2.0|                 2.0|
|                   3|3.0|                 3.0|                 3.0|
|                   4|4.0|                 4.0|                 4.0|
+--------------------+---+--------------------+--------------------+

I've been trying to figure this out with union and join and just not having any luck yet.  I first created a new dataframe based on filtering for the ID of df1 and that works and I called that dataframe matchedDF that looks like:
matchedDF (dataframe based on finding a match of ID 1 in df1)
+--------------------+---+--------------------+--------------------+
|                 ID |B  |C                   |            D       |
+--------------------+---+--------------------+--------------------+
|                   1|1.0|                 1.0|                 1.0|
+--------------------+---+--------------------+--------------------+

But I don't know if I just want to delete the original ID 1 in df1 and add the new matchedDF or do I somehow want to update the original ID 1 with the matchedDf?  Or am I approaching this all wrong?
Thanks

How to replace a spark dataframe row with another spark dataframe's row using java
To stay computationally efficient, it's always a good idea to avoid joins/shuffles where possible.
This looks like a case where it is possible to avoid joining, have a look at the following code (it is in Scala, but the principles remain the same):
// Constructing the 2 dfs
val df = Seq(
  (1, 1.0, 1.0, 1.0),
  (2, 2.0, 2.0, 2.0),
  (3, 3.0, 3.0, 3.0),
  (4, 4.0, 4.0, 4.0)
).toDF("ID", "B", "C", "D")

val df2 = Seq(
  (1, 100, 1.0, 100),
  (2, 100, 1.0, 100)
).toDF("ID", "B", "C", "D")

// Collecting the IDs to be updated into a single Array
// IMPORTANT: we make the assumption that this array is not large (in your
// example there is only 1 row here, so the array only has 1 element which is
// totally fine)
val newIds = df2.select("ID").collect.map(_.getInt(0))

// Removing the original rows with the unwanted IDs and unioning the result with
// the new rows
val output = df
  .filter(not(col("ID").isin(newIds: _*)))
  .union(df2)

scala> output.show
+---+-----+---+-----+                                                           
| ID|    B|  C|    D|
+---+-----+---+-----+
|  3|  3.0|3.0|  3.0|
|  4|  4.0|4.0|  4.0|
|  1|100.0|1.0|100.0|
|  2|100.0|1.0|100.0|
+---+-----+---+-----+

So basically, if we can make the assumption that df2 (with the new values) is small like in your example, you can do something like the following:

collect the ID values into a single (undistributed) Array. From your example it seems like this is OK. If the amount of new rows is really large this might not be the best approach
filter the original df using the isin method of a column and negating using not (basically removing the rows with the new IDs)
union the filtered df and df2, resulting in the rows being updated WITHOUT any expensive operation like a shuffle

