How to decode a Base64 string in Scala or Java?

There is unfortunately not just one Base64 encoding. The - character doesn't have the same representation in all encodings. For example, in the MIME encoding, it's not used at all. In the encoding for URLs, it is a value of 62--and this is the one that Python is using. The default sun.misc decoder wants + for 62. If you change the - to +, you get the correct answer (i.e. the Python answer).

In Scala, you can convert the string s to MIME format like so:

s.map{ case '-' => '+'; case '_' => '/'; case c => c }

and then the Java MIME decoder will work.

In Scala, Encoding a String to Base64 and decoding back to the original String using Java APIs:

import java.util.Base64
import java.nio.charset.StandardCharsets

scala> val bytes = "foo".getBytes(StandardCharsets.UTF_8)
bytes: Array[Byte] = Array(102, 111, 111)

scala> val encoded = Base64.getEncoder().encodeToString(bytes)
encoded: String = Zm9v

scala> val decoded = Base64.getDecoder().decode(encoded)
decoded: Array[Byte] = Array(102, 111, 111)

scala> val str = new String(decoded, StandardCharsets.UTF_8)
str: String = foo

Both Python and Java are correct in terms of the decoding. They are just using a different RFC for this purpose. Python library is using RFC 3548 and the used java library is using RFC 4648 and RFC 2045.

Changing the hyphen(-) into a plus(+) from your input string will make the both decoded byte data are similar.