This question with regard to JDK 5 says, there is no implementation provided with JDK 5, but JDK 6 is supposed to have a sun.misc.Base64Decoder.
As far
Just like Joachim Sauer said in a previous comment JDK1.6 already comes bundled with it's own Base64 implementation (sun.misc.*), here's an example:
String toEncode = "Encoding and Decoding in Base64 Test";
//Encoding in b64
String encoded = new BASE64Encoder().encode(toEncode.getBytes());
System.out.println(encoded);
//Decoding in b64
byte[] decodeResult = new BASE64Decoder().decodeBuffer(encoded);
System.out.println(new String(decodeResult));