Appending a byte into a byte array in Java Card - byte

Question

I have a method below to insert a single byte into a byte buffer and during building and cleaning for a Java Card CAP file it throws an error.
Code:
private void appendOutputBuffer(byte msg) {
ArrayLogic.arrayCopyRepack(msg, (short) 0, (short) 0, outputBuffer, (short) outputBuffer.length);
}
Error:
error: line 163: sctest: class java.lang.Byte not found in export file lang.exp.
error: line 163: sctest: method valueOf(byte) of class java.lang.Byte not found in export file lang.exp or the method signature has changed.
error: line 163: sctest: class java.lang.Byte not found in export file lang.exp.
error: line 163: sctest: class java.lang.Byte in return type of method java.lang.Byte.valueOf(byte) not found.
How do I resolve it ?

Answer 1

This is not how arrayCopyRepack works. Read the documentation: http://www.win.tue.nl/pinpasjc/docs/apis/jc222/javacardx/framework/util/ArrayLogic.html#arrayCopyRepack%28java.lang.Object,%20short,%20short,%20java.lang.Object,%20short%29
Its signature is:
public static final short arrayCopyRepack(Object src,
short srcOff,
short srcLen,
Object dest,
short destOff)
but the src argument is meant to be an array - it is an Object just because there is no common ancestor class for all primitive arrays in Java Card. Not everything in Java Card is an Object: byte is a primitive. That causes your troubles.
The first step of Java Card build is a common Java compiler creating a standard .class file. This compiler does not know anything about Java Card and it sees a byte used as an Object, so it uses autoboxing, casting your byte to java.lang.Byte and adding a dependence to java.lang.Byte in your .class file. So far so good. This is just plain Java, so it works.
However, in Java Card libraries there is no java.lang.Byte in java.lang package. That causes the error when creating a .cap file.
Appending a byte to an existing array (and thus creating a new array) is a very bad idea, btw. You should create your buffer long enough and store the effective length (how long is the used part of the buffer):
private static final short BUF_LEN = (short) 256;
byte[] outputBuffer = new byte[BUF_LEN];
...
private void appendOutputBuffer(byte msg) {
if (effectiveLen == BUF_LEN)
ISOException.throwIt(ISO7816.SW_UNKNOWN);
outputBuffer[effectiveLen] = msg;
++effectiveLen;
}
and think about RAM vs EEPROM storing of both outputBuffer and effectiveLen.