### Question:
Counter extends Applet { private void debit(APDU apdu, byte[] buffer) { short debit; if (apdu.setIncomingAndReceive() != 2) ISOException.throwIt(ISO7816.SW_WRONG_LENGTH); debit = Util.getShort(buffer, ISO7816.OFFSET_CDATA); if ((debit > balance) || (debit <= 0) || (debit > MAX_DEBIT)) ISOException.throwIt(ISO7816.SW_WRONG_DATA); balance -= debit; getBalance(apdu, buffer); } private  Counter(); static void install(byte[] buffer, short offset, byte length); void process(APDU apdu); static final byte INS_GET_BALANCE; static final byte INS_CREDIT; static final byte INS_DEBIT; static final short MAX_BALANCE; static final short MAX_CREDIT; static final short MAX_DEBIT; }### Answer:
@Test public void debitTest() throws CardException { sendCredit(new byte[]{0x00, 0x05}, 0x9000, new byte[]{0x00, 0x05}); assertArrayEquals(new byte[]{0x00, 0x05}, getBalance().getData()); sendDebit(new byte[]{0x00, 0x05}, 0x9000, new byte[]{0x00, 0x00}); assertArrayEquals(new byte[]{0x00, 0x00}, getBalance().getData()); }