Output changes although input doesn't

Question

Hi guys, I have a query.
Firstly, I'm using JDK 1.3.1_02 and Forte for Java, Community Edition version 3 build 010925.
I have written a class with a method that reads a String of 2 characters. Being hexadecimals, I use a switch statement with the checking of each character. As with my normal practice, I would compile both source codes (my main calls this particular class), and run my execution.
The source codes below have worked before, even moments just before i commented out my method call in main. When I uncomment it next, it returns the wrong result. Checking each character of the string doesn't turn up any anomaly. Currently, it is churning out the right result again. However, it doesn't feel stable at all and this has me worried.
1) is this problem i face software related?
2) does it have anything to do with buffers?
3) is this problem code related?
4) are there any better practice i should add in?
how can i ensure the application i'm writing is stable? constantly testing it currently is slowing my progress alot. =(
public class errorMsg {
    /** Creates new Message */
    public errorMsg() {
    public static void printErrorCode(String str) {
        if (str.length() == 2)
        {   StringBuffer strBuf = new StringBuffer();
            String       strErrMsg = new String();
            strBuf.append(str).append(": ");
            for (int n=0; n<str.length(); n++)
            {   System.out.println(str.charAt(n));
            switch (str.charAt(0))
            {   case '0':
                    System.out.println("first char is '0'");
                    switch (str.charAt(1))
                    {   case '1': strErrMsg = "The radio modem has received an order from the network to stop all user traffic.";
                        break;
                        case '2': strErrMsg = "The radio modem has received an order from the network to resume user traffic.";
                        break;
                        case 'A': strErrMsg = "Reception buffer is full, i.e., the radio modem cannot receive more MPAKs from the network.";
                        break;
                        case 'B': strErrMsg = "Buffer/memory free, i.e., the radio modem starts to receive MPAKs from the network again.";
                        break;
                    } break;
                case '1':
                    switch (str.charAt(1))
                    {   case '0': strErrMsg = "The radio modem returns an MPAK to the PC because it has received an order from the network to stop all user traffic.";
                        break;
                        case '3': strErrMsg = "The radio modem returns an MPAK to the PC because the transmission buffer is full.";
                        break;
                        case '6': strErrMsg = "Login Request denied, MAN already exists in the flexlist.";
                        break;
                        case '7': strErrMsg = "Login Request denied, the flexlist is full.";
                        break;
                        case '8': strErrMsg = "The MPAK's sender MAN is neither the TMAN(Terminal subscription MAN), nor in the flexlist.";
                        break;
                    } break;
                case '5':
                    switch (str.charAt(1))
                    {   case '1': strErrMsg = "Invalid MPAK type, must not be transmitted.";
                        break;
                        case '2': strErrMsg = "Invalid MPAK state.";
                        break;
                        case '3': strErrMsg = "Invalid MPAK flags.";
                        break;
                        case '4': strErrMsg = "Invalid send list.";
                        break;
                        case '5': strErrMsg = "Invalid MPAK length.";
                        break;
                        case '6': strErrMsg = "Invalid addressee in MPAK.";
                        break;
                    } break;
                case '6':
                    switch (str.charAt(1))
                    {   case '1': strErrMsg = "TEMP_DEFAULT_LIST received from PC is incorrect.";
                        break;
                        case '2': strErrMsg = "MPAK returned on command (frame F1 or F0).";
                        break;
                        case '3': strErrMsg = "User tries to change into already active mode (express/battery save).";
                        break;
                        case '5': strErrMsg = "Mode change already activated. The message will be sent if two equal mode change orders are given consecutively.";
                        break;
                        case '6': strErrMsg = "Invalid network ID.";
                        break;
                        case '7': strErrMsg = "Mode MPAK with invalid parameter.";
                        break;
                    } break;
                case 'A':
                    if (str.charAt(1) == '0')
                    {   strErrMsg = "Transmission buffer is full, i.e., memory in buffer is not large enough for an MPAK of maximum length.";
                        break;
                    } break;
                case 'E':
                    if (str.charAt(1) == '0')
                    {   strErrMsg = "Radio transmission has failed.";
                        break;
                    } break;
                case 'F':
                    switch (str.charAt(1))
                    {   case '0': strErrMsg = "Transmit queue is full, i.e., number of MPAKs has reached the upper limit.";
                        break;
                        case 'D': strErrMsg = "Other error, the radio modem is not able to transmit any MPAKs.";
                        break;
                    } break;
                default: strErrMsg = "Unidentifiable Error!";
                break;
            strBuf.append(strErrMsg);
            System.out.println(strBuf);
        else System.out.println("Incorrect Error Code Identifier!");
// in main, my method call
        errorMsg.printErrorCode("0A");

Answer 1

it did not provide the proper result of 0x0A but instead, printed Unidentifiable Error! in the "Unidentifiable Error" case, it never went into the inner switch, but it prints 0A, then the only possible cause is the 0 and O confusion. Make sure 0 (zero) and O (letter O) not mixed when calling it in your main method.
Yet, better use a HEX parser method to get the error code in decimal and do a simple switch on all case.
--lichu