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Trouble receiving images from an 8 modem server

I have implemented this java app communicating with a remote 8 modem server and when I request for an image from a security camera (error free or not) the outcome is a 1 byte image (basically a small white square). Can you help me find the error?
Here is my code:
import java.io.*;
import java.util.Scanner;
import ithakimodem.Modem;
public class g {
private static Scanner scanner = new Scanner(System.in);
private static String EchoCode = "E3369";
private static String noImageErrorscode = "M2269";
private static String imageErrorsCode = "G6637";
private static String GPS_Code = "P7302";
private static String ACK_Code = "Q2591";
private static String NACK_Code = "R4510";
public static void main(String[] args) throws IOException, InterruptedException {
int timeout = 2000;
int speed = 80000;
Modem modem = new Modem(speed);
modem.setTimeout(timeout);
modem.write("ATD2310ITHAKI\r".getBytes());
getConsole(modem);
modem.write(("test\r").getBytes());
getConsole(modem);
while (true) {
System.out.println("\nChoose one of the options below :");
System.out.print("Option 0: Exit\n" + "Option 1: Echo packages\n" + "Option 2: Image with no errors\n" + "Option 3: Image with errors\n" + "Option 4: Tracks of GPS\n"
+ "Option 5: ARQ\n" );
System.out.print("Insert option : ");
int option = scanner.nextInt();
System.out.println("");
switch (option) {
case 0: {
// Exit
System.out.println("\nExiting...Goodbye stranger");
modem.close();
scanner.close();
return;
}
case 1: {
// Echo
getEcho(modem, EchoCode);
break;
}
case 2: {
// Image with no errors
getImage(modem, noImageErrorscode, "no_error_image.jpg");
break;
}
case 3: {
// Image with errors
getImage(modem, imageErrorsCode, "image_with_errors.jpg");
}
case 4: {
// GPS
getGPS(modem, GPS_Code);
break;
}
case 5: {
// ARQ
getARQ(modem, ACK_Code, NACK_Code);
break;
}
default:
System.out.println("Try again please\n");
}
}
}
public static String getConsole(Modem modem) {
int l;
StringBuilder stringBuilder= new StringBuilder();
String string = null;
while (true) {
try {
l = modem.read();
if (l == -1) {
break;
}
System.out.print((char) l);
stringBuilder.append((char) l);
string = stringBuilder.toString();
} catch (Exception exc) {
break;
}
}
System.out.println("");
return string;
}
private static void getImage(Modem modem, String password, String fileName) throws IOException {
int l;
boolean flag;
FileOutputStream writer = new FileOutputStream(fileName, false);
flag = modem.write((password + "\r").getBytes());
System.out.println("\nReceiving " + fileName + "...");
if (!flag) {
System.out.println("Code error or end of connection");
writer.close();
return;
}
while (true) {
try {
l = modem.read();
writer.write((char) l);
writer.flush();
if (l == -1) {
System.out.println("END");
break;
}
}
catch (Exception exc) {
break;
}
}
writer.close();
}
private static void getGPS(Modem modem, String password) throws IOException {
int rows = 99;
String Rcode = "";
Rcode = password + "R=10200" + rows;
System.out.println("Executing R parameter = " + Rcode + " (XPPPPLL)");
modem.write((Rcode + "\r").getBytes());
String stringConsole = getConsole(modem);
if (stringConsole == null) {
System.out.println("Code error or end of connection");
return;
}
String[] stringRows = stringConsole.split("\r\n");
if (stringRows[0].equals("n.a")) {
System.out.println("Error : Packages not received");
return;
}
System.out.println("**TRACES**\n");
float l1 = 0, l2 = 0;
int difference = 8;
difference *= 100 / 60;
int traceNumber = 7;
String[] traces = new String[traceNumber + 1];
int tracesCounter = 0, flag = 0;
for (int i = 0; i < rows; i++) {
if (stringRows[i].startsWith("$GPGGA")) {
if (flag == 0) {
String str = stringRows[i].split(",")[1];
l1 = Integer.valueOf(str.substring(0, 6)) * 100 / 60;
flag = 1;
}
String str = stringRows[i].split(",")[1];
l2 = Integer.valueOf(str.substring(0, 6)) * 100 / 60;
if (Math.abs(l2 - l1) >= difference) {
traces[tracesCounter] = stringRows[i];
if (tracesCounter == traceNumber)
break;
tracesCounter++;
l1 = l2;
}
}
}
for (int i = 0; i < traceNumber; i++) {
System.out.println(traces[i]);
}
String w = "", T_cd_fnl = password + "T=";
int p = 1;
System.out.println();
for (int i = 0; i < traceNumber; i++) {
String[] strSplit = traces[i].split(",");
System.out.print("T parameter = ");
String str1 = strSplit[4].substring(1, 3);
String str2 = strSplit[4].substring(3, 5);
String str3= String.valueOf(Integer.parseInt(strSplit[4].substring(6, 10)) * 60 / 100).substring(0, 2);
String str4= strSplit[2].substring(0, 2);
String str5= strSplit[2].substring(2, 4);
String str6= String.valueOf(Integer.parseInt(strSplit[2].substring(5, 9)) * 60 / 100).substring(0, 2);
w = str1 + str2 + str3 + str4 + str5 + str6 + "T";
p = p + 5;
System.out.println(w);
T_cd_fnl = T_cd_fnl + w + "=";
}
T_cd_fnl = T_cd_fnl.substring(0, T_cd_fnl.length() - 2);
System.out.println("\nSending code: " + T_cd_fnl);
getImage(modem, T_cd_fnl, "traces GPS.jpg");
}
private static void getEcho(Modem modem, String strl) throws InterruptedException, IOException {
FileOutputStream echo_time_writer = new FileOutputStream("echoTimes.txt", false);
FileOutputStream echo_counter_writer = new FileOutputStream("echoCounter.txt", false);
int h;
int runtime = 5, packetCounter = 0;
String str = "";
System.out.println("\nRuntime (mins) = " + runtime + "\n");
long start_time = System.currentTimeMillis();
long stop_time = start_time + 60 * 1000 * runtime;
long send_time = 0, receiveTime = 0;
String time = "", ClockTime = "";
echo_time_writer.write("Clock Time\tSystem Time\r\n".getBytes());
while (System.currentTimeMillis() <= stop_time) {
packetCounter++;
send_time = System.currentTimeMillis();
modem.write((strl + "\r").getBytes());
while (true) {
try {
h = modem.read();
System.out.print((char) h);
str += (char) h;
if ( h== -1) {
System.out.println("\nCode error or end of connection");
return;
}
if (str.endsWith("PSTOP")) {
receiveTime = System.currentTimeMillis();
ClockTime = str.substring(18, 26) + "\t";
time = String.valueOf((receiveTime - send_time) + "\r\n");
echo_time_writer.write(ClockTime.getBytes());
echo_time_writer.write(time.getBytes());
echo_time_writer.flush();
str = "";
break;
}
} catch (Exception e) {
break;
}
}
System.out.println("");
}
echo_counter_writer.write(("\r\nRuntime: " + String.valueOf(runtime)).getBytes());
echo_counter_writer.write(("\r\nPackets Received: " + String.valueOf(packetCounter)).getBytes());
echo_counter_writer.close();
echo_time_writer.close();
}
private static void getARQ(Modem modem, String strl, String nack_code) throws IOException, InterruptedException {
FileOutputStream arq_time_writer = new FileOutputStream("ARQtimes.txt", false);
FileOutputStream arq_counter_writer = new FileOutputStream("ARQcounter.txt", false);
int runtime = 5;
int xor = 1;
int f = 1;
int m;
int packageNumber = 0;
int iterationNumber = 0;
int[] nack_times_counter = new int[15];
int nack_to_package = 0;
String time = "", clock_time = "", s = "";
long start_time = System.currentTimeMillis();
long stop_time = start_time + 60 * 1000 * runtime;
long send_time = 0, receive_time = 0;
System.out.printf("Runtime (mins) = " + runtime + "\n");
arq_time_writer.write("Clock Time\tSystem Time\tPacket Resends\r\n".getBytes());
while (System.currentTimeMillis() <= stop_time) {
if (xor == f) {
packageNumber++;
nack_times_counter[nack_to_package]++;
nack_to_package = 0;
send_time = System.currentTimeMillis();
modem.write((strl + "\r").getBytes());
} else {
iterationNumber++;
nack_to_package++;
modem.write((nack_code + "\r").getBytes());
}
while (true) {
try {
m = modem.read();
System.out.print((char) m);
s += (char) m;
if (m == -1) {
System.out.println("\nCode error or end of connection");
return;
}
if (s.endsWith("PSTOP")) {
receive_time = System.currentTimeMillis();
break;
}
} catch (Exception e) {
break;
}
}
System.out.println("");
String[] string = s.split("<");
string = string[1].split(">");
f = Integer.parseInt(string[1].substring(1, 4));
xor = string[0].charAt(0) ^ string[0].charAt(1);
for (int i = 2; i < 16; i++) {
xor = xor ^ string[0].charAt(i);
}
if (xor == f) {
System.out.println("Packet ok");
receive_time = System.currentTimeMillis();
time = String.valueOf((receive_time - send_time) + "\t");
clock_time = s.substring(18, 26) + "\t";
arq_time_writer.write(clock_time.getBytes());
arq_time_writer.write(time.getBytes());
arq_time_writer.write((String.valueOf(nack_to_package) + "\r\n").getBytes());
arq_time_writer.flush();
} else {
xor = 0;
}
s = "";
}
arq_counter_writer.write(("\r\nRuntime: " + String.valueOf(runtime)).getBytes());
arq_counter_writer.write("\r\nPackets Received (ACK): ".getBytes());
arq_counter_writer.write(String.valueOf(packageNumber).getBytes());
arq_counter_writer.write("\r\nPackets Resent (NACK): ".getBytes());
arq_counter_writer.write(String.valueOf(iterationNumber).getBytes());
arq_counter_writer.write("\r\nNACK Time Details".getBytes());
for (int i = 0; i < nack_times_counter.length; i++) {
arq_counter_writer.write(("\r\n" + i + ":\t" + nack_times_counter[i]).getBytes());
}
arq_counter_writer.close();
arq_counter_writer.close();
System.out.println("Packets Received: " + packageNumber);
System.out.println("Packets Resent: " + iterationNumber);
System.out.println("\n\nFile arqTimes.txt is created.");
System.out.println("File arqCounter.txt is created.");
}
}
I know the problem is most probably in the getImage() function but I haven't figured it out yet.

Display the Shortest Path using BFS in java

This is my BFS algorithm code.
I can calculate the shortest path distance, but somehow I am not able to display the shortest path.
Like for example, I calculated the shortest path from source to destination is 2. But i would like to also display the pathway. (PlaceA -> PlaceB -> PlaceC) for example.
May i know how do i display out the shortest path out using Java?
Please do help me! Thank you!
public static void main(String[] args) {
// TODO Auto-generated method stub
chooseNumOfVertices();
chooseNumOfEdges();
generateGraph();
in.close();
}
private static void generateGraph() {
int source = 0;
int destination = 0;
int edge = chooseEdge;
// TODO Auto-generated method stub
ArrayList<LinkedList<Integer>> graph = new ArrayList<LinkedList<Integer>>();
for (int i = 0; i < limit; i++) {
LinkedList<Integer> vertex = new LinkedList<Integer>();
vertex.add(i);
graph.add(vertex);
}
while (chooseEdge > 0) {
// Randomize the value
int value = new Random().nextInt(cityMapping.size());
int value2 = new Random().nextInt(cityMapping.size());
//
if (value != value2 && !graph.get(value).contains(value2) && !graph.get(value2).contains(value)) {
graph.get(value).add(value2);
graph.get(value2).add(value);
chooseEdge--;
}
}
// Printing out the Nodes
for (int i = 0; i < graph.size(); i++) {
// Return each LinkedList nodes
// System.out.println(graph.get(i));
for (int j = 0; j < graph.get(i).size(); j++) {
// Return each individual nodes inside LinkedList
for (Entry<Integer, String> entry : cityMapping.entrySet()) {
if (entry.getKey() == graph.get(i).get(j)) {
//System.out.print(graph.get(i).get(j) + "-> ");
System.out.print(graph.get(i).get(j) + ". " + entry.getValue() + " -> ");
}
}
}
System.out.println();
}
do {
for (
int i = 0; i < limit; i++) {
int[] newArray = new int[limit];
distance.add(newArray);
predecessor.add(newArray);
}
long time = System.nanoTime();
System.out.println("Searching BFS");
System.out.println("--------------------------------------------");
for (int i = 0; i < limit; i++) {
BFS(graph, i);
}
long CPUTime = (System.nanoTime() - time);
System.out.println("CPU Time for BFS for " + limit + "vertices and " + edge + "edges (in ns): " + CPUTime);
System.out.print("Enter -1 to exit! Enter source vertex (between 0 to " + (limit - 1) + ") : ");
source = in.nextInt();
if (source == -1) {
System.out.print("System terminating...");
break;
}
System.out.print("Enter destination vertex (between 0 to " + (limit - 1) + ") : ");
destination = in.nextInt();
System.out.println("Distance from " + source + " to " + destination + " is: " +
getDistance(source, destination));
System.out.println("The Predecessor of the path from " + source + " to " + destination + " is: "
+ getPredecessor(source, destination));
} while (source != -1);
}
private static void BFS(ArrayList<LinkedList<Integer>> graph, int i) {
// TODO Auto-generated method stub
boolean[] mark = new boolean[graph.size()];
Queue<Integer> L = new ArrayBlockingQueue<Integer>(graph.size()); //Queue
L.add(i);
mark[i] = true;
Arrays.fill(predecessor.get(i), -1);
Arrays.fill(distance.get(i), -1);
distance.get(i)[i] = 0;
while (!L.isEmpty()) {
int vertex = L.remove();
for (int i1 = 0; i1 < graph.get(vertex).size(); i1++) {
int v = graph.get(vertex).get(i1);
if (!mark[v]) {
mark[v] = true;
predecessor.get(i)[v] = vertex;
L.add(v);
distance.get(i)[v] = distance.get(i)[predecessor.get(i)[v]] + 1;
}
}
}
}
public static int getDistance(int start, int end) {
return (distance.get(start)[end]);
}
public static int getPredecessor(int start, int end) {
return (predecessor.get(start)[end]);
}
private static void chooseNumOfEdges() {
System.out.println("Please input the number of Edges:");
chooseEdge = in.nextInt();
}
// Number of Vertices
private static void chooseNumOfVertices() {
in = new Scanner(System.in);
System.out.println("Please input the number of Vertices:");
limit = in.nextInt();
// Read CSV
List<String[]> content = readCsvFile();
// Map each number to a city name
cityMapping = new HashMap<>();
for (int i = 0; i < limit; i++) {
cityMapping.put(i, content.get(i)[0]);
}
// System.out.println(cityMapping);
}
// Read CSV file
public static List<String[]> readCsvFile() {
String csvFile = "./Lab 4/country.csv";
BufferedReader br = null;
ArrayList<String> names = new ArrayList<String>();
List<String[]> content = new ArrayList<>();
String cvsSplitBy = ",";
try {
String line = "";
br = new BufferedReader(new FileReader(csvFile));
while ((line = br.readLine()) != null) {
content.add(line.split(cvsSplitBy));
}
} catch (Exception e) {
e.printStackTrace();
}
Random r = new Random();
Collections.shuffle(content);
return content;
}
}

Cannot read a txt file

import java.io.*;
public class Point {
private double x;
private double y;
public Point(double x_coord, double y_coord) {
x = x_coord;
y = y_coord;
}
}
public class PointArray {
private Point points[];
public PointArray(FileInputStream fileIn) throws IOException {
try {
BufferedReader inputStream = new BufferedReader(new InputStreamReader(fileIn));
int numberOfPoints = Integer.parseInt(inputStream.readLine())...points = new Point[numberOfPoints];
int i = 0;
String line;
while ((line = inputStream.readLine()) != null) {
System.out.print(line);
double x = Double.parseDouble(line.split(" ")[0]);
double y = Double.parseDouble(line.split(" ")[1]);
points[i] = new Point(x, y);
i++;
}
inputStream.close();
} catch (IOException e) {
System.out.println("Error");
System.exit(0);
}
}
}
public String toString() {
String format = "{";
for (int i = 0; i < points.length; i++) {
if (i < points.length - 1) format = format + points[i] + ", ";
else format = format + points[i];
}
format = format + "}";
return format;
}
public static void main(String[] args) {
FileInputStream five = new FileInputStream(new File("fivePoints.txt"));
PointArray fivePoints = new PointArray(five);
System.out.println(fivePoints.toString());
}
The txt file fivePoints is as shown below:
5
2 7
3 5
11 17
23 19
150 1
The first number in the first line means the number of points in the text file.
There is an error when I read the file. The output I want to get is {(2.0, 7.0), (3.0, 5.0), (11.0,17.0), (23.0, 19.0), (150.0, 1.0)}. How can I fix it?

Add a toString method to your Point which formats the Point as x, y
public class Point {
//...
#Override
public String toString() {
return x + ", " + y;
}
}
Move your toString method so it's defined in PointArray, you might also consider making use of StringJoiner to make your life simpler
public class PointArray {
//...
#Override
public String toString() {
StringJoiner sj = new StringJoiner(", ", "{", "}");
for (int i = 0; i < points.length; i++) {
sj.add("(" + points[i].toString() + ")");
}
return sj.toString();
}
}

Java Serialization issues

I have this code that writes an object:
FileOutputStream fileOut = new FileOutputStream("Model.ser");
ObjectOutputStream out = new ObjectOutputStream(fileOut);
out.writeObject(this);
out.close();
fileOut.close();
And this code that loads the object:
Model m = null;
try {
FileInputStream fileIn = new FileInputStream("Model.ser");
ObjectInputStream in = new ObjectInputStream(fileIn);
m = (Model) in.readObject();
in.close();
fileIn.close();
} catch (Exception e) {}
setStudentList(m.getStudentList());
setModuleList(m.getModuleList());
I'm pretty sure that saving works, as when I opened the file in notepad++ I saw most of the data that I had saved, but when I load there is no data in module list.
Full source code:
import java.io.*;
import java.util.*;
public class Model implements java.io.Serializable {
private Student[] studentList = new Student[0];
private Module[] moduleList = new Module[0];
public void menu() {
while (true) {
System.out.println ("MENU");
System.out.println ("");
System.out.println (" 1 - Run Tests");
System.out.println (" 2 - Add Student");
System.out.println (" 3 - Add Module");
System.out.println (" 4 - Add Student To Module");
System.out.println (" 5 - Save System (Text file)");
System.out.println (" 6 - Load System (Text file)");
System.out.println (" 7 - Save System (Serialized)");
System.out.println (" 8 - Load System (Serialized)");
System.out.println (" 9 - Print Report");
System.out.println ("");
System.out.print ("Enter choice: ");
String input = keyboard.readString();
switch (input) {
case "1" :
runTests();
break;
case "2" :
System.out.print("First Name : ");
String fN = keyboard.readString();
System.out.print("Surname : ");
String sN = keyboard.readString();
System.out.print("Course Code : ");
String c = keyboard.readString();
System.out.print("User ID : ");
String iD = keyboard.readString();
AddStudent(iD, sN, fN, c);
break;
case "3" :
System.out.print("Module Code : ");
String code = keyboard.readString();
String[] temp = new String[0];
AddModule(code,temp);
break;
case "4" :
System.out.print("Module Code : ");
code = keyboard.readString();
Module m = findAModule(code);
if (m != null) {
System.out.print("User ID : ");
iD = keyboard.readString();
Student s = findAStudent(iD);
if (s != null) {
m.addThisStudent(s);
} else {
System.out.println("Student not found");
}
} else {
System.out.println("Module not found");
}
break;
case "5" :
saveToTextFiles();
break;
case "6" :
loadFromTextFiles();
break;
case "7" :
saveSerialized();
break;
case "8" :
break;
case "9" :
printReport();
break;
}
}
}
public void runTests() {
loadFromTextFiles();
saveSerialized();
printReport();
}
public void loadFromTextFiles() {
studentList = new Student[0];
moduleList = new Module[0];
try {
Scanner fileReader = new Scanner(new InputStreamReader(new FileInputStream("students.txt")));
int num = fileReader.nextInt(); fileReader.nextLine();
for (int i = 0; i < num; i++) {
String u = fileReader.nextLine();
String sn = fileReader.nextLine();
String fn = fileReader.nextLine();
String c = fileReader.nextLine();
AddStudent(u, sn, fn, c);
}
fileReader.close();
fileReader = new Scanner(new InputStreamReader(new FileInputStream("modules.txt")));
num = fileReader.nextInt(); fileReader.nextLine();
for (int i = 0; i < num; i++) {
String code = fileReader.nextLine();
int numOfStudents = fileReader.nextInt(); fileReader.nextLine();
String[] students = new String[numOfStudents];
for (int j = 0; j < numOfStudents; j++) {
students[j] = fileReader.nextLine();
}
AddModule(code, students);
}
fileReader.close();
} catch (IOException e) {}
}
public void saveToTextFiles () {
try {
PrintWriter outfile = new PrintWriter(new OutputStreamWriter (new FileOutputStream("students.txt")));
outfile.println(studentList.length);
for (int i = 0; i < studentList.length; i++) {
outfile.println(studentList[i].getUID());
outfile.println(studentList[i].getSN());
outfile.println(studentList[i].getFN());
outfile.println(studentList[i].getDegree());
}
outfile.close();
outfile = new PrintWriter(new OutputStreamWriter (new FileOutputStream("modules.txt")));
outfile.println(moduleList.length);
for (int i = 0; i < moduleList.length; i++) {
outfile.println(moduleList[i].getCode());
outfile.println(moduleList[i].getStudents().length);
for (int j = 0; j < moduleList[i].getStudents().length; j++) {
outfile.println(moduleList[i].getStudents()[j]);
}
}
outfile.close();
} catch (IOException e) {}
}
public void saveSerialized() {
try {
FileOutputStream fileOut = new FileOutputStream("Model.ser");
ObjectOutputStream out = new ObjectOutputStream(fileOut);
out.writeObject(this);
out.close();
fileOut.close();
FileOutputStream fileOut2 = new FileOutputStream("Module.ser");
ObjectOutputStream out2 = new ObjectOutputStream(fileOut2);
out2.writeObject(studentList);
out2.close();
fileOut2.close();
FileOutputStream fileOut3 = new FileOutputStream("Student.ser");
ObjectOutputStream out3 = new ObjectOutputStream(fileOut3);
out3.writeObject(moduleList);
out3.close();
fileOut3.close();
} catch (IOException e) {}
}
public void loadSerialized() {
Model m = null;
try {
FileInputStream fileIn = new FileInputStream("Model.ser");
ObjectInputStream in = new ObjectInputStream(fileIn);
m = (Model) in.readObject();
in.close();
fileIn.close();
} catch (Exception e) {}
setStudentList(m.getStudentList());
setModuleList(m.getModuleList());
}
private Module[] getModuleList() {
return moduleList;
}
private Student[] getStudentList() {
return studentList;
}
private void setModuleList(Module[] m) {
moduleList = m.clone();
}
private void setStudentList(Student[] s) {
studentList = s.clone();
}
private void AddModule(String code, String[] students) {
int length = moduleList.length;
Module NewArray[] = new Module[length + 1];
for (int i = 0; i < length + 1; i++) {
if (i < length) {
NewArray[i] = new Module(moduleList[i]);
}
}
NewArray[length] = new Module(code, students);
moduleList = NewArray.clone();
}
private void AddStudent(String u, String sn, String fn, String c) {
int length = studentList.length;
Student NewArray[] = new Student[length + 1];
for(int i = 0; i < length + 1; i++) {
if (i < length) {
NewArray[i] = new Student(studentList[i]);
}
}
NewArray[length] = new Student(u, sn, fn, c);
studentList = NewArray.clone();
}
public void printReport() {
for (int i = 0; i < moduleList.length; i++) {
System.out.println(moduleList[i].toString(this));
}
}
public Student findAStudent(String uid) {
for (int i = 0; i < studentList.length; i++) {
if (studentList[i].getUID().compareTo(uid) == 0) {
return studentList[i];
}
}
return null;
}
public Module findAModule(String code) {
for (int i = 0; i < moduleList.length; i++) {
if (moduleList[i].getCode().compareTo(code) == 0) {
return moduleList[i];
}
}
return null;
}
}

Look at your code sample, in particular the switch statement:
switch (input) {
...
case "8" :
break;
...
}
I'd assume the method loadSerialized should be called there but it's missing and is not called anywhere else in the code.
Once you actually call the method that does the loading, the code will work, assuming you have declared a serialVersionUID for both Student and Module.
Edit: why using serialization to persist objects is a bad idea
In simple terms, using serialization to persist objects is brittle.
An object's serialized form is tied to its class. Classes tend to change over time, meaning the serialized instance of the old cannot be loaded into the new.
While you can work round this by setting a serialVersionUID doing so introduces a reverse of the problem where, in the case new fields are introduced, the new cannot be read into objects of the old class, which can be a problem if you do rolling updates to the deployed system.
There's a host of other reasons - it's not easily readable (meaning you can't update it like you would a database or XML/JSON doc), it's inefficient, etc.

How to sort a file by grade?

This is a complete program that sorts the file by ID. However, I would like to sort it by grade. I modified it several times but it doesn't seem to work. Could someone please direct me where to change the ID to grade. Also, do you think this code can be simplified or are there any other code simpler than this code.
Sorry for the bad indentation, this source code can also be found here.
student.txt file:
4 A 87 A
5 B 99 A+
1 C 75 A
2 D 55 C
3 E 68 B
source:
import java.io.*;
import java.util.*;
class ShowData implements Comparable {
int id;
String name;
int marks;
String grade;
public void setId(int id) {
this.id = id;
}
public int getId() {
return id;
}
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setMarks(int marks) {
this.marks = marks;
}
public int getMarks() {
return marks;
}
public void setGrade(String grade) {
this.grade = grade;
}
public String getGrade() {
return grade;
}
public int compareTo(Object Student) throws ClassCastException {
if (!(Student instanceof ShowData))
throw new ClassCastException("Error");
int ide = ((ShowData) Student).getId();
return this.id - ide;
}
}
public class SortFile {
SortFile() {
int j = 0;
ShowData data[] = new ShowData[5];
try {
FileInputStream fstream = new FileInputStream("C:/student.txt");
// Use DataInputStream to read binary NOT text.
BufferedReader br = new BufferedReader(new InputStreamReader(fstream));
String strLine;
ArrayList list = new ArrayList();
while ((strLine = br.readLine()) != null) {
list.add(strLine);
}
Iterator itr;
for (itr = list.iterator(); itr.hasNext();) {
String str = itr.next().toString();
String[] splitSt = str.split(" ");
String id = "", name = "", marks = "", grade = "";
for (int i = 0; i < splitSt.length; i++) {
id = splitSt[0];
name = splitSt[1];
marks = splitSt[2];
grade = splitSt[3];
}
data[j] = new ShowData();
data[j].setId(Integer.parseInt(id));
data[j].setName(name);
data[j].setMarks(Integer.parseInt(marks));
data[j].setGrade(grade);
j++;
}
Arrays.sort(data);
File file = new File("C:/new.txt");
FileWriter fw = new FileWriter(file, true);
BufferedWriter out = new BufferedWriter(fw);
System.out.println("********Sorted by id********");
String strVal = "";
for (int i = 0; i < 5; i++) {
ShowData show = data[i];
int no = show.getId();
String name = show.getName();
int marks = show.getMarks();
String grade = show.getGrade();
System.out.println(no + " " + name + " " + marks + " " + grade);
String d = no + " " + name + " " + marks + " " + grade;
ArrayList al = new ArrayList();
al.add(d + "\n");
Iterator itr1 = al.iterator();
while (itr1.hasNext()) {
out.write(itr1.next().toString());
out.newLine();
}
}
out.close();
} catch (Exception e) {
}
}
public static void main(String[] args) {
SortFile data = new SortFile();
}
}

The compareTo() method is currently comparing the IDs, not the grades. Try compare the
marks instead.