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I want to convert Json file to CSV file. I passed data from json to java object then I try to write Java object to CSV file but it gives NPE. It probably not correctly gets the data from JSON. I'm not sure what it is.

Class for reading json to java object

public Student convertJson(File jsonFile)  {
    Gson gson = new Gson();
    Student std;

    try{
        std = gson.fromJson(new FileReader(jsonFile),Student.class);
        return std;

    }
    catch (FileNotFoundException e) {
        return null;
    }
}

Class for converting object to csv

private final String SEPARATOR = ", ";
public void toCSV(List<Student> student) {

try{
    BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream("group.csv"),"UTF-8"));

    for(Student std: student){

        StringBuffer oneLine = new StringBuffer();
        oneLine.append(std.getEmail());
        oneLine.append(SEPARATOR);
        oneLine.append(std.getName());
        oneLine.append(SEPARATOR);
        oneLine.append(std.getScoreGroup());
        oneLine.append(SEPARATOR);
        oneLine.append(std.getCommentGroup());

        bw.write(oneLine.toString());
        bw.newLine();

    }
    bw.flush();
    bw.close();

} catch (IOException e) {
    e.printStackTrace();
}

Main

 File newfile = new File(pathName);
    List<File> files= file.findFile(newfile);  // step 1 find files and return arralist of files
    System.out.println(files);




    List<Student> students = new ArrayList<>(); // array list of objects Student
    ReadJson fiel = new ReadJson();             // parses passed json file and returns Student file

    List<Group> info = new ArrayList<>();
    java.util.Collections.sort(info);

    for(int i=0;i<files.size();i++){
        Student temp = fiel.convertJson(new File(files.get(i).getName()));
        students.add(temp);
    }


    ConvertJSONFiles json = new ConvertJSONFiles();
    json.toCSV(students);

Error

Exception in thread "main" java.lang.NullPointerException
at ca.cmpt213.as2.ConvertJSONFiles.toCSV(ConvertJSONFiles.java:16)
at com.company.Main.main(Main.java:44)

Disconnected from the target VM, address: '127.0.0.1:63683', transport: 'socket'

New problem: Now it says it cannot read because there should be object not array.

Exception in thread "main" com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 2 column 13 path $.group at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:224) at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:129) at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220) at com.google.gson.Gson.fromJson(Gson.java:888) at com.google.gson.Gson.fromJson(Gson.java:826) at ca.cmpt213.as2.ReadJson.convertJson(ReadJson.java:17) at com.company.Main.main(Main.java:36)

Kassymkhan Bekbolatov
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1 Answers1

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The ultimate cause resides here:

new File(files.get(i).getName())

File.getName() returns the last portion of the file's path, so the newly created file basically looses all path information.

The resulting path most probably does not exist which makes convertJson(File) return null, which is passed into List<Student> students and from there to toCSV(List), where finally the NPE occurs.

Possible fix:

Student temp = fiel.convertJson(files.get(i));
Izruo
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  • A file or path not existing should not cause a `NullPointerException`. A far better fix would be to let `convertJson()` throw the `FileNotFoundException` back to the caller. – user207421 Jan 26 '18 at 01:22
  • @EJP That's totally not a fix, because then there's a `FileNotFoundException` for a file that was seemingly listed while reading a directory. It is however an improvement in design, so feel free to post it as a comment to the question. – Izruo Jan 26 '18 at 14:06