NullPointer Exception handling in the code [closed]

Question

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.List;

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

public class FileOperations<T> implements Operations<T> {

    private ObjectMapper objectMapper;
    //this is going to come from the property file.
    private String filePath = "C:\\Users\\phyadavi\\workspace\\ConfigurationService\\src\\main\\resources\\seed.txt" ;
    private File file;

    public FileOperations() {
        this.objectMapper = new ObjectMapper();
        this.file = new File(filePath);
        if (!file.exists()) {
            try {
                file.createNewFile();
            } catch (IOException e) {
                e.getMessage();
            }
        }
    }

    private void saveImpl(Object serializableObject) throws IOException {
        String jsonData = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(serializableObject);
        BufferedWriter br = new BufferedWriter(new FileWriter(file));
        br.write(jsonData);
        br.close();
    }

    @Override
    public void save(List<T> serializableObjects) throws IOException {
        saveImpl(serializableObjects);
    }

    @Override
    public void save(T serializableObject) throws IOException {
        saveImpl(serializableObject);
    }

    @Override
    public List<T> get() throws IOException {
        List<T> list = objectMapper.readValue(file, new TypeReference<List<T>>() {
        });

        return list;
    }

}


import java.io.IOException;
import java.util.List;

import com.cisco.daas.configservice.entities.User;
import com.cisco.daas.configservice.utilities.FileOperations;
import com.cisco.daas.configservice.utilities.Operations;

public class Data {

    private static final Operations<User> operations = new FileOperations<User>();

    private static List<User> users;

    public static List<User> getUsers() {
        try {
            users = operations.get();
        } catch (IOException e) {
            e.getMessage();
        }
        return users;
    }
}

The above code reads json data from a text file using the Jacksons ObjectMapper API.

The public List<T> get() method reads the data from the files and converts the data into a list of objects of type T.

The public void save(T serializableObject) and public void save(List<T> serializableObjects) both saves the json data to the file.

The Data creates a static list objects which is meant for performing operations in-memory.

Now, I would like someone to review the exception handling in the code. I need some idea on how to handle exceptions like NullPointer and FileNotFound exceptions in the above code.

Answer 1

The general rule is:

never return null except it is a valid member of the result set.

Usually it is not as in your example.

Instead of returning a null after the IOException occurred you should either return an empty List or throw your own (Runtime-) Exception. Whch option to choose depends if havin no users is an expected use case whithin your application (e.g. resulting in an empty list presented to the user) or if it is an error condition with the user gets presented as an explicit error message.

Your class Data should look like this:

public class Data {

    private static final Operations<User> operations = new FileOperations<User>();

    private static final List<User> users = new ArrList<>();

    public static List<User> getUsers() {
        users.clear();
        try {
            users.addAll(operations.get());
        } catch (IOException e) {
            e.getMessage();
        }
        return users;
    }
}

---

BTW:

avoid the use of static

declaring your methods static has a lot of Problems.

It makes your code inflexible, had to change and to maintain. It also throws away polymorphism which is the most powerful feature of OOP.

If you refure to work with objects you should ask yourself why you chose an OO language...