How to read a LinkedHashMap written in file?

Question

I want to read a particular value from LinkedHashMap written in a .txt file, but is showing "java.io.StreamCorruptedException: invalid stream header: 7B495020"

For writing in LinkedHashMap i tried the method;

public void WriteBasicInfo(String name, String pass) throws IOException, ParseException {
    Map<String, String> m;
        try {
            m = new LinkedHashMap<String, String>();
            m.put("Name", name);
            m.put("Password", pass);

            BufferedWriter bw = new BufferedWriter(new FileWriter(FILE_NAME, false));
            bw.write(m.toString());
            bw.flush();
            bw.close();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

And it successfully writes in file. But when Iam trying to read the above hashmap using this method

public void readBasicInfo() throws IOException, ParseException, InvocationTargetException, ClassNotFoundException 
{
        ObjectInputStream is = new ObjectInputStream(new FileInputStream(FILE_NAME));
        Map<String, String> myMap=(LinkedHashMap<String, String>) is.readObject();

        for(Map.Entry<String,String> m :myMap.entrySet()){
                 System.out.println(m.getKey()+" : "+m.getValue());
        //  String val1=m.get("Name"); 
        }     
     ois.close();
    }

it is showing "java.io.StreamCorruptedException: invalid stream header: 7B495020" , and no data is read

I tried to read all the entries written in hashmap, to check whether it is reading or not; but actually I just want read only "name" entry stored in hashmap.

Answer 1

You are simply: writing as string, but reading back expecting a binary serialized object!

You know, that is like: you put an egg in a box, and then you expect you can open that box and pour milk from it into a glass! That wont work.

Here:

bw.write(m.toString());

You write the map into that file as raw string. This means that your file now contains human readable strings!

But then you do:

Map<String, String> myMap=(LinkedHashMap<String, String>) is.readObject();

Which expects that the file contains serialized objects.

Long story short, these are your options:

1. keep writing these strings, but then you need to implement your own parser that reads such text files, and turns them back into objects within maps
2. instead of writing raw text strings, use a library such as gson or jackson and serialize your map as JSON string (which requires that all keys/values can be serialized as JSON)
3. instead of writing raw text or JSON, use the default Java serialization mechanism and serialize to binary content which requires that all keys/values implement the Serializable interface. See here for a nice tutorial how to do that in detail.

My recommendation: go for option 2, or 3. 2 adds a dependency to a 3rd party library, but I think it is the more "common" practice these days.