CODEWITHSUNDEEP
javaserializationhashmap
bhavya
bhavya

Reputation: 719

serialization and deserialization of multiple java objects in a single file

I have a scenario where I have multiple HashMap objects which need to be stored in the same text file. for example :

    a.put("01jan", 13);
    a.put("02feb", 13);
    a.put("03march", 13);
    a.put("04apr", 13);
    a.put("05may", 13);

    b.put("06june", 12);
    b.put("07july", 12);
    b.put("08aug", 12);
    b.put("09sept", 12);
    b.put("10oct", 12);

I would like to use java serialization to save the objects in the same txt file. Is there any way to do this one after the other I have tried using the FileOutputStream( file_name,true). Also when I try to retrieve objects say HashMap b now and HashMap a when the need arises. Is there any way of achieving this? How do I retrieve out of order objects and retrieve the correct object?

Thanks, Bhavya

Upvotes: 1

Views: 4230

Answers (4)

Peter Lawrey
Peter Lawrey

Reputation: 533780

There is no advantage is retrieving objects out of order, just save all the data to disk and when you need them load all the data from disk. Unless the file is at least 64KB it won't matter.

If your data is larger, you could place a and b in separate files but its unlikely to make more than a few milli-seconds difference but will make your code more complex.

FileOutputStream( file_name,true)

You cannot append ObjectOutputStream's to a file. When you read the data back, you will only ever get the first. (You can retrieve the data with a lot of pain, but its best not to try)

Upvotes: 1

Andrew Thompson
Andrew Thompson

Reputation: 168845

Another approach is to put each HashMap as an entry in a Zip archive.

Upvotes: 0

Buhake Sindi
Buhake Sindi

Reputation: 89189

If you have a many maps to store, maps uses key/value pair so I would suggest storing the data in key/value format. JSON is human-readable data interchangeable format which is key/value based. It can be stored in a file and you can use JSONObject(javadoc) or GSON to serialize/unserialize data.

Example:

{"hashmap1": 
    [
        {"01jan": 13}, 
        {"02feb": 13}, 
        {"03march", 13}
    ]
}

Upvotes: 2

Param
Param

Reputation: 2540

You can put all the hashmap objects in an ArrayList and then serialize the ArrayList object. Later on when you deserealize the ArrayList object, you can get individual HashMap objects back:

List<HashMap> list = new ArrayList<HashMap>();
list.add(a);
list.add(b);

// serialize the list object

Upvotes: 7

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