Load objects from file to arrayList

Question

I wonder how I should do to read objects from a list if I don't know how many object there are? To save is easier because than I use the number of objects that I have in the ArrayList where I store all objects. Code like this:

// Save all customer object from customerList
for(int j=0; j < customerList.size(); j++) {
    outObjectStream.writeObject(customerList.get(j));
}

My thought was to use something similar to load all objects in the file and add them one after one to the ArrayList after it has been cleared. But as I wrote, it's not possible when I don't know the number of object inside the file. Any ideas how I can solve this in simple way?

Bohemian · Accepted Answer · 2012-01-28 12:25:21Z

6

Your design is flawed: You should be serializing the List to file, not each Customer object. Then you would just deserialize the whole List.

After deserializing you will have a new instance of the list. If you absolutely must load the customers into your list, use customerList.addAll(list);

All of the common Collections are themselves Serializable: Use and trust the JDK API!

edited Jan 28, 2012 at 12:25

answered Jan 28, 2012 at 12:10

Bohemian♦

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2 Comments

3D-kreativ Over a year ago

OK, Then I perhaps I should change my code and save the customer the arraylist then, but would I load the arraylist just like this: inObjectStream.readObject(customerList) and the be ready to use the arraylist again, and no need to do soemthing else?

Stephen C Over a year ago

@3D-kreativ - I suggest you try it and see rather than asking a "did you really mean that?" style follow-up question.

Aravind Yarram · Accepted Answer · 2012-01-28 12:10:36Z

3

You should use outObjectStream.writeObject(customerList); and let the ObjectInputStream do the job while reading it back.

answered Jan 28, 2012 at 12:10

Aravind Yarram

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1 Comment

3D-kreativ Over a year ago

Hmm, but if I want to load the customerList, how does it "connect" to the customerList that I have in the code, or do I not need to do anything else after that I have loaded the customerList?! Is it enough to just use somethong like: inObjectStream.readObject(customerList); and then just be happy and continue working with the arrayList!? :)

hmjd · Accepted Answer · 2012-01-28 12:09:47Z

2

Use ArrayList.add(). The number of elements is not required to be known beforehand.

answered Jan 28, 2012 at 12:09

hmjd

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Comments

Scott Bennett-McLeish · Accepted Answer · 2012-01-28 12:20:42Z

1

To read objects from a file into a list you need to read it back in the opposite way you wrote it:

readObject(...)

rather than:

writeObject(...)

However, you really wouldn't normally want to save each object separately. One should either save the whole list at one as one object or ideally save it to a more readable format such as CSV, XML or JSON etc.

Writing and Reading objects gets tricky when you start changing your objects (adding new fields etc.).

answered Jan 28, 2012 at 12:20

Scott Bennett-McLeish

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