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Saving data structures in Java

Please give me some advice regarding data saving in Java.
I’m new in Java. Previously I wrote programs on C++.
It was easy to save data in C++: just declare structure, fill its fields and then save it like byte array. As far as Java doesn’t have structures I suppose that Java programming means another save concept.
I will appreciate you for ideas and ideas what to read regarding this.

It's simpler in Java.
Use a class in place of your C++ struct, and implement java.io.Serializable. Your IDE will help you build any methods that you need to implement. Conceptually Java uses reflection to capture the values of the fields (aside from ones you've marked Transient) in your class, so, by and large, the process is automated.

I am guessing, by 'saving' you mean saving to a disc.
If that is the case, you should be looking at the concept of serialization in Java.
Make your class implement the Serializable interface.
Then take cue from the following code to serialize.
FileOutputStream fileOut =
new FileOutputStream("/tmp/employee.ser");
ObjectOutputStream out = new ObjectOutputStream(fileOut);
out.writeObject(e);
out.close();
fileOut.close();
Code on similar lines to deserialize.

In java instead of structure create a class corresponding to your storage criteria then create object for that class and then you can store the required datas in that object then store that object in array or any collections.
for eg create Student class which has name,regno etc.. then create object of student class store the datas in that object,so you can have your data in an object which is similar to structure then you can save students object in array or collections.

Use of Serializable other than Writing& Reading object to/from File

In Which Cases it is a good coding practice to use implements serializable other than Writing & Reading object to/from file.In a project i went through code. A class using implements serializable even if in that class/project no any Writing/Reading objects to/from file?

If the object leaves the JVM it was created in, the class should implement Serializable.
Serialization is a method by which an object can be represented as a sequence of bytes that includes the object's data as well as information about the object's type and the types of data stored in the object.
After a serialized object has been written into a file, it can be read from the file and deserialized that is, the type information and bytes that represent the object and its data can be used to recreate the object in memory.
This is the main purpose of de-serialization. To get the object information, object type, variable type information from a written(loosely speaking) representation of an object. And hence serialization is required in the first place, to make this possible.
So, whenever, your object has a possibility of leaving the JVM, the program is being executed in, you should make the class, implement Serializable.
Reading/Writing objects into files (Memory), or passing an object over internet or any other type of connection. Whenever the object, leaves the JVM it was created in, it should implement Serializable, so that it can be serialized and deserialized for recognition once it enters back into another/same JVM.
Many good reads at :
1: Why Java needs Serializable interface?
2: What is the purpose of Serialization in Java?

Benefits of serialization:
To persist data for future use.
To send data to a remote computer using client/server Java technologies like RMI , socket programming etc.
To flatten an object into array of bytes in memory.
To send objects between the servers in a cluster.
To exchange data between applets and servlets.
To store user session in Web applications
To activate/passivate enterprise java beans.
You can refer to this article for more details.

If you ever expect your object to be used as data in a RMI setting, they should be serializable, as RMI either needs objects Serializable (if they are to be serialized and sent to the remote side) or to be a UnicastRemoteObject if you need a remote reference.

In earlier versions of java (before java 5) marker interfaces were good way to declare meta data but currently we having annotation which are more powerful to declare meta data for classes.
Annotation provides the very flexible and dynamic capability and we can provide the configuration for annotation meta deta that either we want to send that information in byte code or at run time.
Here If you are not willing to read & write object then there is one purpose left of serialization is, declare metadata for class and if you are goint to declare meta data for class then personally I suggest you don't use serialization just go for annotation.
Annotation is better choice than marker interface and JUnit is a perfect example of using Annotation e.g. #Test for specifying a Test Class. Same can also be achieved by using Test marker interface.
There is one more example which indicate that Annotations are better choice #ThreadSafe looks lot better than implementing ThraedSafe marker interface.

There are other cases in which you want to send an object by value instead of by reference:
Sending objects over the network.
Can't really send objects by reference here.
Multithreading, particularly in Android
Android uses Serializable/Parcelable to send information between Activities. It has something to do with memory mapping and multithreading. I don't really understand this though.

Along with Martin C's answer I want to add that - if you use Serializable then you can easily load your Object graph to memory. For example you have a Student class which have a Deportment. So if you serialize your Student then the Department also be saved. Moreover it also allow you -
1. to rename variables in a serialized class while maintaining backwards-compatibility.
2. to access data from deleted fields in a new version (in other words, change the internal representation of your data while maintaining backwards-compatibility).

Some frameworks/environments might depend upon data objects being serializable. For example in J2EE, the HttpSession attributes must be serializable in order to benefit from Session Persistence. Also RMI and other dark ages artifacts use serialization.
Therefore, though you might not immediately need your data objects to be serializable, it might make sense to declare Serializable just in case (It is almost free, unless you need to go through the pain of declaring readObject/writeObject methods)

Serializing objects with changing class source code

Note: Due to the lack of questions like this on SO, I've decided to put one up myself as a Q&A
Serializing objects (using an ObjectOutputStream and an ObjectInputStream) is a method for storing an instance of a Java Object as data that can be later deserialized for use. This can cause problems and frustration when the Class used to deserialize the data does not remain the same (source-code changes; program updates).
So how can an Object be serialized and deserialized with an updated / downgraded version of a Class?

Here are a few common ways of serializing an object that can be deserialized in a backwards-compatible way.
1. Store the data in the JSON format using import and export methods designed to save all fields needed to recreate the instance. This can be made backwards-compatible by including a version key that allows for an update algorithm to be called if the version is too low. A common library for this is the Google Gson library which can represent Java objects in JSON as well as normally editing a JSON file.
2. Use the built-in java Properties class in a way similar to the method described above. Properties objects can be later stored using a stream (store()) written as a regular Java Properties file, or saved in XML (storeToXML()).
3. Sometimes simple objects can be easily represented with key-value pairs in a place where storing them in a JSON, XML, or Properties file is either too complicated or not neccessary (overkill one could say). In this case, an effective way of serializing the object could be using the ObjectOutputStream class to serialize a HashMap object containing key-value pairs where the key could be a String and the value could be an Object (HashMap<String,Object>). This allows for all of the object's fields to be stored as well as including a version key while providing much versatility.
Note: Although serializing an object using the ObjectOutputStream for persistence storage is normally considered bad convention, it can be used either way as long as the class' source code remains the same.
Also Note about versioning: Changes to a class can be safely made without disrupting deserialization using an ObjectOutputStream as long as they are a compatible change. As mentioned in the Versioning of Serializable Objects chapter of the Object Serialization Specification:
A compatible change is a change that does not affect the contract
between the class and its callers.

Transfer of a Java Serialized Object

Is it possible to declare an instance of a serializable object in one Java program / class, then repeat the definitions of the internal objects in a different program /class entirely, and load in a big complex object from a data file? The goal is to be able to write an editor for items that's kept locally on my build machine, then write the game itself and distribute it to people who would like to play the game.
I'm writing a game in Java as a hobbyist project. Within my game, there's an a family of classes that extend a parent class, GameItem. Items might be in various families like HealingPotion, Bomb, KeyItem, and so on.
class GameItem implements Serializable {
String ItemName
String ImageResourceLocation
....}
What I want to do is include definitions of how to create each item in a particularly family of items, but then have a big class called GameItemList, which contains all possible items that can occur as you play the game.
class GameItemList implements Serializable {
LinkedList<GameItem>gameItemList;
//methods here like LookUpByName, LookUpByIndex that return references to an item
}
Maybe at some point - as the player starts a new game, or as the game launches, do something like:
//create itemList
FileInputStream fileIn = new FileInputStream("items.dat");
ObjectInputStream in = new ObjectInputStream(fileIn);
GameItemList allItems = (GameItemList)in.readObject();
in.close();
//Now I have an object called allItems that can be used for lookups.
Thanks guys, any comments or help would be greatly appreciated.

When you serialize an object, every field of the object is serialized, unless marked with transient. And this behavior is of course recursive. So yes, you can serialize an object, then deserialize it, and the deserialized object will have the same state as the serialized one. A different behavior would make serialization useless.
I wouldn't use native serialization for long-term storage of data, though. Serialized objects are hard to inspect, impossible to modify using a text editor, and maintaining backward compatibility with older versions of the classes is hard. I would use a more open format like XML or JSON.

Yes, that is possible. If an object is correctly serialized, it can be deserialized in any other machine as long as the application running there knowns the definition of the class to be deserialized.

This will work, but Java serialization is notorious for making it hard to "evolve" classes -- the internal representation is explicitly tied to the on-disk format. You can work around this with custom reader / writer methods, but you might consider a more portable format like JSON or XML instead of object serialization.

will serialized object contains metadata?

When we are deserializing an object, its very difficult to understand that, how it is retriving the object in some certain state? Does it contain any Meta data of the object?

When an object is serialized, the object's class is written to the stream along with the contents of the object's non-transient fields. The deserializer will attempt to load that class (and there are several mechanisms for it to do that), then populate the non-transient fields.
The protocol spec is here: http://java.sun.com/javase/6/docs/platform/serialization/spec/protocol.html
If by "metadata" you're referring to annotations on the class, then no, they are not serialized with the object itself, but are available on the class. If you mean something else, please describe what you mean.

At a high level, the serialization stream contains the data inside the object and the name of the classes involved, as well as a version number to ensure the class didn't change. It uses that information to make a new instance of an object and fills it with the same data as the old instance. It does this avoiding all of the usual constraints on object creation (the need to call constructors, for example).
One confusing point people have is that they can think the class definition itself is serialized. It is not, just the data it contains with enough information to know which objects to recreate when deserilalized. When the object is deserialized, it has to match the existing class on the class path, the serialization binary data does not contain the class.