Serialization Java Interview Questions – Set 03

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Why doesn’t Collection extend Cloneable and Serializable

From Sun FAQ Page: Many Collection implementations (including all of the ones provided by the JDK) will have a public clone method, but it would be mistake to require it of all Collections. For example, what does it mean to clone a Collection that’s backed by a terabyte SQL database? Should the method call cause the company to requisition a new disk farm? Similar arguments hold for serializable. If the client doesn’t know the actual type of a Collection, it’s much more flexible and less error prone to have the client decide what type of Collection is desired, create an empty Collection of this type, and use the addAll method to copy the elements of the original collection into the new one. Note on Some Important Terms

  • Synchronized means only one thread can modify a hash table at one point of time. Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.

Fail-fast is relevant from the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object “structurally”, a concurrent modification exception will be thrown. It is possible for other threads though to invoke “set” method since it doesn’t modify the collection “structurally”. However, if prior to calling “set”, the collection has been modified structurally, “IllegalArgumentException” will be thrown

How to Serialize a collection in java? How to serialize a ArrayList, Hashmap or Hashset object in Java

All standard implementations of collections List, Set and Map interface already implement java.io.Serializable. All the commonly used collection classes like java.util.ArrayList, java.util.Vector, java.util.Hashmap, java.util.Hashtable, java.util.HashSet, java.util.TreeSet do implement Serializable. This means you do not really need to write anything specific to serialize collection objects. However you should keep following things in mind before you serialize a collection object – Make sure all the objects added in collection are Serializable. – Serializing the collection can be costly therefore make sure you serialize only required data isntead of serializing the whole collection. – In case you are using a custom implementation of Collection interface then you may need to implement serialization for it.

What is a Serial Version UID (serialVersionUID) and why should I use it? How to generate one

The serialVersionUID represents your class version, and you should change it if the current version of your class is not backwards compatible with its earlier versions. This is extract from Java API Documentation

The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization.

Most of the times, we probably do not use serialization directly. In such cases, I would suggest to generate a default serializable uid by clicking the quick fix option in eclipse.

Is it possible to customize the serialization process? How can we customize the Serialization process

Yes, the serialization process can be customized. When an object is serialized, objectOutputStream.writeObject (to save this object) is invoked and when an object is read, ObjectInputStream.readObject () is invoked. What most people do not know is that Java Virtual Machine provides you with an option to define these methods as per your needs. Once this is done, these two methods will be invoked by the JVM instead of the application of the default serialization process. Classes that require special handling during the serialization and deserialization process must implement special methods with these exact signatures:

private void writeObject(java.io.ObjectOutputStream out) throws IOException

private void readObject(java.io.ObjectInputStream in)

throws IOException, ClassNotFoundException;

private void readObjectNoData()throws ObjectStreamException;

Define Serialization? What do you mean by Serialization in Java

Serialization is a mechanism by which you can save or transfer the state of an object by converting it to a byte stream. This can be done in java by implementing Serialiazable interface. Serializable is defined as a marker interface which needs to be implemented for transferring an object over a network or persistence of its state to a file. Since its a marker interface, it does not contain any methods. Implementation of this interface enables the conversion of object into byte stream and thus can be transferred. The object conversion is done by the JVM using its default serialization mechanism.

What would happen if the SerialVersionUID of an object is not defined

If you don’t define serialVersionUID in your serilizable class, Java compiler will make one by creating a hash code using most of your class attributes and features. When an object gets serialized, this hash code is stamped on the object which is known as the SerialVersionUID of that object. This ID is required for the version control of an object. SerialVersionUID can be specified in the class file also. In case, this ID is not specified by you, then Java compiler will regenerate a SerialVersionUID based on updated class and it will not be possible for the already serialized class to recover when a class field is added or modified. Its recommended that you always declare a serialVersionUID in your Serializable classes.