Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.
Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
38. What is the common usage of serialization?
Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.
39. When you serialize an object, what happens to the object references included in the object?
The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process.
Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.
40. What one should take care of while serializing the object?
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
41. What happens to the static fields of a class during serialization?
There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.
42. Does Java provide any construct to find out the size of an object?
No, there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.
43. What are wrapper classes?
Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes.
They are example: Integer, Character, Double etc.
44. Why do we need wrapper classes?
It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also.
Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.
45. What are checked exceptions?
Checked exception are those which the Java compiler forces you to catch.
Example: IOException are checked exceptions.
46. What are runtime exceptions?
Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.
47. What is the difference between error and an exception?
An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error.
These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. Example: FileNotFoundException will be thrown if the specified file does not exist. Or aNullPointerException will take place if you try using a null reference.
In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).
48. How to create custom exceptions?
Your class should extend class Exception, or some more specific type thereof.