38.
Question :Objects are passed by value or by reference?
39. Question :What is serialization?
40. Question :How do I serialize an object to a file?
41. Question :Which methods of Serializable interface should I implement?
42. Question :How can I customize the seralization process? i.e. how can one have a control over the serialization process?
43. Question :What is the common usage of serialization?
44. Question :What is Externalizable interface?
45. Question :When you serialize an object, what happens to the object references included in the object?
46. Question :What one should take care of while serializing the object?
47. Question :What happens to the static fields of a class during serialization?
48. Question :Does Java provide any construct to find out the size of an object?
49. Question :Give a simplest way to find out the time a method takes for execution without using any profiling tool?
50. Question :What are wrapper classes?
51. Question :Why do we need wrapper classes?
52. Question :What are checked exceptions?
53. Question :What are runtime exceptions?
54. Question :What is the difference between error and an exception?
55. Question :How to create custom exceptions?
56. Question :If I want an object of my class to be thrown as an exception object, what should I do?
57. Question :If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
58. Question :How does an exception permeate through the code?
Answer :
Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object39. Question :What is serialization?
Answer :
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream40. Question :How do I serialize an object to a file?
Answer :
The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file41. Question :Which methods of Serializable interface should I implement?
Answer :
The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods42. Question :How can I customize the seralization process? i.e. how can one have a control over the serialization process?
Answer :
Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process43. Question :What is the common usage of serialization?
Answer :
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 serilazed44. Question :What is Externalizable interface?
Answer :
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 methods45. Question :When you serialize an object, what happens to the object references included in the object?
Answer :
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 along with the original object46. Question :What one should take care of while serializing the object?
Answer :
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException47. Question :What happens to the static fields of a class during serialization?
Answer :
There are three exceptions in which serialization does not necessarily read and write to the stream. These are 1. Serialization ignores static fields, because they are not part of ay particular state . 2. Base class fields are only hendled if the base class itself is serializable. 3. Transient fields48. Question :Does Java provide any construct to find out the size of an object?
Answer :
No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java49. Question :Give a simplest way to find out the time a method takes for execution without using any profiling tool?
Answer :
Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution. To put it in code... long start = System.currentTimeMillis (); method (); long end = System.currentTimeMillis (); System.out.println ("Time taken for execution is " + (end - start)); Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing50. Question :What are wrapper classes?
Answer :
Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc51. Question :Why do we need wrapper classes?
Answer :
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 object52. Question :What are checked exceptions?
Answer :
Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions53. Question :What are runtime exceptions?
Answer :
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 time54. Question :What is the difference between error and an exception?
Answer :
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. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException 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.)55. Question :How to create custom exceptions?
Answer :
Your class should extend class Exception, or some more specific type thereof56. Question :If I want an object of my class to be thrown as an exception object, what should I do?
Answer :
The class should extend from Exception class. Or you can extend your class from some more precise exception type also57. Question :If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
Answer :
One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well58. Question :How does an exception permeate through the code?
Answer :
An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates
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