HCL Interview Questions-Part 3
- Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
- Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
- What is the difference between declaring a variable and defining a variable?
- What is the default value of an object reference declared as an instance variable?
- Can a top level class be private or protected?
- What type of parameter passing does Java support?
- Primitive data types are passed by reference or pass by value?
- Objects are passed by value or by reference?
- What is serialization?
- How do I serialize an object to a file?
- Which methods of Serializable interface should I implement?
- How can I customize the seralization process? i.e. how can one have a control over the serialization process?
- What is the common usage of serialization?What is Externalizable interface?
- When you serialize an object, what happens to the object references included in the object? What one should take care of while serializing the object?
- What happens to the static fields of a class during serialization?
Are the imports checked for validity
at compile time? e.g. will the code containing an import such as java.lang.ABCD
compile?
Yes the
imports are checked for the semantic validity at compile time. The code
containing above line of import will not compile. It will throw an error
saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;
[capptitudebank.blogspot.com]
Does importing a package imports the
subpackages as well? e.g. Does importing com.MyTest.* also import
com.MyTest.UnitTests.*?
No you will
have to import the subpackages explicitly. Importing com.MyTest.* will import
classes in the package MyTest only. It will not import any class in any of it's
subpackage.
[capptitudebank.blogspot.com]
What is the difference between
declaring a variable and defining a variable?
In
declaration we just mention the type of the variable and it's name. We do not
initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new
String ("abcd"); Or String s = "abcd"; are both
definitions.
[capptitudebank.blogspot.com]
What is the default value of an
object reference declared as an instance variable?
null unless
we define it explicitly.
[capptitudebank.blogspot.com]
Can a top level class be private or
protected?
No. A top
level class can not be private or protected. It can have either
"public" or no modifier. If it does not have a modifier it is
supposed to have a default access.If a top level class is declared as private
the compiler will complain that the "modifier private is not allowed
here". This means that a top level class can not be private. Same is the
case with protected.
[capptitudebank.blogspot.com]
What type of parameter passing does
Java support?
In Java the
arguments are always passed by value .
[ Update
from Eki and Jyothish Venu]
Primitive data types are passed by
reference or pass by value?
Primitive
data types are passed by value.
[capptitudebank.blogspot.com]
Objects are passed by value or by
reference?
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 object .
[ Update
from Eki and Jyothish Venu]
What is serialization?
Serialization
is a mechanism by which you can save the state of an object by converting it to
a byte stream.
[capptitudebank.blogspot.com]
How do I serialize an object to a
file?
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 file.
[capptitudebank.blogspot.com]
Which methods of Serializable
interface should I implement?
The
serializable interface is an empty interface, it does not contain any methods.
So we do not implement any methods.
[capptitudebank.blogspot.com]
How can I customize the seralization
process? i.e. how can one have a control over the serialization process?
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 process.
[capptitudebank.blogspot.com]
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.
[capptitudebank.blogspot.com]
What is Externalizable interface?
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.
[capptitudebank.blogspot.com]
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.
[capptitudebank.blogspot.com]
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.
[capptitudebank.blogspot.com]
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.
[
Received from Sandesh Sadhale Modified after P.John David comments.] 
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