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;
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.
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.
What is the default value of an object
reference declared as an instance variable?
null unless
we define it explicitly.
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.
What type of parameter passing does Java
support?
In Java the arguments are always passed by value .
Primitive data types are passed by reference
or pass by value?
Primitive
data types are passed by value.
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 .
What is serialization?
Serialization
is a mechanism by which you can save the state of an object by converting it to
a byte stream.
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.
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.
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.
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.
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.
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.
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