JAVA BASIC QUESTIONS
1. What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to
create objects of that class. It has the
same name as the class itself, has no return type, and is invoked
using the new operator.
A method is an ordinary member function of a class. It has its own
name, a return type (which may be
void), and is invoked using the dot operator.
2. What is the purpose of garbage collection in Java, and when is
it used?
The purpose of garbage collection is to identify and discard
objects that are no longer needed by a
program so that their resources can be reclaimed and reused.
A Java object is subject to garbage collection when it becomes
unreachable to the program in which it
is used.
3. Describe synchronization in respect to multithreading.
With respect to multithreading, synchronization is the capability
to control the access of multiple
threads to shared resources.
Without synchonization, it is possible for one thread to modify a
shared variable while another thread
is in the process of using or updating same shared variable. This
usually leads to significant errors.
4. What is an abstract class?
Abstract class must be extended/subclassed (to be useful). It
serves as a template. A class that is
abstract may not be instantiated (ie. you may not call its
constructor), abstract class may contain
static data.
Any class with an abstract method is automatically abstract
itself, and must be declared as such. A
class may be declared abstract even if it has no abstract methods.
This prevents it from being
instantiated.
5. What is the difference between an Interface and an Abstract
class?
An abstract class can have instance methods that implement a
default behavior. An Interface can only
declare constants and instance methods, but cannot implement
default behavior and all methods are
implicitly abstract.
An interface has all public members and no implementation. An
abstract class is a class which may
have the usual flavors of class members (private, protected,
etc.), but has some abstract
methods.
6. Explain different way of using thread?
The thread could be implemented by using runnable interface
or by inheriting from the Thread class.
The former is more advantageous, 'cause when you are going for
multiple inheritance, the only
interface can help.
7. What is an Iterator?
Some of the collection classes provide traversal of their contents
via
a java.util.Iterator interface. This interface allows you to walk
through a collection of objects,
operating on each object in turn.
Remember when using Iterators that they contain a snapshot of the
collection at the time the Iterator
was obtained; generally it is not advisable to modify the
collection itself while traversing an Iterator.
8. State the significance of public, private, protected, default
modifiers both singly and in
combination and state the effect of package relationships on
declared items qualified by
these modifiers.
public: Public class is visible in other packages, field is
visible everywhere (class must be public
too)
private : Private variables or methods may be used only by an
instance of the same class that
declares the variable or method, A private feature may only be
accessed by the class that owns the
feature.
protected : Is available to all classes in the same package and also
available to all subclasses of
the class that owns the protected feature. This access is provided
even to subclasses that reside in a
different package from the class that owns the protected feature.
What you get by default ie, without any access modifier (ie,
public private or protected). It means that
it is visible to all within a particular package.
9. What is static in java?
Static means one per class, not one for each object no matter how
many instance of a class might
exist. This means that you can use them without creating an
instance of a class.Static methods are
implicitly final, because overriding is done based on the type of
the object, and static methods are
attached to a class, not an object.
A static method in a superclass can be shadowed by another static
method in a subclass, as long as
the original method was not declared final. However, you can't
override a static method with a
nonstatic method. In other words, you can't change a static method
into an instance method in a
subclass.
10. What is final class?
A final class can't be extended ie., final class may not be
subclassed. A final method can't be
overridden when its class is inherited. You can't change value of
a final variable (is a constant).
11. What if the main() method is declared as private?
The program compiles properly but at runtime it will give
"main() method not public." message.
12. What if the static modifier is removed from the signature of
the main() method?
Program compiles. But at runtime throws an error "NoSuchMethodError".
13. What if I write static public void instead of public static
void?
Program compiles and runs properly.
14. What if I do not provide the String array as the argument to
the method?
Program compiles but throws a runtime error
"NoSuchMethodError".
15. What is the first argument of the String array in main()
method?
The String array is empty. It does not have any element. This is
unlike C/C++ where the first element
by default is the program name.
16. If I do not provide any arguments on the command line, then
the String array of main()
method will be empty or null?
It is empty. But not null.
17. How can one prove that the array is not null but empty using
one line of code?
Print args.length. It will print 0. That means it is empty. But if
it would have been null then it would
have thrown a NullPointerException on attempting to print args.length.
18. What environment variables do I need to set on my machine in
order to be able to run
Java programs?
CLASSPATH
and PATH are the two variables.
19. Can an application have multiple classes having main() method?
Yes it is possible. While
starting the application we mention the class name to be run. The JVM will
look for the Main method only in the class whose name you have mentioned.
Hence there is not conflict amongst the multiple classes having main()
method.
20. Can I have multiple main() methods in the same class?
No the program fails to compile. The compiler says that the main()
method is already defined in the
class.
21. Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.
22. Can I import same package/class twice? Will the JVM load the
package twice at
runtime?
One can import the same package or same class multiple times.
Neither compiler nor JVM complains
about it. And the JVM will internally load the class only once no
matter how many times you import
the same class.
23. What are Checked and UnChecked Exception?
A checked exception is some subclass of Exception (or Exception
itself), excluding class
RuntimeException and its subclasses. Making an exception checked
forces client programmers to deal
with the possibility that the exception will be thrown.
Example: IOException thrown by java.io.FileInputStream's read() method·
Unchecked exceptions are RuntimeException and any of its
subclasses. Class Error and its subclasses also are unchecked. With an
unchecked exception, however, the compiler doesn't force client
programmers either to catch the exception or declare it in a
throws clause. In fact, client programmers
may not even know that the exception could be thrown.
Example: StringIndexOutOfBoundsException thrown by String's charAt()
method· Checked
exceptions must be caught at compile time. Runtime exceptions do
not need to be. Errors often
cannot be.
24. What is Overriding?
When a class defines a method using the same name, return type,
and arguments as a method in its
superclass, the method in the class overrides the method in the
superclass.
When the method is invoked for an object of the class, it is the
new definition of the method that is
called, and not the method definition from superclass. Methods may
be overridden to be more public,
not more private.
25. Are the imports checked for validity at compile time? Example:
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;
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