Java interview questions and answers:
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;
26. Does importing a package imports the
subpackages as well? Example: 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.
27. 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.
Example: String s; is just a declaration
while String s = new String ("abcd"); Or String s =
"abcd"; are both definitions.
28. What is the default value of an object
reference declared as an instance variable?
The default value will be null unless we
define it explicitly.
29. Can a top level class be private or protected?
No. A top level class cannot 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.
30. What type of parameter passing does Java
support?
In Java the arguments are always passed by value.
