JAVA Interview Questions
|
Previous |
1 |
2 |
3 |
4 |
Next |
Sample JAVA Programs |
|
|
| 1. What if the main method is declared as private? |
| The program compiles properly
but at runtime it will give “Main method not public.” message. |
| 2. What is meant by
pass by reference and pass by value in Java? |
| Pass by reference means,
passing the address itself rather than passing the value. Pass by value means
passing a copy of the value. |
| 3. If you’re
overriding the method equals() of an object, which other method you might also
consider? |
| hashCode().
|
| 4. What is Byte Code? (or) What gives java it’s
“write once and run anywhere” nature? |
| All Java programs are compiled
into class files that contain bytecodes. These byte codes can be run in any
platform and hence java is said to be platform independent.
|
| 5. Expain the reason
for each keyword of public static void main(String args[])? |
public-
main(..) is the first method called by java environment when a program is
executed so it has to accessible from java environment. Hence the access
specifier has to be public.
static:
Java environment should be able to call this method without creating an instance
of the class , so this method must be declared as static.
void:
main does not return anything so the return type must be void.
The argument String indicates
the argument type which is given at the command line and arg is an array for
string given during command line.
|
| 6. What is encapsulation? |
|
Encapsulation describes the
ability of an object to hide its data and methods from the rest of the world.
|
7. What if the
static modifier is removed from the signature of the main method?
(or)
What if I do not
provide the String array as the argument to the method?
|
| Program compiles. But at
runtime throws an error “NoSuchMethodError”.
|
| 8. Why oracle Type 4 driver is named as oracle thin driver? |
| Oracle provides a Type 4 JDBC
driver, referred to as the Oracle “thin” driver. This driver includes its
own implementation of a TCP/IP version of Oracle’s Net8 written entirely in
Java, so it is platform independent, can be downloaded to a browser at runtime,
and does not require any Oracle software on the client side. This driver
requires a TCP/IP listener on the server side, and the client connection string
uses the TCP/IP port address, not the TNSNAMES entry for the database name.
|
9. What is the difference between final, finally and finalize?
(or)
What does it mean that a class or member is final?
|
o final -declare constant
o finally - handles exception
o finalize - helps in garbage collection.
Variables defined in an
interface are implicitly final. A final class can’t be extended i.e., final
class may not be subclassed. This is done for security reasons with basic
classes like String and Integer. It also allows the compiler to make some
optimizations, and makes thread safety a little easier to achieve. A final
method can’t be overridden when its class is inherited. You can’t change
value of a final variable (is a constant). finalize() method is used just before
an object is destroyed and garbage collected. finally, a key word used in
exception handling and will be executed whether or not an exception is thrown.
For example, closing of open connections is done in the finally method.
|
| 10. What is the Java API? |
| The Java API is a large
collection of ready-made software components that provide many useful
capabilities, such as graphical user interface (GUI) widgets.
|
| 11. What is the GregorianCalendar class? |
| The GregorianCalendar provides
support for traditional Western calendars. |
| 12. What is the ResourceBundle class? |
| The ResourceBundle class is
used to store locale-specific resources that can be loaded by a program to
tailor the program’s appearance to the particular locale in which it is being
run.
|
| 13. Why there are no global variables in Java? |
Global variables are globally
accessible. Java does not support globally accessible variables due to following
reasons:
» The global variables breaks the referential transparency.
» Global variables creates collisions in namespace.
|
| 14. How to convert
String to Number in java program? |
The valueOf() function of
Integer class is is used to convert string to Number. Here is the code example:
String numString = “1000″;
int id=Integer.valueOf(numString).intValue(); |
| 15. What is the
SimpleTimeZone class? |
|
The SimpleTimeZone class
provides support for a Gregorian calendar.
|
| 16. What is the
difference between a while statement and a do statement? |
| A while statement (pre test)
checks at the beginning of a loop to see whether the next loop iteration should
occur. A do while statement (post test) checks at the end of a loop to see
whether the next iteration of a loop should occur. The do statement will always
execute the loop body at least once.
|
| 17. What is the Locale
class? |
| The Locale class is used to
tailor a program output to the conventions of a particular geographic,
political, or cultural region.
|
| 18. Describe the
principles of OOPS. |
| There are three main principals
of oops which are called Polymorphism, Inheritance and Encapsulation.
|
| 19. Explain the
Inheritance principle. |
| Inheritance is the process by
which one object acquires the properties of another object. Inheritance allows
well-tested procedures to be reused and enables changes to make once and have
effect in all relevant places.
|
| 20. What is implicit
casting? |
Implicit casting is the process
of simply assigning one entity to another without any transformation guidance to
the compiler. This type of casting is not permitted in all kinds of
transformations and may not work for all scenarios.
Example:
int i = 1000;
ong j = i; //Implicit casting
|
| 21. Is sizeof a keyword
in java? |
|
The sizeof operator is not a
keyword.
|
| 22. What is a native
method? |
| A native method is a method
that is implemented in a language other than Java.
|
| 23. In
System.out.println(), what is System, out and println? |
|
System is a predefined final
class, out is a PrintStream object and println is a built-in overloaded method
in the out object.
|
24. What are
Encapsulation, Inheritance and Polymorphism
(or)
Explain the
Polymorphism principle. Explain the different forms of Polymorphism.
|
Polymorphism in simple terms
means one name many forms. Polymorphism enables one entity to be used as a
general category for different types of actions. The specific action is
determined by the exact nature of the situation.
Polymorphism exists in three distinct forms in Java:
• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface.
|
| 25. What is explicit
casting? |
Explicit casting in the process
in which the complier are specifically informed to about transforming the
object.
Example
long i = 700.20;
int j = (int) i; //Explicit
casting.
|
| 26. What is the Java
Virtual Machine (JVM)? |
| The Java Virtual Machine is
software that can be ported onto various hardware-based platforms.
|
| 27. What do you
understand by downcasting? |
| The process of Downcasting
refers to the casting from a general to a more specific type, i.e. casting down
the hierarchy. |
28. What are Java Access Specifiers?
(or)
What is the difference between public, private, protected and default Access Specifiers?
(or)
What are different
types of access modifiers?
|
Access specifiers are keywords
that determine the type of access to the member of a class. These keywords are
for allowing,
privileges to parts of a program such as functions and variables. These are:
• Public : accessible to all classes
• Protected : accessible to the classes within the same package and
any subclasses.
• Private : accessible only to the class to which they belong
• Default : accessible to the class to which they belong and to
subclasses within the same package.
|
| 29. Which class is the superclass of every class? |
| Object. |
| 30. Name primitive Java types. |
| The 8 primitive types are byte,
char, short, int, long, float, double, and boolean. |
31. What is the difference between static and non-static variables?
(or)
What is a static method?
|
A static variable is associated
with the class as a whole rather than with specific instances of a class. Each
object will share a common copy of the static variables i.e. there is only one
copy per class, no matter how many objects are created from it. Class variables
or static variables are declared with the static keyword in a class. These are
declared outside a class and stored in static memory. Class variables are mostly
used for constants. Static variables are always called by the class name. This
variable is created when the program starts and gets destroyed when the programs
stops. The scope of the class variable is same an instance variable. Its initial
value is same as instance variable and gets a default value when its not
initialized corresponding to the data type. Similarly, a static method is a
method that belongs to the class rather than any object of the class and
doesn’t apply to an object or even require that any objects of the class have
been instantiated.
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 non-static method. In other words, you
can’t change a static method into an instance method in a subclass.
Non-static variables take on
unique values with each object instance.
|
| 32. What is the
difference between the boolean & operator and the && operator? |
| If an expression involving the
boolean & operator is evaluated, both operands are evaluated, whereas the
&& operator is a short cut operator. When an expression involving the
&& operator is evaluated, the first operand is evaluated. If the first
operand returns a value of true then the second operand is evaluated. If the
first operand evaluates to false, the evaluation of the second operand is
skipped. |
| 33. How does Java handle integer overflows and underflows? |
| It uses those low order bytes
of the result that can fit into the size of the type allowed by the operation. |
| 34. What if I write static public void instead of public static void? |
| Program compiles and runs
properly. |
| 35. What is the difference between declaring a variable and defining a variable? |
| In declaration we only mention
the type of the variable and its name without initializing it. Defining means
declaration + initialization. E.g. String s; is just a declaration while String
s = new String (”bob”); Or String s = “bob”; are both definitions. |
| 36. What type of parameter passing does Java support? |
| In Java the arguments
(primitives and objects) are always passed 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. |
| 37. Explain the
Encapsulation principle. |
| Encapsulation is a process of
binding or wrapping the data and the codes that operates on the data into a
single entity. This keeps the data safe from outside interface and misuse.
Objects allow procedures to be encapsulated with their data to reduce potential
interference. One way to think about encapsulation is as a protective wrapper
that prevents code and data from being arbitrarily accessed by other code
defined outside the wrapper. |
| 38. What do you
understand by a variable? |
| Variable is a named memory
location that can be easily referred in the program. The variable is used to
hold the data and it can be changed during the course of the execution of the
program. |
| 39. What do you
understand by numeric promotion? |
|
The Numeric promotion is the
conversion of a smaller numeric type to a larger numeric type, so that integral
and floating-point operations may take place. In the numerical promotion process
the byte, char, and short values are converted to int values. The int values are
also converted to long values, if necessary. The long and float values are
converted to double values, as required.
|
| 40. What do you
understand by casting in java language? What are the types of casting? |
| The process of converting one
data type to another is called Casting. There are two types of casting in Java;
these are implicit casting and explicit casting. |
| 41. 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. If we do not provide any arguments on the command
line, then the String array of main method will be empty but not null. |
| 42. How can one prove
that the array is not null but empty? |
| Print array.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 array.length. |
| 43. Can an application
have multiple classes having main method? |
| Yes. 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. |
| 44. When is static
variable loaded? Is it at compile time or runtime? When exactly a static block
is loaded in Java? |
| Static variable are loaded when
classloader brings the class to the JVM. It is not necessary that an object has
to be created. Static variables will be allocated memory space when they have
been loaded. The code in a static block is loaded/executed only once i.e. when
the class is first initialized. A class can have any number of static blocks.
Static block is not member of a class, they do not have a return statement and
they cannot be called directly. Cannot contain this or super. They are primarily
used to initialize static fields. |
| 45. Can I have multiple
main methods in the same class? |
We can have multiple
overloaded main methods but there can be only
one main method with the following signature :
public static void main(String[] args) {}
No the program fails to
compile. The compiler says that the main method is already defined in the
class.
|
| 46. Explain working of
Java Virtual Machine (JVM)? |
| JVM is an abstract computing
machine like any other real computing machine which first converts .java file
into .class file by using Compiler (.class is nothing but byte code file.) and
Interpreter reads byte codes. |
| 47. Why is
UTFDataFormatException thrown by DataOutputStream.writeUTF() when serializing a
String? |
| DataOutputStream.writeUTF()
does not support
writing out strings larger than 64K. The first two bytes of a UTF string in the
stream are the length of the string. If a java.lang.String is larger than 64K,
it needs to be stored in the stream by an alternative method rather than
depending on the default method of storing a String in the stream, writeUTF. |
| 48. Why is OutOfMemoryError thrown after writing a large number of objects into an ObjectOutputStream? |
| The ObjectOutputStream
maintains a table mapping objects written into the stream to a handle. The first
time an object is written to a stream, its contents are written into the stream;
subsequent writes of the object result in a handle to the object being written
into the stream. This table maintains references to objects that might otherwise
be unreachable by an application, thus, resulting in an unexpected situation of
running out of memory. A call to the ObjectOutputStream.reset() method resets
the object/handle table to its initial state, allowing all previously written
objects to be eligible for garbage collection. |
| 49. How can I get the serialVersionUID of a class? |
| By
running the serialver tool with the name of the class as the command line
argumet, as shown in the example that follows: serialver java.lang.String. |
| 50. What is abstraction? |
| An abstraction is an idea,
concept, or word which defines the phenomena which make up the concrete events
or things which the abstraction refers to, the referents. |
|
Previous |
1 |
2 |
3 |
4 |
Next |
Sample JAVA Programs |
Back to Top |
