Q. What if the main method is
declared as private?
Answer: The program compiles properly but at runtime it will give
“Main method not public.” message.
Q. What is meant by pass by
reference and pass by value in Java?
Answer: Pass by reference means, passing the address itself rather than passing the value. Pass by value
means passing a copy of the
value.
Q. If you’re overriding the method
equals() of an object, which other method you might also consider?
Answer: hashCode()
Q. What is Byte Code?
Or
Q. What gives java it’s “write once
and run anywhere” nature?
Answer: 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.
Q. Expain the reason for each
keyword of public static void main(String args[])?
Answer:
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.
Q. What are the differences between
== and .equals() ?
Or
Q. what is difference between == and
equals
Or
Q. Difference between == and equals
method
Or
Q. What would you use to compare two
String variables - the operator == or the method equals()?
Or
Q. How is it possible for two String
objects with identical values not to be equal under the == operator?
Answer:
The == operator compares two objects
to determine if they are the same object in memory i.e. present in the same
memory location. It is possible for two String objects to have the same value,
but located in different areas of memory.
== compares references while .equals
compares contents. The method public boolean equals(Object obj) is provided by
the Object class and can be overridden. The default implementation returns true
only if the object is compared with itself, which is equivalent to the equality
operator == being used to compare aliases to the object. String, BitSet, Date,
and File override the equals() method. For two String objects, value equality
means that they contain the same character sequence. For the Wrapper classes,
value equality means that the primitive values are equal.
public class EqualsTest {
public
static void main(String[] args) {
String
s1 = “abc”;
String
s2 = s1;
String
s5 = “abc”;
String
s3 = new String(”abc”);
String
s4 = new String(”abc”);
System.out.println(”==
comparison : ” + (s1 == s5));
System.out.println(”==
comparison : ” + (s1 == s2));
System.out.println(”Using
equals method : ” + s1.equals(s2));
System.out.println(”==
comparison : ” + s3 == s4);
System.out.println(”Using
equals method : ” + s3.equals(s4));
}
}
|
Output
== comparison : true
== comparison : true
== comparison : true
Using equals method : true
false
Using equals method : true
Using equals method : true
Q. What if the static modifier is
removed from the signature of the main method?
Or
Q. What if I do not provide the
String array as the argument to the method?
Answer: Program compiles. But at runtime throws an error
“NoSuchMethodError”.
Q. Why oracle Type 4 driver is named
as oracle thin driver?
Answer: 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.
Q. What is the difference between
final, finally and finalize? What do you understand by the java final keyword?
Or
Q. What is final, finalize() and
finally?
Or
Q. What is finalize() method?
Or
Q. What does it mean that a class or
member is final?
Answer:
final - declare constant
finally - handles exception
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.
Q. What is the Java API?
Answer: The Java API is a large collection of ready-made software
components that provide many useful capabilities, such as graphical user
interface (GUI) widgets.
Q. What is the GregorianCalendar
class?
Answer: The GregorianCalendar provides support for traditional
Western calendars.
Q. What is the ResourceBundle class?
Answer: 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.
Q. Why there are no global variables
in Java?
Answer: 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 create collisions
in namespace.
Q. How to convert String to Number
in java program?
Answer: 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();
Q. What is the SimpleTimeZone class?
Answer: The SimpleTimeZone class provides support for a Gregorian
calendar.
Q. What is the difference between a
while statement and a do statement?
Answer: 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.
Q. What is the Locale class?
Answer: The Locale class is used to tailor a program output to the
conventions of a particular geographic, political, or cultural region.
Q. Describe the principles of OOPS.
Answer: There are three main principals of oops which are called
Polymorphism, Inheritance and Encapsulation.
Q. Explain the Inheritance
principle.
Answer: 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
Q. What is implicit casting?
Answer: 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;
long j = i; //Implicit casting
Q. Is sizeof a keyword in java?
Answer: The sizeof is not a keyword.
Q. What is a native method?
Answer: A native method is a method that is implemented in a
language other than Java.
Q. In System.out.println(), what is
System, out and println?
Answer: System is a predefined final class, out is a PrintStream object
and println is a built-in overloaded method in the out object.
Q. What are Encapsulation,
Inheritance and Polymorphism
Or
Q. Explain the Polymorphism
principle. Explain the different forms of Polymorphism.
Answer: 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
Q. What is explicit casting?
Answer: 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
Q. What is the Java Virtual Machine
(JVM)?
Answer: The Java Virtual Machine is software that can be ported onto
various hardware-based platforms
Q. What do you understand by
downcasting?
Answer: The process of Downcasting refers to the casting from a
general to a more specific type, i.e. casting down the hierarchy
Q. What are Java Access Specifiers?
Or
Q. What is the difference between
public, private, protected and default Access Specifiers?
Or
Q. What are different types of
access modifiers?
Answer: 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
• 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
Q. Which class is the superclass of
every class?
Answer: Object.
Q. Name primitive Java types.
Answer: The 8 primitive types are byte, char, short, int, long,
float, double, and boolean. Additional is String.
Q. What is the difference between
static and non-static variables?
Or
Q. What are “class variables”?
Or
Q. What is static in java?
Or
Q. What is a static method?
Answer: 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 it’s 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.
Q. What is the difference between
the boolean & operator and the && operator?
Answer: 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.
Q. How does Java handle integer
overflows and underflows?
Answer: It uses those low order bytes of the result that can
fit into the size of the type allowed by the operation.
Q. What if I write static public
void instead of public static void?
Answer: Program compiles and runs properly.
Q. What is the difference between
declaring a variable and defining a variable?
Answer: 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.
Q. What type of parameter passing
does Java support?
Answer: 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.
Q. Explain the Encapsulation
principle.
Answer: 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.
Q. What do you understand by a variable?
Answer: 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.
Q. What do you understand by numeric
promotion?
Answer: 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.
Q. What do you understand by casting
in java language? What are the types of casting?
Answer: 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.
Q. What is the first argument of the
String array in main method?
Answer: 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.
Q. How can one prove that the array
is not null but empty?
Answer: 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.
Q. Can an application have multiple
classes having main method?
Answer: 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.
Q. When is static variable loaded?
Is it at compile time or runtime? When exactly a static block is loaded in
Java?
Answer: 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.
Q. Can I have multiple main methods
in the same class?
Answer: 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.
Q. Explain working of Java Virtual
Machine (JVM)?
Answer: 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.
Q. How can I swap two variables
without using a third variable?
Answer: Add two variables and assign the value into First variable.
Subtract the Second value with the result Value. and assign to Second variable.
Subtract the Result of First Variable With Result of Second Variable and Assign
to First Variable. Example:
int a=5,b=10;a=a+b; b=a-b; a=a-b;
An other
approach to the same question
You use an XOR swap. (BEST
APPROACH) as in case of using above approach it may goes over/under flow.
for example:
int a = 5; int b = 10;
a = a ^ b;
b = a ^ b;
a = a ^ b;
Q. What is data encapsulation?
Answer: Encapsulation may be used by creating ‘get’ and ’set’
methods in a class (JAVABEAN) which are used to access the fields of the
object. Typically the fields are made private while the get and set methods are
public. Encapsulation can be used to validate the data that is to be stored, to
do calculations on data that is stored in a field or fields, or for use in
introspection (often the case when using javabeans in Struts, for instance).
Wrapping of data and function into a single unit is called as data encapsulation.
Encapsulation is nothing but wrapping up the data and associated methods into a
single unit in such a way that data can be accessed with the help of associated
methods. Encapsulation provides data security. It is nothing but data hiding.
Q. What is reflection API? How are
they implemented?
Answer: Reflection is the process of introspecting the features and
state of a class at runtime and dynamically manipulate at run time. This is
supported using Reflection API with built-in classes like Class, Method, Fields,
Constructors etc. Example: Using Java Reflection API we can get the class name,
by using the getName method.
Q. Does JVM maintain a cache by
itself? Does the JVM allocate objects in heap? Is this the OS heap or the heap
maintained by the JVM? Why
Answer: Yes, the JVM maintains a cache by itself. It creates the
Objects on the HEAP, but references to those objects are on the STACK.
Q. What is phantom memory?
Answer: Phantom memory is false memory. Memory that does not exist
in reality.
Q. Can a method be static and synchronized?
Answer: A static method can be synchronized. If
you do so, the JVM will obtain a lock on the java.lang.
Class instance associated with the object. It is similar to saying:
Class instance associated with the object. It is similar to saying:
synchronized(XYZ.class) {
}
Q. What is difference between String
and StringTokenizer?
Answer: A StringTokenizer is utility class used to break up string.
Example:
StringTokenizer st = new
StringTokenizer(”Hello World”);
while (st.hasMoreTokens()) {
System.out.println(st.nextToken());
}
Output:
Hello
World
Question: What
is transient variable?
Answer: Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.
Answer: Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.
Note :
transient
identifies a variable not to be written out when an
instance is serialized (It can't be copied to remove
system)
volatile
indicates that the field is used by synchronized threads
and that the compiler should not attempt to perform
optimizations with it.
When more than one thread share a (volatile) data it is
checked every time. Every thread keeps the latest value of volatile variable
Question: Name the containers which uses Border
Layout as their default layout?
Answer: Containers which uses Border Layout as their default are: window, Frame and Dialog classes.
Answer: Containers which uses Border Layout as their default are: window, Frame and Dialog classes.
Question: What do you understand by Synchronization?
Answer: Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}
Answer: Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}
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