Core Java important
interview questions and answers
1.
What is the difference between an executable file and a .class file?
2) It eliminates a lot of security protocols for data on internet.
import statement makes the jvm go to the java standard library, execute
the code there and substitute the result into the program.
1) String s = “Hello”;
2) String s = new String(“Hello”);
In the second statement, new operator is used to create the String object.
In this case, JVM always creates a new object without looking in the string
constant pool.
How can you find the hash code of an object?
The hashcode() method of ‘Object’ class in java.lang package is useful to find
the hash code of an object.
The object reference (hashcode) internally represents heap
memory where
instance variables are stored.
Instance variables are created in the objects on heap memory. Class variables
are stored on method area.
1) Using new operator.
Employee obj = new Employee();
2) Using factory methods:
NumberFormat obj = NumberFormat.getNumberInstance();
Here, we are creating NumberFormat object using the factory method
getNumberInstance();
3) Using newInstance() method. Here we should follow two steps as:
a) First store the class name Employee as a string into an object. For this
purpose, factory method forName() of the class ‘Class’ will be useful.
Class c = Class.forName(‘Employee’);
We should note that there is a class with the name Class in java.lang package.
b) Next, create another object to the class whose name is in the object c. For
this purpose, we need newInstance() method of the class ‘Class’ as:
Employee obj = (Employee)c.newInstance();
4) By cloning an already available object, we can create another object.
Employee obj1 = new Employee();
Employee obj2 = (Employee)obj1.clone();
2) It eliminates a lot of security protocols for data on internet.
import statement makes the jvm go to the java standard library, execute the code there and substitute the result into the program.
1) String s = “Hello”;
2) String s = new String(“Hello”);
In the second statement, new operator is used to create the String object. In this case, JVM always creates a new object without looking in the string constant pool.
How can you find the hash code of an object?
The hashcode() method of ‘Object’ class in java.lang package is useful to find the hash code of an object.
The object reference (hashcode) internally represents heap memory where instance variables are stored.
Instance variables are created in the objects on heap memory. Class variables are stored on method area.
1) Using new operator.
Employee obj = new Employee();
2) Using factory methods:
NumberFormat obj = NumberFormat.getNumberInstance();
Here, we are creating NumberFormat object using the factory method getNumberInstance();
3) Using newInstance() method. Here we should follow two steps as:
a) First store the class name Employee as a string into an object. For this purpose, factory method forName() of the class ‘Class’ will be useful.
Class c = Class.forName(‘Employee’);
We should note that there is a class with the name Class in java.lang package.
b) Next, create another object to the class whose name is in the object c. For this purpose, we need newInstance() method of the class ‘Class’ as:
Employee obj = (Employee)c.newInstance();
4) By cloning an already available object, we can create another object.
Employee obj1 = new Employee();
Employee obj2 = (Employee)obj1.clone();
Method Overloading
|
Method Overriding
|
Two or more methods with
the same name but with different signatures.
|
Two or more methods with
the same name and same signatures.
|
Method overloading is done
in the same class.
|
Method overriding is
done in super and sub class.
|
Return type can be same or
different.
|
Return types should also
be same or overriding method can also return a subtype of the type returned
by the overridden method. This is called a "covariant return type".
|
JVM decides which method
is called depending on the difference in the method signatures.
|
JVM decides which method
is called depending on the data type(class) of the object used to call the
method.
|
It is done when the
programmer wants to extend the already available feature.
|
It is done when the
programmer wants to provide a different implementation(body) for the same
feature.
|
Overloading is code
refinement. Same method is refined to perform a different task.
|
Overriding is code
replacement. The sub class method overrides (replaces) the super class
method.
|
49.
Can you override private methods?
Answer: No. Private methods are not available in
the sub classes, so they cannot be overridden.
50.
Can we take private methods and final methods as
same?
Answer: Yes.
The java compiler assigns the value for the private methods at the time of
compilation. Also, private methods cannot be modified at the run time. This is
the same casewith
final methods also. Neither the private methods nor the final methods can be
overriden. So, private methods can be taken as final methods.
51.
What is final?
Answer: It is used to declare constants as:
final double PI = 3.14159;
It is used to prevent inheritance, as:
final class A //sub class to A cannot be created.
final double PI = 3.14159;
It is used to prevent inheritance, as:
final class A //sub class to A cannot be created.
52.
What is the difference between dynamic
polymorphism and static polymorphism?
Dynamic polymorphism is the polymorphism exhibited
at runtime. Here, java compiler does not understand which method is called at
compilation time. Only JVM decides which method is called at runtime. Method
overloading and method overriding using instance methods are the examples for
dynamic polymorphism.
Static polymorphism is the polymorphism exhibited at compile time. Here, java compiler knows which method is called. Method overloading and method overriding using static methods; method overriding using private or final methods are examples for static polymorphism.
Static polymorphism is the polymorphism exhibited at compile time. Here, java compiler knows which method is called. Method overloading and method overriding using static methods; method overriding using private or final methods are examples for static polymorphism.
53.
Which is the super class for all the classes
including your classes also?
Answer: Object
class
There is a class with the name ‘Object’ in java.lang package which is the super class of all classes in java. Every class in java is a direct or indirect sub class of the Object class.
There is a class with the name ‘Object’ in java.lang package which is the super class of all classes in java. Every class in java is a direct or indirect sub class of the Object class.
54.
Can you write an interface without any methods?
Answer: Yes
55.
What do you call the interface without any
members?
Answer: An interface without any methods is called
marking interface or tagging interface. It marks the class objects for a
special purpose. For example, Cloneable (java.lang) and Serializable (java.io)
are two marking interfaces. Cloneable interface indicates that a particular
class objects are cloneable.
56.
Can you declare a class as abstract and final
also?
Answer: No. abstract class needs sub classes.
Final key word represents sub classes which cannot be created.
57.
What is an interface?
Answer: It is a specification of method
prototypes. All the methods of the interface are public and abstract.
58.
Why the methods of interface are public and
abstract by default?
Answer: Since they should be available to third
party vendors to provide implementation. They are abstract because their
implementation is left for third party vendors.
59.
Can you implement one interface from another?
Answer: No, we can’t. Implementing an interface
means writing body for the methods. This cannot be done again in an interface,
since none of the methods of the interface can have body.
60.
Can you write a class within an interface?
Answer: Yes, it is possible to write a class
within an interface.
61.
What is the difference between an abstract class
and an interface?
Answer:
Abstract class
|
Interface
|
It is written when there
are some common features shared by all the objects.
|
It is written when all the
features are implemented differently in different objects.
|
It contains some abstract
methods and also some concrete methods.
|
It contains only abstract
methods.
|
An abstract class
can contain instance variables also.
|
An interface cannot
contain instance variables. It contains only constants.
|
All the methods of the
abstract class should be implemented in its subclass.
|
All the abstract methods
of the interface should be implemented in this implementation classes.
|
62.
How can you call the garbage collector?
Answer: We can call garbage collector of JVM to
delete any unused variables and unreferenced objects from memory using gc()
method. This gc() method appears in both Runtime and System classes of
java.lang package. For example:
System.gc();
Runtime.getRuntime.gc();
System.gc();
Runtime.getRuntime.gc();
63.
What is CLASSPATH?
Answer: The CLASSPATH is an environment variable
that tells the java compiler where to look for class files to import. CLASSPATH
is generally set to a directory or a JAR.
64.
What is the scope of default access specifier?
Answer: Default members are available within the
same package, but not outside of the package. So their scope is package scope.
65.
What happens if main() method is written without
String args[]?
Answer: The code compiles but JVM cannot run it,
as it cannot see the main method with String args[].
66.
What are checked exceptions?
Answer: The exceptions that are checked at compile
time by the java compiler are called ‘checked exceptions’. The exceptions that
are checked by the JVM are called ‘unchecked exceptions’.
67.
What is Throwable?
Answer: Throwable is a class that represents all
errors and exceptions which may occur in java.
68.
Which is the super class for all exceptions?
Answer: Exception is the super class of all
exceptions in java.
69.
What is the difference between an exception and an
error?
Answer: An exception is an error which can be
handled. It means that when an exception happens, the programmer can do
something to avoid any harm. But an error is an error which cannot be handled.
70.
What is the difference between throws and throw?
Answer: throws clause is used when the programmer
does not want to handle the exception and throw it out of a method. Throw
clause is used when the programmer wants to throw an exception explicitly and
wants to handle it using catch block. Hence, throws and throw are
contradictory.
71.
Is it possible to re-throw the exception?
Answer: Yes, we can re-throw an exception from
catch block to another class where it can be handled.
72.
Why do we need wrapper classes?
Answer: 1. They convert primitive data types into
objects and this is needed on Internet to communicate between two applications.
2. The classes in java.util package handle only objects and hence wrapper classes help in this case also.
2. The classes in java.util package handle only objects and hence wrapper classes help in this case also.
73.
Which of the wrapper classes contains only one
constructor? (Or) which of the wrapper classes does not contain a constructor
with String as parameter?
Answer: Character
74.
What is boxing?
Answer: Converting a primitive data type into an
object is called ‘boxing’.
75.
What is unboxing?
Answer: Converting an object into its
corresponding primitive data type is called unboxing.
76.
What happens if a string like “Hello” is passed to
parseInt() method?
Answer: Ideally, a string with an integer value
should be passed to parseInt() method. So, on passing “Hello” an exception
called ‘NumberFormatException’ occurs since the parseInt() method cannot
convert the given string “Hello” into an integer value.
77.
What is a collection framework?
Answer: A collection framework is a class library
to handle groups of objects. Collection framework is implemented in java.util
package.
78.
Does a collection object store copies of other
objects or their references?
Answer: A collection object stores references of
other objects.
79.
Can you store a primitive data types into a
collection?
Answer: No, collections store only objects.
80.
For-each loop: for-each loop is like a for loop
which repeatedly executes a group of statements for each element of collection.
for(variable: collection-object){
Statements;
}
for(variable: collection-object){
Statements;
}
81.
What is the difference between Iterator and
ListIterator?
Answer: Both are useful to retrieve elements from
a collection. Iterator can retrieve the elements only in forward direction. But
ListIterator can retrieve the elements in forward and backward direction also.
So ListIterator is preferred.
82.
What is the difference between Iterator and
Enumeration?
Answer: Both are useful to retrieve elements from
a collection, but Iterator has an option to remove elements from the
collection.
83.
HashSet class:It represents set of
elements(objects). It does not guarantee the order of elements. Also it does
not allow the duplicate elements to be stored.
It has two constructors:
HashSet();
HashSet(int capacity);
HashSet(int capacity, float loadFactor);
Capacity: Represents how many elements can be stored in the HashSet initially.
LoadFactor: Determines the point where the capacity of the HashSet would be increased internally. The default initial capacity is 16 and the default load factor is 0.75.
It has two constructors:
HashSet();
HashSet(int capacity);
HashSet(int capacity, float loadFactor);
Capacity: Represents how many elements can be stored in the HashSet initially.
LoadFactor: Determines the point where the capacity of the HashSet would be increased internally. The default initial capacity is 16 and the default load factor is 0.75.
84.
What is auto boxing?
Answer: Converting a primitive data type into an
object form automatically is called ‘auto boxing’. Atuo boxing is done in
generic types.
85.
ArrayList class:It is like an array which can grow
in memory dynamically. ArrayList is not synchronized.
86.
Vector class: A vector also stores elements
(objects) similar to ArrayList, but Vector is synchronized.
87.
What is the difference between ArrayList and
Vector?
Answer:
ArrayList
|
Vector
|
ArrayList object is not
synchronized by default.
|
Vector object is
synchronized by default.
|
In case of a single
thread, using ArrayList is faster than vector.
|
In case of multiple
threads, using Vector is advisable. With a single thread, Vector becomes
slow.
|
ArrayList increases its
size every time by 50 percent (half).
|
Vector increases its size
every time by doubling it.
|
88.
Can you synchronize the ArrayList object?
Answer: Yes, we can use synchronizedList() method
to synchronize the ArrayList, as:
Collections.synchronizedList(new ArrayList());
Collections.synchronizedList(new ArrayList());
89.
HashMap class: HashMap is a collection that stores
elements in the form of key-value pairs. If key is provided later, its
corresponding value can be easily retrieved from the HashMap. Keys should be
unique. This means we cannot use duplicate data for keys in the HashMap.
HashMap is not synchronized.
The default initial capacity of this HashMap will be taken as 16 and the load factor as 0.75. Load factor represents at what level the HashMap capacity should be doubled.
The default initial capacity of this HashMap will be taken as 16 and the load factor as 0.75. Load factor represents at what level the HashMap capacity should be doubled.
90.
What is the load factor of HashMap or Hashtable?
Answer: 0.75
91.
Hashtable class: Hashtable is similar to HashMap
which can store elements in the form of key-value pairs. But Hashtable is
synchronized.
92.
What is the difference between HashMap and
Hashtable?
Answer:
HashMap
|
Hashtable
|
HashMap object is not
synchronized by default.
|
Hashtable object is
synchronized by default.
|
HashMap is faster in case
of single thread.
|
It is advisable in case of
multiple threads.
|
HashMap allows null keys
and null values to be stored.
|
Hashtable does not allow
null keys or values.
|
Iterator in the HashMap is
fail-fast. This means Iterator will produce exception if concurrent updates
are made to the HashMap.
|
Enumeration for the
Hashtable is not fail-fast. Then means even if concurrent updations are done
to Hashtable, there will not be any incorrect results produced by the
Enumeration.
|
93.
What is the difference between a Set and a List?
Answer:
Set
|
List
|
A set represents a
collection of elements. Order of the elements may change in the set.
|
A list represents ordered
collection of elements. List preserves the order of elements in which they
are entered.
|
Set will not allow
duplicate values to be stored.
|
List will allow duplicate
values.
|
Accessing elements by
their index (position number) is not possible in case of sets.
|
Accessing elements by
index is possible in lists.
|
Sets will not allow null
elements.
|
Lists will allow null
elements to be stored.
|
94.
What is the difference between System.out and
System.err?
Answer: Both are used to display messages on the
monitor. System.out is used to display normal messages as:
System.out.println(“Hello”);
System.err is used to display any error messages in the program as:
System.err.println(“This is an error”);
System.err is used to display any error messages in the program as:
System.err.println(“This is an error”);
95.
What is the advantage of stream concept?
Answer: Streams are mainly useful to move data
from one place to another place.
96.
What is the default buffer size used by any
buffered class?
Answer: 512 bytes
97.
What is serialization?
Answer: Serialization is the process of storing
object contents into a file. The class whose objects are stored in the file
should implement ‘Serializable’ interface of java.io package.
98.
Which type of variables cannot be serialized?
Answer: static and transient variables cannot be
serialized.
Once the objects re stored into a file, they can be later retrieved and used as and when needed. This is called de-serialization.
Once the objects re stored into a file, they can be later retrieved and used as and when needed. This is called de-serialization.
99.
What is deserialization?
Answer: De-Serialization is a process of reading
back the objects from a file.
100.
What is IP address?
Answer: An IP address is a unique identification
number allotted to every computer on a network or internet. IP address contains
some bytes which identify the network and actual computer inside the network.
101.
What is DNS?
Answer: Domain Naming Service is a service on
Internet that maps the IP addresses with corresponding website names.
102.
What is a socket?
Answer: A socket is a point of connection between
a server and a client on a network.
103.
What is a port number?
Answer: Port number is a 2 byte number which is
used to identify a socket uniquely.
104.
Which thread always runs in a Java program by
default?
Answer: main thread
105.
Why threads are called light weight?
Answer: Threads are light weight because they
utilize minimum resources of the system. This means they take less than memory
and less processor time.
106.
What is the difference between single tasking and
multi tasking?
Answer: Executing only one job at a time is called
single tasking. Executing several jobs at a time is called multi tasking. In
single tasking, the processor time is wasted, but in multi tasking, we can
utilize the processor time in an optimum way.
107.
How can you stop a thread in java?
Answer: First of all, we should create a boolean
type variable which stores ‘false’. When the user wants to stop the thread, we
should store ‘true’ into the variable. The status of the variable is checked in
the run() method and if it is true, the thread executes ‘return’ statement and
then stops.
108.
What is the difference between ‘extends Thread’
and ‘implements Runnable’? Which one is advantageous?
Answer: extends Thread and implements Runnable –
both are functionally same. But when we write extends Thread, there is no scope
to extend class,as multiple inheritance is not supported in java.
109.
What is thread synchronization?
Answer: When a thread is already acting on an
object, preventing any other thread from acting on the same object is called
‘Thread Synchronization’. Or ‘Thread Safe’. The object on ehich the threads are
synchronized is called ‘synchronized object’.
110.
Explain thread life cycle.
Answer: The two diagrams below explain the life
cycle of thread.
New: A new thread begins its life cycle in the new state. It
remains in this state until the program starts the thread. It is also referred
to as a born thread.Runnable (Ready-to-run): After a newly born thread is started, the thread becomes
runnable. A thread in this state is considered to be executing its task.Running: A thread is in running state that means the thread is
currently executing. There are several ways to enter in Runnable state but
there is only one way to enter in Running state: the scheduler select a thread
from runnable pool.Waiting/Blocking: Sometimes, a thread transitions to the waiting state while
the thread waits for another thread to perform a task.A thread transitions back
to the runnable state only when another thread signals the waiting thread to
continue executing.Dead: A thread can be considered dead when its run() method
completes. If any thread comes on this state that means it cannot ever run
again.
Answer: Synchronized block is useful to
synchronize a block of statements. Synchronized keyword is useful to
synchronize an entire method.
112.
What is thread deadlock?
Answer: When a thread has locked an object and
waiting for another object to be released by another thread and the other
thread is also waiting for the first thread to release the first object, both
the threads will continue waiting forever. This is called ‘Thread deadlock’.
113.
What is the difference between the sleep() and
wait() methods?
Answer: Both the sleep() and wait() methods are
used to suspend a thread execution for a specified time. When slepp() is
executed inside a synchronized block, the object is still under lock. When
wait() method is executed, it breaks the synchronized block, so that the object
lock is removed and it is available.
114.
What is the default priority of a thread?
Answer: When a thread is created, by default its
priority will be 5.
115.
What is a daemon thread?
Answer: A daemon thread is a thread that executes
continuously. Daemon threads are service providers for other threads or
objects. It generally provides a background processing.
116.
What is JDBC?
Answer: JDBC is an API that is useful to write
java programs to connect to any database, retrieve the data from the database
and utilize the data in a java program.
117.
What is the database driver?
Answer: A database is a set of classes and
interfaces, written according to JDBC API to communicate with a database.
118.
How can you register a driver?
Answer: To
register a database driver, we can follow one of the 4 options:
- By creating an object to driver class.
- By sending driver class object to DriverManager.registerDriver() method.
- By sending the driver class name to class.forName() method.
- By using System class getProperty() method.
- By creating an object to driver class.
- By sending driver class object to DriverManager.registerDriver() method.
- By sending the driver class name to class.forName() method.
- By using System class getProperty() method.
119.
What is DSN?
Answer: Data Source Name is a name given to the
database to identify it in the java program. The DSN is linked with the actual
location of the database.
120.
What is resultset?
Answer: ResultSet is an object that contains the
results (rows) fo executing a SQL statement on a database.
121.
Will the performance of a JDBC program depend on
the driver?
Answer: Yes, each driver offers a different
performance.
122.
What is parsing?
Answer: Parsing represents checking the syntax and
grammar of a statement as a whole and also word by word.
123.
What is the differece between Statement and
PreparedStatement?
Answer: Statement parses a statement before its
execution on the database. This parsing is done every time the statement is
executed and hence it may take more time when the same statement gets executed
repeatedly. PreparedStatement conducts parsing only once when the same
statement is executed repeatedly and hence it gives better performance.
124.
What are stored procedures?
Answer: A stored procedure represents a set of
statements that is stored and executed at database server sending the results
to the client.
create or replace procedure myproc(no in int, isal out float) as
salary float;
begin
select sal into salary from emptab where eno = no;
isal := salary + 500;
end;
CallableStatement stmt = con.prepareCall(“{call myproc(?,?)}”);
stmt.setInt(1,1004);
stmt.registerOutParameter(2,Types.FLOAT);
stmt.execute();
float incsal = stmt.getFloat(2);
create or replace procedure myproc(no in int, isal out float) as
salary float;
begin
select sal into salary from emptab where eno = no;
isal := salary + 500;
end;
CallableStatement stmt = con.prepareCall(“{call myproc(?,?)}”);
stmt.setInt(1,1004);
stmt.registerOutParameter(2,Types.FLOAT);
stmt.execute();
float incsal = stmt.getFloat(2);
125.
What is the use of CallableStatement?
Answer: CallableStatement is useful to call stored
procedures and functionswhich run at a database server and get the results into
the client.
126.
What is scrollable result set?
Answer: It represents a resultset object where
moving in forward and backward direction is possible. It also provides methods
to update the rows in the resultset.
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