Table
of Contents
- Basic Questions
- OOPs interview Questions
- Exception handling interview Questions
- Java Multithreading interview Questions
- Serialization interview Questions
- String Interview Questions
- Java Collections interview Questions
- Applet interview Questions
Basic
Questions
Q) Is Java platform
independent?
Yes. Java is a platform independent language. We can write java code on one
platform and run it on another platform. For e.g. we can write and compile the
code on windows and can run it on Linux or any other supported platform. This
is one of the main features of java.
Q) What all memory
areas are allocated by JVM?
Heap, Stack, Program Counter Register and Native Method Stack
Q) Java vs. C ++?
The following features of java make it different from the C++:
- Simple
- Multi-threaded
- Distributed Application
- Robust
- Security
- Complexities are removed
(Pointers, Operator overloading, Multiple inheritance).
Q) What is javac ?
It produces the java byte code from *.java file. It is the intermediate
representation of your source code that contains instructions.
Q) What is class?
Class is nothing but a template that describes the data and behavior associated
with instances of that class
Q) What is the base
class of all classes?
java.lang.Object
Q) Path and ClassPath
Path specifies the location of .exe files while classpath is used for
specifying the location of .class files.
Q) Different Data
types in Java.
- byte – 8 bit (are esp. useful when
working with a stream of data from a network or a file).
- short – 16 bit
- char – 16 bit Unicode
- int – 32 bit (whole number)
- float – 32 bit (real number)
- long – 64 bit (Single precision)
- double – 64 bit (double precision)
Note: Any time you have an integer expression involving bytes,
shorts, ints and literal numbers, the entire expression is promoted to int
before the calculation is done.
Q) What is Unicode?
Java uses Unicode to represent the characters. Unicode defines a fully
international character set that can represent all of the characters found in
human languages.
Q) What are Literals?
A literal is a value that may be assigned to a primitive or string variable or
passed as an argument to a method.
Q) Dynamic
Initialization?
Java allows variables to be initialized dynamically, using any expression valid
at the time the variable is declared.
Q) What is Type
casting in Java?
To create a conversion between two incompatible types, we must use a cast.
There are two types of casting in java: automatic casting (done automatically)
and explicit casting (done by programmer).
Q) Arrays?
An array is a group of fixed number of same type values. Read more about Arrays
here.
Q) What is BREAK
statement in java?
It is also referred as terminator. In Java, the break statement can be used in
following two cases:
- It terminates a statement sequence
in a switch-case statement.
- It can be used to come out of a
loop
Q) Why can’t I do
myArray.length () ? Arrays are just objects, right?
Yes, the specification says that arrays are object references just like classes
are. You can even invoke the methods of Object such as toString () and hashCode
() on an array. However, length is a data item of an array and not a method. So
you have to use myArray.length.
Q) How can I put all
my classes and resources into one file and run it?
Use a JAR file. Put all the files in a JAR, then run the app like this:
Java -jar [-options] jarfile [args...]
Q) Can I declare a
data type inside loop in java?
Any Data type declaration should not be inside the loop.
Q) Advantage over jdk
1.0 vs. jdk 1.1 ?
Jdk1.1 release consists of Java Unicode character to support the multiple
language fonts, along with Event Handling, Java security, Java Beans, RMI, SQL
are the major feature provided.
Q) java.lang.* get
imported by default. For using String and Exception classes, you don’t need to
explicitly import this package. The major classes inside this package are
- Object class
- Data type wrapper classes
- Math class
- String class
- System and Runtime classes
- Thread classes
- Exception classes
- Process classes
- Class classes
Q) Arrays can be
defined in different ways. Write them down.
int arr[] = null;
int arr[][] = new int arr[][];
int [][] arr = new arr [][];
int [] arr [] = new arr[][];
OOPs
Interview Questions
Q) Four main
principles of OOPS language?
- Inheritance
- Polymorphism
- Data Encapsulation
- Abstraction
Q) What is
inheritance?
The process by which one class acquires the properties and functionalities of
another class. Inheritance brings reusability of code in a java application. Read
more here.
Q) Does Java support
Multiple Inheritances?
When a class extends more than one classes then it is called multiple
inheritance. Java doesn’t support multiple inheritance whereas C++ supports it,
this is one of the difference between java and C++. Refer this: Why java doesn’t support multiple inheritance?
Q) What is
Polymorphism and what are the types of it?
Polymorphism is the ability of an object to take many forms. The most common
use of polymorphism in OOPs is to have more than one method with the same name
in a single class. There are two types of polymorphism: static polymorphism and
dynamic polymorphism, read them in detail here.
Q) What is the method
overriding?
It is a feature using which a child class overrides the method of parent class.
It is only applicable when the method in child class has the signature same as
parent class. Read more about method overriding here.
Q) Can we override a
static method?
No, we cannot override a static method.
Q) What is method
overloading?
Having more than one method with the same name but different number, sequence
or types of arguments is known is method overloading. Read
more about it here.
Q) Does Java support
operator overloading?
Operator overloading is not supported in Java.
Q) Can we overload a
method by just changing the return type and without changing the signature of
method?
No, We cannot do this.
Q) Is it possible to
overload main() method of a class?
Yes, we can overload main() method as well.
Q) What is the
difference between method overloading and method overriding?
There are several differences; You can read them here: Overloading Vs Overriding.
Q) What is static and
dynamic binding?
Binding refers to the linking of method call to its body. A binding that
happens at compile time is known as static binding while binding at runtime is
known as dynamic binding.
Q) What is
Encapsulation?
Encapsulation means the localization of the information or knowledge within an
object.
Encapsulation is also called as “Information Hiding”. Read it
here in detail.
Q) Abstract class?
An abstract class is a class which can’t be instantiated (we cannot create the
object of abstract class), we can only extend such classes. It provides the
generalized form that will be shared by all of its subclasses, leaving it to
each subclass to fill in the details. We can achieve partial abstraction using
abstract classes, to achieve full abstraction we use interfaces.
Q) What is Interface
in java?
An interface is a collection of abstract methods. A class implements an
interface, thereby inheriting the abstract methods of the interface. Read
more about interface here.
Q) What is the
difference between abstract class and interface?
1) abstract class can have abstract and non-abstract methods. An interface can
only have abstract methods.
2) An abstract class can have static methods but an interface cannot have
static methods.
3) abstract class can have constructors but an interface cannot have
constructors.
Q) Which access
modifiers can be applied to the inner classes?
public ,private , abstract, final, protected.
Q) What are
Constructors?
Constructors are used for creating an instance of a class, they are invoked
when an instance of class gets created. Constructor name and class name should
be same and it doesn’t have a return type. Read
more about constructors here.
Q) Can we inherit the
constructors?
No, we cannot inherit constructors.
Q) Can we mark
constructors final?
No, Constructor cannot be declared final.
Q) What is default
and parameterized constructors?
Default: Constructors with no arguments are known as default constructors, when
you don’t declare any constructor in a class, compiler creates a default one
automatically.
Parameterized: Constructor with
arguments are known as parameterized constructors.
Q) Can a constructor
call another constructor?
Yes. A constructor can call the another constructor of same class using this
keyword. For e.g. this() calls the default constructor.
Note: this() must be the first statement in the calling constructor.
Q) Can a constructor
call the constructor of parent class?
Yes. In fact it happens by default. A child class constructor always calls the
parent class constructor. However we can still call it using super keyword. For
e.g. super() can be used for calling super class default constructor.
Note: super() must be the first
statement in a constructor.
Q)THIS keyword?
The THIS keyword is a reference to the current object.
Q) Can this keyword
be assigned null value?
No, this keyword cannot have null values assigned to it.
Q) Explain ways to
pass the arguments in Java?
In java, arguments can be passed in 2 ways,
Pass by value – Changes made to the
parameter of the subroutines have no effect on the argument used to call it.
Pass by reference – Changes made to the parameter will affect the argument used
to call the subroutine.
Q) What is static
variable in java?
Static variables are also known as class level variables. A static variable is
same for all the objects of that particular class in which it is declared.
Q) What is static
block?
A static block gets executed at the time of class loading. They are used for
initializing static variables.
Q) What is a static
method?
Static methods can be called directly without creating the instance (Object) of
the class. A static method can access all the static variables of a class directly
but it cannot access non-static variables without creating instance of class.
Q) Explain super
keyword in Java?
super keyword references to the parent class.
There are several uses of super keyword:
- It can be used to call the
superclass(Parent class) constructor.
- It can be used to access a method
of the superclass that has been hidden by subclass (Calling parent class version,
In case of method overriding).
- To call the constructor of parent
class.
Q) Use of final
keyword in Java?
Final methods – These methods cannot be overridden by any other method.
Final variable – Constants, the value of these variable can’t be changed,
its fixed.
Final class – Such classes cannot be inherited by other classes. These
type of classes will be used when application required security or someone
don’t want that particular class. More details.
Q) What is a Object
class?
This is a special class defined by java; all other classes are subclasses of
object class. Object class is superclass of all other classes. Object class has
the following methods
- objectClone () – to creates a new
object that is same as the object being cloned.
- boolean equals(Object obj) –
determines whether one object is equal to another.
- finalize() – Called by the garbage
collector on an object when garbage collection determines that there are
no more references to the object. A subclass overrides the finalize method
to dispose of system resources or to perform other cleanup.
- toString () – Returns a string
representation of the object.
Q) What are Packages?
A Package can be defined as a grouping of related types (classes, interfaces,
enumerations and annotations )
Q)What is the
difference between import java.util.Date and java.util.* ?
The star form (java.util.* ) includes all the classes of that package and that
may increase the compilation time – especially if you import several packages.
However it doesn’t have any effect run-time performance.
Q) Garbage collection
in java?
Since objects are dynamically allocated by using the new operator, java handles
the de-allocation of the memory automatically when no references to an object
exist for a long time is called garbage collection. The whole purpose of Garbage
collection is efficient memory management.
Q) Use of finalize()
method in java?
finalize() method is used to free the allocated resource.
Q) How many times
does the garbage collector calls the finalize() method for an object?
The garbage collector calls the
finalize() method Only once for an object.
Q) What are two
different ways to call garbage collector?
System.gc()
OR Runtime.getRuntime().gc().
Q) Can the Garbage
Collection be forced by any means?
No, its not possible. you cannot force garbage collection. you can call
system.gc() methods for garbage collection but it does not guarantee that
garbage collection would be done.
Exception
handling Interview Questions
Q) What is an
exception?
Exceptions are abnormal conditions that arise during execution of the program.
It may occur due to wrong user input or wrong logic written by programmer.
Q) Exceptions are
defined in which java package? OR which package has definitions for all the
exception classes?
Java.lang.Exception
This package contains definitions for Exceptions.
Q) What are the types
of exceptions?
There are two types of exceptions: checked and unchecked exceptions.
Checked exceptions: These exceptions must be handled by programmer otherwise
the program would throw a compilation error.
Unchecked exceptions: It is up to the programmer to write the code in such a
way to avoid unchecked exceptions. You would not get a compilation error if you
do not handle these exceptions. These exceptions occur at runtime.
Q) What is the
difference between Error and Exception?
Error: Mostly a system issue. It always occur at run time and must be resolved
in order to proceed further.
Exception: Mostly an input data issue or wrong logic in code. Can occur at
compile time or run time.
Q) What is throw
keyword in exception handling?
The throw keyword is used for throwing user defined or pre-defined exception.
Q) What is throws
keyword?
If a method does not handle a checked exception, the method must declare it
using the throwskeyword. The throws keyword appears at the end of a method’s
signature.
Q) Difference between
throw and throws in Java
Read the difference here: Java – throw vs throws.
Q) Can static block
throw exception?
Yes, A static block can throw exceptions. It has its own limitations: It can
throw only Runtime exception (Unchecked exceptions), In order to throw checked
exceptions you can use a try-catch block inside it.
Q) What is finally
block?
Finally
block is a block of code that always executes, whether an exception
occurs or not. Finally block follows try block or try-catch block.
Q) ClassNotFoundException vs
NoClassDefFoundError?
1) ClassNotFoundException occurs when loader could not find the required class
in class path.
2) NoClassDefFoundError occurs when class is loaded in classpath, but one or
more of the class which are required by other class, are removed or failed to
load by compiler.
Q) Can we have a try
block without catch or finally block?
No, we cannot have a try block without catch or finally block. We must have
either one of them or both.
Q) Can we have
multiple catch blocks following a single try block?
Yes we can have multiple catch blocks in order to handle more
than one exception.
Q) Is it possible to
have finally block without catch block?
Yes, we can have try block followed by finally block without even using catch
blocks in between.
When a finally block
does not get executed?
The only time finally won’t be called is if you call System.exit() or if the
JVM crashes first.
Q) Can we handle more
than one exception in a single catch block?
Yes we can do that using if-else statement but it is not considered as a good
practice. We should have one catch block for one exception.
Q) What is a Java
Bean?
A JavaBean is a Java class that follows some simple conventions including
conventions on the names of certain methods to get and set state called
Introspection. Because it follows conventions, it can easily be processed by a
software tool that connects Beans together at runtime. JavaBeans are reusable
software components.
Java
Multithreading Interview Questions
Q) What is
Multithreading?
It is a process of executing two or more part of a program simultaneously. Each
of these parts is known as threads. In short the process of executing multiple
threads simultaneously is known as multithreading.
Q) What is the main
purpose of having multithread environment?
Maximizing CPU usage and reducing CPU idle time
Q) What are the main
differences between Process and thread? Explain in brief.
1) One process can have multiple threads. A thread is a smaller part of a
process.
2) Every process has its own memory space, executable code and a unique
process identifier (PID) while every thread has its own stack in Java but it
uses process main memory and shares it with other threads.
3) Threads of same process can communicate with each other using keyword like
wait and notify etc. This process is known as inter process communication.
Q) How can we create
a thread in java?
There are following two ways of creating a thread:
1) By Implementing Runnable interface.
2) By Extending Thread class.
Q) Explain yield and
sleep?
yield() – It causes the currently executing thread object to temporarily pause
and allow other threads to execute.
sleep() – It causes the current thread
to suspend execution for a specified period. When a thread goes into sleep
state it doesn’t release the lock.
Q) What is the
difference between sleep() and wait()?
sleep() – It causes the current thread to suspend execution for a specified
period. When a thread goes into sleep state it doesn’t release the lock
wait() – It causes current thread to
wait until either another thread invokes the notify() method or the notifyAll()
method for this object, or a specified amount of time has elapsed.
Q) What is a daemon
thread?
A daemon thread is a thread, that does not prevent the JVM from exiting when
the program finishes but the thread is still running. An example for a daemon
thread is the garbage collection.
Q) What does join( )
method do?
if you use join() ,it makes sure that as soon as a thread calls join,the
current thread(yes,currently running thread) will not execute unless the thread
you have called join is finished.
Q) Preemptive
scheduling vs. time slicing?
1) The preemptive scheduling is prioritized. The highest priority process
should always be the process that is currently utilized.
2) Time slicing means task executes for a defined slice/ period of time and
then enter in the pool of ready state. The scheduler then determines which task
execute next based on priority or other factor.
Q) Can we call run()
method of a Thread class?
Yes, we can call run() method of a Thread class but then it will behave like a
normal method. To actually execute it in a Thread, you should call
Thread.start() method to start it.
Q) What is
Starvation?
Starvation describes a situation where a thread is unable to gain regular
access to shared resources and is unable to make progress. This happens when
shared resources are made unavailable for long periods by “greedy” threads. For
example, suppose an object provides a synchronized method that often takes a
long time to return. If one thread invokes this method frequently, other
threads that also need frequent synchronized access to the same object will
often be blocked.
Q) What is deadlock?
Deadlock describes a situation where two or more threads are blocked forever,
waiting for each other.
Serialization
interview Questions
Q: What is
Serialization and de-serialization?
Serialization is a process of converting an object and its attributes to the
stream of bytes. De-serialization is recreating the object from stream of
bytes; it is just a reverse process of serialization. To know more about
serialization with example program, refer this article.
Q) Do we need to implement
any method of Serializable interface to make an object serializable?
No. In order to make an object serializable we just need to implement the
interface Serializable. We don’t need to implement any methods.
Q) What is a
transient variable?
1) transient variables are not included in the process of serialization.
2) They are not the part of the object’s serialized state.
3) Variables which we don’t want to include in serialization are declared as
transient.
String
interview questions
Q) A string class is
immutable or mutable?
String class is immutable that’s the reason once its object gets created, it
cannot be changed further.
Q) Difference between
StringBuffer and StringBuilder class?
1) StringBuffer is thread-safe but StringBuilder is not thread safe.
2) StringBuilder is faster than StringBuffer.
3) StringBuffer is synchronized whereas StringBuilder is not synchronized.
Q) What is toString()
method in Java?
The toString() method returns the string representation of any object.
Java
collections interview questions
Q) What is List?
Elements can be inserted or accessed by their position in the list, using a
zero-based index.
A list may contain duplicate elements.
Q) What is Map?
Map interface maps unique keys to values. A key is an object that we use to
retrieve a value later. A map cannot contain duplicate keys: Each key can map
to at most one value.
Q) What is Set?
A Set is a Collection that cannot contain duplicate elements.
Q) Why ArrayList is
better than Arrays?
Array can hold fixed number of elements. ArrayList can grow dynamically.
Q) What is the
difference between ArrayList and LinkedList?
1) LinkedList store elements within a doubly-linked list data structure.
ArrayList store elements within a dynamically resizing array.
2) LinkedList is preferred for add and update operations while ArrayList is a
good choice for search operations. Read more here.
Q) For addition and
deletion. Which one is most preferred: ArrayList or LinkedList?
LinkedList. Because deleting or adding a node in LinkedList is faster than
ArrayList.
Q) For searches.
Which one is most preferred: ArrayList or LinkedList?
ArrayList. Searching an element is faster in ArrayList compared to LinkedList.
Q) What is the
difference between ArrayList and Vector?
1) Vector is synchronized while ArrayList is not synchronized.
2) By default, Vector doubles the size of its array when it is re-sized
internally. ArrayList increases by half of its size when it is re-sized. More details.
Q) What is the
difference between Iterator and ListIterator?
Following are the major differences between them:
1) Iterator can be used for traversing Set, List and Map. ListIterator can only
be used for traversing a List.
2) We can traverse only in forward direction using Iterator. ListIterator can
be used for traversing in both the directions(forward and backward). Read more
at: ListIterator vs Iterator.
Q) Difference between
TreeSet and SortedSet?
TreeSet implements SortedSet interface.
Q) What is the
difference between HashMap and Hashtable?
1) Hashtable is synchronized. HashMap is not synchronized.
2) Hashtable does not allow null keys or values. HashMap allows one null key
and any number of null values. Read more here.
Q) What is the
difference between Iterator and Enumeration?
1) Iterator allows to remove elements from the underlying collection during the
iteration using its remove() method. We cannot add/remove elements from a
collection when using enumerator.
2) Iterator has improved method names.
Enumeration.hasMoreElement() -> Iterator.hasNext()
Enumeration.nextElement() -> Iterator.next().
Applet
Interview Questions
Q) How do you do file
I/O from an applet?
Unsigned applets are simply not allowed to read or write files on the local
file system .
Unsigned applets can, however, read
(but not write) non-class files bundled with your applet on the server, called
resource files
Q) What is container
?
A component capable of holding another component is called as container.
Container
Panel
Applet
Window
Frame
Dialog
Learning)
- Flow Layout is default for panel.
- Border Layout is default for
Frames.
Q) On Windows,
generally frames are invisible, how to make it visible. ?
Frame f = new Frame();
f.setSize(300,200); //height
and width
f.setVisible(true) ; //
Frames appears
Q) JFC – Java
Foundation Class
Swing
AWT
Java2D
Drag and Drop
Accessibility
Learning) Listeners and Methods?
ActionListerner – actionPerformed();
ItemListerner – itemStateChanged();
TextListener – textValueChanged();
FocusListener – focusLost(); & FocusGained();
WindowListener – windowActified();
windowDEactified(); windowIconified(); windowDeiconified(); windowClosed();
windowClosing(); windowOpened();
MouseMotionListener – mouseDragged();
& mouseMoved();
MouseListener – mousePressed();
mouseReleased(); mouseEntered(); mouseExited(); mouseClicked();
Learnings)
parseInt – to convert string to int.
getBytes – string to byte array
Q) Applet Life cycle?
Following stage of any applets life cycle, starts with init(), start(),
paint(), stop() and destroy().
Q) showStatus() ?–
To display the message at the bottom of the browser when applet is started.
Q) What is the Event
handling?
Is irrespective of any component, if any action performed/done on Frame, Panel
or on window, handling those actions are called Event Handling.
Q) What is Adapter
class?
Adapter class is an abstract class.
Advantage of adapter: To perform any
window listener, we need to include all the methods used by the window listener
whether we use those methods are not in our class like Interfaces whereas with
adapter class, its sufficient to include only the methods required to override.
Straight opposite to Interface.
