Java Questions Frequently asked in Interview

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Java :: Frequently Asked Questions
  • Can a main() method of class be invoked in another class?
  • What is the difference between java command line arguments and C command  line arguments?
  • What is the difference between == & .equals
  • What is the difference between abstract class & Interface.
  • What is singleton class & it's implementation.
  • Use of static,final variable
  • Examples of final class
  • Difference between Event propagation & Event delegation
  • Difference between Unicast & Multicast model
  • What is a java bean
  • What is synchronized keyword used for.
  • What are the restrictions of an applet & how to make the applet access the local machines resources.
  • What is reflect package used for & the methods of it.
  • What is serialization used for
  • Can methods be overloaded based on the return types ?
  • Why do we need a finalze() method when Garbage Collection is there ?
  • Difference between AWT and Swing compenents ?
  • Is there any heavy weight component in Swings ?
  • Can the Swing application if you upload in net, be compatible with your browser?
  • What should you do get your browser compatible with swing  components?
  • What are the methods in Applet ?
  • When is init(),start() called ?
  • When you navigate from one applet to another what are the methods called?
  • What is the difference between Trusted and Untrusted Applet ?
  • What is Exception ?
  • What are the ways you can handle exception ?
  • When is try,catch block used ?
  • What is finally method in Exceptions ?
  • What are the types of access modifiers ?
  • What is protected and friendly ?
  • What are the other modifiers ?
  • Is synchronised modifier ?
  • What is meant by polymorphism ?
  • What is inheritance ?
  • What is method Overloading ? What is this in OOPS ?
  • What is method Overriding ? What is it in OOPS ?
  • Does java support multi dimensional arrays ?
  • Is multiple inheritance used in Java ?
  • How do you send a message to the browser in JavaScript ?
  • Does javascript support multidimensional arrays ?
  • Is there any tool in java that can create reports ?
  • What is meant by Java ?
  • What is meant by a class ?
  • What is meant by a method ?
  • What are the OOPS concepts in Java ?
  • What is meant by encapsulation ? Explain with an example
  • What is meant by inheritance ? Explain with an example
  • What is meant by polymorphism ? Explain with an example
  • Is multiple inheritance allowed in Java ? Why ?
  • What is meant by Java interpreter ?
  • What is meant by JVM ?
  • What is a compilation unit ?
  • What is meant by identifiers ?
  • What are the different types of modifiers ?
  • What are the access modifiers in Java ?
  • What are the primitive data types in Java ?
  • What is meant by a wrapper class ?
  • What is meant by static variable and static method ?
  • What is meant by Garbage collection ?
  • What is meant by abstract class
  • What is meant by final class, methods and variables ?
  • What is meant by interface ?
  • What is meant by a resource leak ?
  • What is the difference between interface and abstract class ?
  • What is the difference between public private, protected and static
  • What is meant by method overloading ?
  • What is meant by method overriding ?
  • What is singleton class ?
  • What is the difference between an array and a vector ?
  • What is meant by constructor ?
  • What is meant by casting ?
  • What is the difference between final, finally and finalize ?
  • What is meant by packages ?
  • What are all the packages ?
  • Name 2 calsses you have used ?
  • Name 2 classes that can store arbitrary number of objects ?
  • What is the difference between java.applet.* and java.applet.Applet ?
  • What is a default package ?
  • What is meant by a super class and how can you call a super class ?
  • What is anonymous class ?
  • Name interfaces without a method ?
  • What is the use of an interface ?
  • What is a serializable interface ?
  • How to prevent field from serialization ?
  • What is meant by exception ?
  • How can you avoid the runtime exception ?
  • What is the difference between throw and throws ?
  • What is the use of finally ?
  • Can multiple catch statements be used in exceptions ?
  • Is it possible to write a try within a try statement ?
  • What is the method to find if the object exited or not ?
  • What is meant by a Thread ?
  • What is meant by multi-threading ?
  • What is the 2 way of creating a thread ? Which is the best way and why?
  • What is the method to find if a thread is active or not ?
  • What are the thread-to-thread communcation ?
  • What is the difference between sleep and suspend ?
  • Can thread become a member of another thread ?
  • What is meant by deadlock ?
  • How can you avoid a deadlock ?
  • What are the three typs of priority ?
  • What is the use of synchronizations ?
  • Garbage collector thread belongs to which priority ?
  • What is meant by time-slicing ?
  • What is the use of 'this' ?
  • How can you find the length and capacity of a string buffer ?
  • How to compare two strings ?
  • What are the interfaces defined by Java.lang ?
  • What is the purpose of run-time class and system class
  • What is meant by Stream and Types ?
  • What is the method used to clear the buffer ?
  • What is meant by Stream Tokenizer ?
  • What is serialization and de-serialisation ?
  • What is meant by Applet ?
  • How to find the host from which the Applet has originated ?
  • What is the life cycle of an Applet ?
  • How do you load an HTML page from an Applet ?
  • What is meant by Applet Stub Interface ?
  • What is meant by getCodeBase and getDocumentBase method ?
  • How can you call an applet from a HTML file
  • What is meant by Applet Flickering ?
  • What is the use of parameter tag ?
  • What is audio clip Interface and what are all the methods in it ?
  • What is the difference between getAppletInfo and getParameterInfo ?
  • How to communicate between applet and an applet ?
  • What is meant by event handling ?
  • What are all the listeners in java and explain ?
  • What is meant by an adapter class ?
  • What are the types of mouse event listeners ?
  • What are the types of methods in mouse listeners ?
  • What is the difference between panel and frame ?
  • What is the default layout of the panel and frame ?
  • What is meant by controls and types ?
  • What is the difference between a scroll bar and a scroll panel.
  • What is the difference between list and choice ?
  • How to place a component on Windows ?
  • What are the different types of Layouts ?
  • What is meant by CardLayout ?
  • What is the difference between GridLayout and GridBagLayout
  • What is the difference between menuitem and checkboxmenu item.
  • What is meant by vector class, dictionary class , hash table class,and property class ?
  • Which class has no duplicate elements ?
  • What is resource bundle ?
  • What is an enumeration class ?
  • What is meant by Swing ?
  • What is the difference between AWT and Swing ?
  • What is the difference between an applet and a Japplet
  • What are all the components used in Swing ?
  • What is meant by tab pans ?
  • What is the use of JTree ?
  • How can you add and remove nodes in Jtree.
  • What is the method to expand and collapse nodes in a Jtree
  • What is the use of JTable ?
  • What is meant by JFC ?
  • What is the class in Swing to change the appearance of the Frame in Runtime.
  • How to reduce flicking in animation ?
  • What is meant by Javabeans ?
  • What is JAR file ?
  • What is meant by manifest files ?
  • What is Introspection ?
  • What are the steps involved to create a bean ?
  • Say any two properties in Beans ?
  • What is persistence ?
  • What is the use of beaninfo ?
  • What are the interfaces you used in Beans ?
  • What are the classes you used in Beans ?
  • What is the diffrence between an Abstract class and Interface
  • What is user defined exception ?
  • What do you know about the garbate collector ?
  • What is the difference between C++ & Java ?
  • How do you communicate in between Applets & Servlets ?
  • What is the use of Servlets ?
  • In an HTML form I have a Button which makes us to open another page in 15 seconds. How will do you that ?
  • What is the difference between Process and Threads ?
  • How will you initialize an Applet ?
  • What is the order of method invocation in an Applet ?
  • When is update method called ?
  • How will you communicate between two Applets ?
  • Have you ever used HashTable and Dictionary ?
  • What are statements in JAVA ?
  • What is JAR file ?
  • What is JNI ?
  • What is the base class for all swing components ?
  • What is JFC ?
  • What is Difference between AWT and Swing ?
  • Considering notepad/IE or any other thing as process, What will Happen if you start notepad or IE 3 times? Where 3 processes are started or 3 threads are started ?
  • How does thread synchronization occurs inside a monitor ?
  • How will you call an Applet using a Java Script function ?
  • Is there any tag in HTML to upload and download files ?
  • Why do you Canvas ?
  • How can you push data from an Applet to Servlet ?
  • What are the benefits of Swing over AWT ?
  • Where the CardLayout is used ?
  • What is the Layout for ToolBar ?
  • What is the difference between Grid and GridbagLayout ?
  • How will you add panel to a Frame ?
  • What is the corresponding Layout for Card in Swing ?
  • What is light weight component ?
  • What is bean ? Where it can be used ?
  • What is difference in between Java Class and Bean ?
  • What is the mapping mechanism used by Java to identify IDL language ?
  • Diff between Application and Applet ?
  • What is serializable Interface ?
  • What is the difference between CGI and Servlet ?
  • What is the use of Interface ?
  • Why Java is not fully objective oriented ?
  • Why does not support multiple Inheritance ?
  • What it the root class for all Java classes ?
  • What is polymorphism ?
  • Suppose If we have variable ' I ' in run method, If I can create one or  More thread each thread will occupy a separate copy or same variable will be shared ?
  • What is Constructor and Virtual function? Can we call Virtual
  • Funciton in a constructor ?
  • Why we use OOPS concepts? What is its advantage ?
  • What is the difference in between C++ and Java ? can u explain in detail?
  • What is the exact difference in between Unicast and Multicast object ? Where we will use ?
  • How do you sing an Applet ?
  • In a Container there are 5 components. I want to display the all  the components names, how will you do that one ?
  • Why there are some null interface in java ? What does it mean ?
  • Give me some null interfaces in JAVA ?
  • Tell me the latest versions in JAVA related areas ?
  • What is meant by class loader ? How many types are there? When will we use them ?
  • What is meant by flickering ?
  • What is meant by cookies ? Explain ?
  • Problem faced in your earlier project
  • How OOPS concept is achieved in Java
  • Features for using Java
  • How does Java 2.0 differ from Java 1.0
  • Public static void main - Explain
  • What are command line arguments
  • Explain about the three-tier model
  • Difference between String & StringBuffer
  • Wrapper class. Is String a Wrapper Class
  • What are the restriction for static method Purpose of the file class
  • Default modifier in Interface
  • Difference between Interface & Abstract class
  • Can abstract be declared as Final
  • Can we declare variables inside a method as Final Variables
  • What is the package concept and use of package
  • How can a dead thread be started
  • Difference between Applet & Application
  • Life cycle of the Applet
  • Can Applet have constructors
  • Differeence between canvas class & graphics class
  • Explain about Superclass & subclass
  • What is AppletStub
  • Explain Stream Tokenizer
  • What is the difference between two types of threads
  • Checked & Unchecked exception
  • Use of throws exception
  • What is finally in exception handling  Vector class
  • What will happen to the Exception object after exception handling
  • Two types of multi-tasking
  • Two ways to create the thread
  • Synchronization
  • I/O Filter
  • Can applet in different page communicate with each other
  • Why Java is not 100 % pure OOPS ? ( EcomServer )
  • When we will use an Interface and Abstract class ?
  • How to communicate 2 threads each other ?
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Java Interview Questions with Answers

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1.what is a transient variable?

A transient variable is a variable that may not be serialized.

2.which containers use a border Layout as their default layout?
The window, Frame and Dialog classes use a border layout as their default layout.

3.Why do threads block on I/O?
Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.

4. How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

5. What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

6. Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object..

7. What's new with the stop(), suspend() and resume() methods in JDK 1.2?
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.

8. Is null a keyword?
The null value is not a keyword.

9. What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display normally.

10. What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout.

11. Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.

12. What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.

13. What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.

14. Which characters may be used as the second character of an identifier, but not as the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.

15. What is the List interface?
The List interface provides support for ordered collections of objects.

16. 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.

17. What is the Vector class?
The Vector class provides the capability to implement a growable array of objects

18. What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

19. What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.

20. What is the difference between the >> and >>> operators?
The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.

21. Which method of the Component class is used to set the position and size of a component?
setBounds()

22. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.

23.What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

24. Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.

25. Is sizeof a keyword?
The sizeof operator is not a keyword.

26. What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.

27. Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create
objects that are not subject to garbage collection. 

28. What restrictions are placed on the location of a package statement within a source code file?
A package statement must appear as the first line in a source code file (excluding blank lines and comments).

29. Can an object's finalize() method be invoked while it is reachable?
An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.

30. What is the immediate superclass of the Applet class?
Panel

31. What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

32. Name three Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.

33. What value does readLine() return when it has reached the end of a file?
The readLine() method returns null when it has reached the end of a file.

34. What is the immediate superclass of the Dialog class?
Window

35. What is clipping?
Clipping is the process of confining paint operations to a limited area or shape.

36. What is a native method?
A native method is a method that is implemented in a language other than Java.

37. Can a for statement loop indefinitely?
Yes, a for statement can loop indefinitely. For example, consider the following:
for(;;) ;

38. What are order of precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left.

39. When a thread blocks on I/O, what state does it enter?
A thread enters the waiting state when it blocks on I/O.

40. To what value is a variable of the String type automatically initialized?
The default value of an String type is null.

41. What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

42. What is the difference between a MenuItem and a CheckboxMenuItem?
The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or unchecked.

43. What is a task's priority and how is it used in scheduling?
A task's priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.

44. What class is the top of the AWT event hierarchy?
The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.

45. When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.

46. Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.


47. What is the range of the short type?
The range of the short type is - (2^15) to 2^15 - 1.

48. What is the range of the char type?
The range of the char type is 0 to 2^16 - 1.

49. In which package are most of the AWT events that support the event-delegation model defined?
Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package. The AWTEvent class is defined in the java.awtpackage.

50. What is the immediate superclass of Menu?
MenuItem

51. What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

52. Which class is the immediate superclass of the MenuComponent class.
Object

53. What invokes a thread's run() method?
After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

54. What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. 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. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.

55. Name three subclasses of the Component class.
Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent

56. What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.

57. Which Container method is used to cause a container to be laid out and redisplayed?
validate()
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