Core Java


  1. Can there be an abstract class with no abstract methods in it? - Yes
  2. Can an Interface be final? - No
  3. Can an Interface have an inner class? - Yes. 
     public interface abc
 {
  static int i=0; void dd();
  class a1
  {
   a1()
   {
    int j;
    System.out.println("inside");
   };
   public static void main(String a1[])
   {
    System.out.println("in interfia");
   }
  }
 }
  4. Can we define private and protected modifiers for variables in interfaces? - No
  5. What is Externalizable? - Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
  6. What modifiers are allowed for methods in an Interface? - Only public and abstract modifiers are allowed for methods in interfaces.
  7. What is a local, member and a class variable? - Variables declared within a method are “local” variables. Variables declared within the class i.e not within any methods are “member” variables (global variables). Variables declared within the class i.e not within any methods and are defined as “static” are class variables
  8. What are the different identifier states of a Thread? - The different identifiers of a Thread are: R - Running or runnable thread, S - Suspended thread, CW - Thread waiting on a condition variable, MW - Thread waiting on a monitor lock, MS - Thread suspended waiting on a monitor lock
  9. What are some alternatives to inheritance? - Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn’t force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
  10. Why isn’t there operator overloading? - Because C++ has proven by example that operator overloading makes code almost impossible to maintain. In fact there very nearly wasn’t even method overloading in Java, but it was thought that this was too useful for some very basic methods like print(). Note that some of the classes like DataOutputStream have unoverloaded methods like writeInt() and writeByte().
  11. What does it mean that a method or field is “static”? - Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That’s how library methods like System.out.println() work. out is a static field in the java.lang.System class.
  12. How do I convert a numeric IP address like 192.18.97.39 into a hostname like java.sun.com? 
     String hostname = InetAddress.getByName("192.18.97.39").getHostName();
  13. Difference between JRE/JVM/JDK?
  14. 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.
  15. 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.
  16. Is null a keyword? - The null value is not a keyword.
  17. 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.
  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. 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.
  20. What are wrapped classes? - Wrapped classes are classes that allow primitive types to be accessed as objects.
  21. 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).
  22. 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.
  23. What is a native method? - A native method is a method that is implemented in a language other than Java.
  24. 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
  25. 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.
  26. 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.
  27. What is the range of the char type? - The range of the char type is 0 to 2^16 - 1.




OOPS Concepts with real time examples 

1. Abstraction

Abstraction helps to hide the non essential features from the user and makes the application user friendly.

Eg. Take your mobile phone. You are having buttons and know the purpose but no need to understand how it prints number on the screen when it is pressed.

2. Encapsulation

It bundles the code into a single unit which makes the code very easy to handle.

Eg.

Take computer keyboard. All the buttons have been enclosed.

3. Inheritance

It helps to inherit the superclass properties to subclasses.

Eg. Father - Son relationship.

4. Polymorphism

A entity which behaves differntly at different places.

Eg. A person behaves as a good husband in family.
He behaves as a good employee at company.
He also behaves as a good citizen in public
 

 

 

Difference Between Interface and Abstract Class

  1. Main difference is methods of a Java interface are implicitly abstract and cannot have implementations. A Java abstract class can have instance methods that implements a default behavior.
  2. Variables declared in a Java interface is by default final. An  abstract class may contain non-final variables.
  3. Members of a Java interface are public by default. A Java abstract class can have the usual flavors of class members like private, protected, etc..
  4. Java interface should be implemented using keyword “implements”; A Java abstract class should be extended using keyword “extends”.
  5. An interface can extend another Java interface only, an abstract class can extend another Java class and implement multiple Java interfaces.
  6. A Java class can implement multiple interfaces but it can extend only one abstract class.
  7. Interface is absolutely abstract and cannot be instantiated; A Java abstract class also cannot be instantiated, but can be invoked if a main() exists.
  8. In comparison with java abstract classes, java interfaces are slow as it requires extra indirection.

 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




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?

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.awt package.

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()

58. What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java
runtime system.

59. How many times may an object's finalize() method be
invoked by the
garbage collector?
An object's finalize() method may only be invoked once by the
garbage collector.

60. What is the purpose of the finally clause of a try-catchfinally
statement?
The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.
61. What is the argument type of a program's main() method?

design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the event-inheritance model.

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