Java Developers Interview Questions and Answers

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Java Developers Interview Questions and Answers

What are the four corner stones of OOP? - Abstraction, Encapsulation, Polymorphism and Inheritance.

Difference between a Class and an Object? - A class is a definition or prototype whereas an object is an instance or living representation of the prototype.

What is the difference between method overriding and overloading? - Overriding is a method with the same name and arguments as in a parent, whereas overloading is the same method name but different arguments.

What is a "stateless" protocol? - Without getting into lengthy debates, it is generally accepted that protocols like HTTP are stateless i.e. there is no retention of state between a transaction which is a single request response combination.

What is constructor chaining and how is it achieved in Java? - A child object constructor always first needs to construct its parent (which in turn calls its parent constructor.). In Java it is done via an implicit call to the no-args constructor as the first statement.

What is passed by ref and what by value? - All Java method arguments are passed by value. However, Java does manipulate objects by reference, and all object variables themselves are references

Can RMI and Corba based applications interact? - Yes they can. RMI is available with IIOP as the transport protocol instead of JRMP.

You can create a String object as String str = "abc"; Why cant a button object be created as Button bt = "abc";? Explain - The main reason you cannot create a button by Button bt1= "abc"; is because "abc" is a literal string (something slightly different than a String object, by the way) and bt1 is a Button object. The only object in Java that can be assigned a literal String is java.lang.String. Important to note that you are NOT calling a java.lang.String constuctor when you type String s = "abc";

What does the "abstract" keyword mean in front of a method? A class? - Abstract keyword declares either a method or a class. If a method has a abstract keyword in front of it,it is called abstract method.Abstract method hs no body.It has only arguments and return type.Abstract methods act as placeholder methods that are implemented in the subclasses. Abstract classes can't be instantiated.If a class is declared as abstract,no objects of that class can be created.If a class contains any abstract method it must be declared as abstract.

How many methods do u implement if implement the Serializable Interface? - The Serializable interface is just a "marker" interface, with no methods of its own to implement. Other 'marker' interfaces are

java.rmi.Remote
java.util.EventListener

What are the practical benefits, if any, of importing a specific class rather than an entire package (e.g. import java.net.* versus import java.net.Socket)? - It makes no difference in the generated class files since only the classes that are actually used are referenced by the generated class file. There is another practical benefit to importing single classes, and this arises when two (or more) packages have classes with the same name. Take java.util.Timer and javax.swing.Timer, for example. If I import java.util.* and javax.swing.* and then try to use "Timer", I get an error while compiling (the class name is ambiguous between both packages). Let's say what you really wanted was the javax.swing.Timer class, and the only classes you plan on using in java.util are Collection and HashMap. In this case, some people will prefer to import java.util.Collection and import java.util.HashMap instead of importing java.util.*. This will now allow them to use Timer, Collection, HashMap, and other javax.swing classes without using fully qualified class names in.

What is the difference between logical data independence and physical data independence? - Logical Data Independence - meaning immunity of external schemas to changeds in conceptual schema. Physical Data Independence - meaning immunity of conceptual schema to changes in the internal schema.

What is a user-defined exception? - Apart from the exceptions already defined in Java package libraries, user can define his own exception classes by extending Exception class.

Describe the visitor design pattern? - Represents an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. The root of a class hierarchy defines an abstract method to accept a visitor. Subclasses implement this method with visitor.visit(this). The Visitor interface has visit methods for all subclasses of the baseclass in the hierarchy.



16.What are the advantages of OOPL?

Ans: Object oriented programming languages directly represent the real life objects. The features of OOPL as inhreitance, polymorphism, encapsulation makes it powerful.

17. What do mean by polymorphisum, inheritance, encapsulation?

Ans: Polymorhisum: is a feature of OOPl that at run time depending upon the type of object the appropriate method is called.
Inheritance: is a feature of OOPL that represents the "is a" relationship between different objects(classes). Say in real life a manager is a employee. So in OOPL manger class is inherited from the employee class.
Encapsulation: is a feature of OOPL that is used to hide the information.

18. What do you mean by static methods?

Ans: By using the static method there is no need creating an object of that class to use that method. We can directly call that method on that class. For example, say class A has static function f(), then we can call f() function as A.f(). There is no need of creating an object of class A.

19. What do you mean by virtual methods?

Ans: virtual methods are used to use the polymorhism feature in C++. Say class A is inherited from class B. If we declare say fuction f() as virtual in class B and override the same function in class A then at runtime appropriate method of the class will be called depending upon the type of the object.

20. Given two tables Student(SID, Name, Course) and Level(SID, level) write the SQL statement to get the name and SID of the student who are taking course = 3 and at freshman level.

Ans: SELECT Student.name, Student.SID
FROM Student, Level
WHERE Student.SID = Level.SID
AND Level.Level = "freshman"
AND Student.Course = 3;

21. What are the disadvantages of using threads?

Ans: DeadLock.


22. Write the Java code to declare any constant (say gravitational constant) and to get its value

Ans: Class ABC
{
static final float GRAVITATIONAL_CONSTANT = 9.8;
public void getConstant()
{
system.out.println("Gravitational_Constant: " + GRAVITATIONAL_CONSTANT);
}
}
23. What do you mean by multiple inheritance in C++ ?

Ans: Multiple inheritance is a feature in C++ by which one class can be of different types. Say class teachingAssistant is inherited from two classes say teacher and Student.

24. Can you write Java code for declaration of multiple inheritance in Java ?

Ans: Class C extends A implements B
{
}