JAVA INTERVIEW QUESTIONS - Packages and interface

Packages and interface

1) What are packages? what is the use of packages?

Ans : The package statement defines a name space in which classes are stored.If you omit the package, the classes are put into the default package.

Signature... package pkg;

Use: * It specifies to which package the classes defined in a file belongs to. * Package is both naming and a visibility control mechanism.

2) What is difference between importing "java.applet.Applet" and "java.applet.*;" ?

Ans :"java.applet.Applet" will import only the class Applet from the package java.applet

Whereas "java.applet.*" will import all the classes from java.applet package.

3) What do you understand by package access specifier?

Ans: public: Anything declared as public can be accessed from anywhere

private: Anything declared in the private can’t be seen outside of its class.

default: It is visible to subclasses as well as to other classes in the same package.

4) What is an interface? What is the use of interface?

Ans: It is similar to class which may contain method’s signature only but not bodies.

Methods declared in interface are abstract methods. We can implement many interfaces on a class which supports the multiple inheritances.

5) Is it is necessary to implement all methods in an interface?

Ans: Yes. All the methods have to be implemented.

6) Which is the default access modifier for an interface method?

Ans: public.

7) Can we define a variable in an interface ?and what type it should be?

Ans: Yes we can define a variable in an interface. They are implicitly final and static.

8) What is the difference between interface and an abstract class?

Ans: All the methods declared inside an Interface are abstract. Whereas abstract class must have at least one abstract method and others may be concrete or abstract.

In Interface, we need not use the keyword abstract for the methods.

9) By default, all program import the java.lang package.

True/False

Ans: True

10) Java compiler stores the .class files in the path specified in CLASSPATH

environmental variable.

True/False

Ans: False

11) The user-defined package can also be imported just like the standard packages.

True/False

Ans: True

12) When a program does not want to handle the exception, the ______class is used.

Ans: Throws

13) The main subclass of the Exception class is _______ class.

Ans: RuntimeException

14) Only subclasses of ______class may be caught or thrown.

Ans: Throwable

15) Any user-defined exception class is a subclass of the _____ class.

Ans: Exception

16) The catch clause of the user-defined exception class should ______ its

Base class catch clause.

Ans: Exception

17) A _______ is used to separate the hierarchy of the class while declaring an

Import statement.

Ans: Package

18) All standard classes of Java are included within a package called _____.

Ans : java.lang

19) All the classes in a package can be simultaneously imported using ____.

Ans: *

20) Can you define a variable inside an Interface? If no, why? If yes, how?

Ans.: YES. final and static

21) How many concrete classes can you have inside an interface?

Ans.: None

22) Can you extend an interface?

Ans.: Yes

23) Is it necessary to implement all the methods of an interface while implementing the interface?

Ans.: No

24) If you do not implement all the methods of an interface while implementing, what specifier should you use for the class?

Ans.: abstract

25) How do you achieve multiple inheritances in Java?

Ans: Using interfaces.

26) How to declare an interface example?

Ans : access class classname implements interface.

27) Can you achieve multiple interface through interface?

a)True

b) false

Ans: a.

28) Can variables be declared in an interface? If so, what are the modifiers?

Ans: Yes. final and static are the modifiers can be declared in an interface.

29) What are the possible access modifiers when implementing interface methods?

Ans: public.

30) Can anonymous classes be implemented an interface?

Ans: Yes.

31) Interfaces can’t be extended.

a)True

b)False

Ans: b.

32) Name interfaces without a method?

Ans : Serializable, Cloneble & Remote.

33) Is it possible to use few methods of an interface in a class ? If so, how?

Ans: Yes. Declare the class as abstract.

JAVA INTERVIEW QUESTIONS - Introduction to Classes and Methods

Introduction to Classes and Methods

1) Which is used to get the value of the instance variables?

Ans: Dot notation.

2) The new operator creates a single instance named class and returns a

reference to that object.

a)True

b)False

Ans: a.

3) A class is a template for multiple objects with similar features.

a)True

b)False

Ans: a.

4) What is mean by garbage collection?

Ans: When an object is no longer referred to by any variable, Java automatically

reclaims memory used by that object. This is known as garbage collection.

5) What are methods and how are they defined?

Ans: Methods are functions that operate on instances of classes in which they are defined.Objects can communicate with each other using methods and can call methods in other classes.

Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method.

A method's signature is a combination of the first three parts mentioned above.

6) What is calling method?

Ans: Calling methods are similar to calling or referring to an instance variable. These methods are accessed using dot notation.

Ex: obj.methodname(param1,param2)

7) Which method is used to determine the class of an object?

Ans: getClass( ) method can be used to find out what class the belongs to. This class is defined in the object class and is available to all objects.

8) All the classes in java.lang package are automatically imported when

a program is compiled.

a)True

b)False

Ans: a.

9) How can a class be imported to a program?

Ans: To import a class, the import keyword should be used as shown.;

import classname;

10) How can a class be imported from a package to a program?

Ans: import java . packagename . classname (or) import java.package name.*;

11) What is a constructor?

Ans: A constructor is a special kind of method that determines how an object is

initialized when created.

12) Which keyword is used to create an instance of a class?

Ans: new.

13) Which method is used to garbage collect an object?

Ans: finalize ().

14) Constructors can be overloaded like regular methods.

a)True

b)False

Ans: a.

15) What is casting?

Ans: Casting is bused to convert the value of one type to another.

16) Casting between primitive types allows conversion of one primitive type to another.

a)True

b)False

Ans: a.

17) Casting occurs commonly between numeric types.

a)True

b)False

Ans: a.

18) Boolean values can be cast into any other primitive type.

a)True

b)False

Ans: b.

19) Casting does not affect the original object or value.

a)True

b)False

Ans: a.

20) Which cast must be used to convert a larger value into a smaller one?

Ans: Explicit cast.

21) Which cast must be used to cast an object to another class?

Ans: Specific cast.

22) Which of the following features are common to both Java & C++?

A.The class declaration

b.The access modifiers

c.The encapsulation of data & methods within objects

d.The use of pointers

Ans: a,b,c.

23) Which of the following statements accurately describe the use of access modifiers within a class definition?

a.They can be applied to both data & methods

b.They must precede a class's data variables or methods

c.They can follow a class's data variables or methods

d.They can appear in any order

e.They must be applied to data variables first and then to methods

Ans: a,b,d.

24) Suppose a given instance variable has been declared private.

Can this instance variable be manipulated by methods outside its class?

a.yes

b.no

Ans: b.

25) Which of the following statements can be used to describe a public method?

a.It is accessible to all other classes in the hierarchy

b.It is accessible only to subclasses of its parent class

c.It represents the public interface of its class

d.The only way to gain access to this method is by calling one of the public class

methods

Ans: a,c.

26) Which of the following types of class members can be part of the internal part of a class?

a.Public instance variables

b.Private instance variables

c.Public methods

d.Private methods

Ans: b,d.

27) You would use the ____ operator to create a single instance of a named class.

a.new

b.dot

Ans: a.

28) Which of the following statements correctly describes the relationship between an object and the instance variable it stores?

a.Each new object has its own distinctive set of instance variables

b.Each object has a copy of the instance variables of its class

c.the instance variable of each object are separated from the variables of other objects

d.The instance variables of each object are stored together with the variables of other objects

Ans: a,b,c.

29) If no input parameters are specified in a method declaration then the declaration will include __.

a.an empty set of parentheses

b.the term void

Ans: a.

30) What are the functions of the dot(.) operator?

a.It enables you to access instance variables of any objects within a class

b.It enables you to store values in instance variables of an object

c.It is used to call object methods

d.It is to create a new object

Ans: a,b,c.

31) Which of the following can be referenced by this variable?

a.The instance variables of a class only

b.The methods of a class only

c.The instance variables and methods of a class

Ans: c.

32) This reference is used in conjunction with ___methods.

a.static

b.non-static

Ans: b.

33) Which of the following operators are used in conjunction with the this and super references?

a.The new operator

b.The instance of operator

c.The dot operator

Ans: c.

34) A constructor is automatically called when an object is instantiated

a. true

b. false

Ans: a.

35) When may a constructor be called without specifying arguments?

a. When the default constructor is not called

b. When the name of the constructor differs from that of the class

c. When there are no constructors for the class

Ans: c.

36) Each class in Java can have a finalizer method

a. true

b.false

Ans: a.

37) When an object is referenced, does this mean that it has been identified by the finalizer method for garbage collection?

a.yes

b.no

Ans: b.

38) Because finalize () belongs to the java.lang.Object class, it is present in all ___.

a.objects

b.classes

c.methods

Ans: b.

39) Identify the true statements about finalization.

a.A class may have only one finalize method

b.Finalizers are mostly used with simple classes

c.Finalizer overloading is not allowed

Ans: a,c.

40) When you write finalize() method for your class, you are overriding a finalizer

inherited from a super class.

a.true

b.false

Ans: a.

41) Java memory management mechanism garbage collects objects which are no longer referenced

a true

b.false

Ans: a.

42) are objects referenced by a variable candidates for garbage collection when the variable goes out of scope?

a yes

b. no

Ans: a.

43) Java's garbage collector runs as a ___ priority thread waiting for __priority threads to relinquish the processor.

a.high

b.low

Ans: a,b.

44) The garbage collector will run immediately when the system is out of memory

a.true

b.false

Ans: a.

45) You can explicitly drop an object reference by setting the value of a variable whose data type is a reference type to ___

Ans: null

46) When might your program wish to run the garbage collector?

a. before it enters a compute-intense section of code

b. before it enters a memory-intense section of code

c. before objects are finalized

d. when it knows there will be some idle time

Ans: a,b,d

47) For externalizable objects, the class is solely responsible for the external format of its contents

a.true

b.false

Ans: a

48) When an object is stored, are all of the objects that are reachable from that object stored as well?

a.true

b.false

Ans: a

49) The default__ of objects protects private and transient data and supports the __ of the classes

a.evolution

b.encoding

Ans: b,a.

50) Which are keywords in Java?

a) NULL

b) sizeof

c) friend

d) extends

e) synchronized

Ans: d and e

51) When must the main class and the file name coincide?

Ans: When class is declared public.

52) What are different modifiers?

Ans: public, private, protected, default, static, transient, volatile, final, abstract.

53) What are access modifiers?

Ans : public, private, protected, default.

54) What is meant by "Passing by value" and " Passing by reference"?

Ans: objects – pass by reference

Methods - pass by value

55) Is a class a subclass of itself?

Ans: A class is a subclass itself.

56) What modifiers may be used with a top-level class?

Ans: public, abstract, final.

57) What is an example of polymorphism?

Inner class

Anonymous classes

Method overloading

Method overriding

Ans: c