Wednesday, 13 July 2011

Java Interview Question 3

Java Interview Questions and Answers are given below

Questions 1.The Java interpreter is used for the execution of the source code.
a)True       b) False
Answers : a.
Questions 2) On successful compilation a file with the class extension is created.
a)True       b) False
Answers : a.
Questions 3) The Java source code can be created in a Notepad editor.
a)True       b) False
Answers : a.
Questions 4) The Java Program is enclosed in a class definition.
a)True       b) False
Answers : a.
Questions 5) What declarations are required for every Java application?
Answers : A class and the main( ) method declarations.
Questions 6) What are the two parts in executing a Java program and their purposes?
Answers : Two parts in executing a Java program are:
Java Compiler and Java Interpreter.
The Java Compiler is used for compilation and the Java Interpreter is used for execution of the application.
Questions7) What are the three OOPs principles and define them?
Answers : Encapsulation, Inheritance and Polymorphism are the three OOPs
Principles.
Encapsulation:
Is the Mechanism that binds together code and the data it manipulates, and keeps both safe from outside interference and misuse.
Inheritance:
Is the process by which one object acquires the properties of another object.
Polymorphism:
Is a feature that allows one interface to be used for a general class of actions.


Questions 8) What is a compilation unit?
Answers : Java source code file.
Questions9) What output is displayed as the result of executing the following statement?
System.out.println("// Looks like a comment.");
// Looks like a comment
The statement results in a compilation error
Looks like a comment
No output is displayed
Answers : a.
Questions 10) In order for a source code file, containing the public class Test, to successfully compile, which of the following must be true?
It must have a package statement
It must be named Test.java
It must import java.lang
It must declare a public class named Test
Answers : b
Questions 11) What are identifiers and what is naming convention?
Answers : Identifiers are used for class names, method names and variable names. An identifier may be any descriptive sequence of upper case & lower case letters,numbers or underscore or dollar sign and must not begin with numbers.
Questions 12) What is the return type of program’s main( ) method?
Answers : void
Questions 13) What is the argument type of program’s main( ) method?
Answers : string array.
Questions 14) Which characters are as first characters of an identifier?
Answers : A – Z, a – z, _ ,$
Questions 15) What are different comments?
Answers : 1) // -- single line comment
2) /* --
*/ multiple line comment
3) /** --
*/ documentation
Questions 16) What is the difference between constructor method and method?
Answers : Constructor will be automatically invoked when an object is created. Whereas method has to be call explicitly.
Questions 17) What is the use of bin and lib in JDK?
Answers : Bin contains all tools such as javac, applet viewer, awt tool etc., whereas Lib
contains all packages and variables.


Java Data types,variables and Arrays questions with answers

> Questions 1) What is meant by variable?
Answers : Variables are locations in memory that can hold values. Before assigning any value to a variable, it must be declared.
Questions 2) What are the kinds of variables in Java? What are their uses?
Answers : Java has three kinds of variables namely, the instance variable, the local variable and the class variable.
Local variables are used inside blocks as counters or in methods as temporary variables and are used to store information needed by a single method.
Instance variables are used to define attributes or the state of a particular object and are used to store information needed by multiple methods in the objects.
Class variables are global to a class and to all the instances of the class and are useful for communicating between different objects of all the same class or keeping track of global states.
Questions3) How are the variables declared?
Answers : Variables can be declared anywhere in the method definition and can be initialized during their declaration.They are commonly declared before usage at the beginning of the definition.
Variables with the same data type can be declared together. Local variables must be given a value before usage.
Questions 4) What are variable types?
Answers : Variable types can be any data type that java supports, which includes the eight primitive data types, the name of a class or interface and an array.
Questions 5) How do you assign values to variables?
Answers : Values are assigned to variables using the assignment operator =.
Questions 6) What is a literal? How many types of literals are there?
Answers : A literal represents a value of a certain type where the type describes how that value behaves.
There are different types of literals namely number literals, character literals,
boolean literals, string literals,etc.
Questions 7) What is an array?
Answers : An array is an object that stores a list of items.
Questions 8) How do you declare an array?
Answers : Array variable indicates the type of object that the array holds.
Ex: int arr[];
Questions 9) Java supports multidimensional arrays.
a)True
b)False
Answers : a.
Questions 10) An array of arrays can be created.
a)True
b)False
Answers : a.
Questions 11) What is a string?
Answers : A combination of characters is called as string.
Questions12) Strings are instances of the class String.
a)True
b)False
Answers : a.
Questions 13) When a string literal is used in the program, Java automatically creates instances of the string class.
a)True
b)False
Answers : a.
Questions 14) Which operator is to create and concatenate string?
Answers : Addition operator(+).
Questions15) Which of the following declare an array of string objects?
String[ ] s;
String [ ]s:
String[ s]:
String s[ ]:
Answers : a, b and d
Questions16) What is the value of a[3] as the result of the following array declaration?
1
2
3
4
Answers : d
Questions 17) Which of the following are primitive types?
byte
String
integer
Float
Answers : a.
18) What is the range of the char type?
0 to 216
0 to 215
0 to 216-1
0 to 215-1
Ans. d
Questions19) What are primitive data types?
Answers : byte, short, int, long
float, double
boolean
char
Questions 20) What are default values of different primitive types?
Answers : int - 0
short - 0
byte - 0
long - 0 l
float - 0.0 f
double - 0.0 d
boolean - false
char - null
Questions21) Converting of primitive types to objects can be explicitly.
a)True
b)False
Answers : b.
Questions 22) How do we change the values of the elements of the array?
Answers : The array subscript expression can be used to change the values of the elements of the array.
Questions 23) What is final varaible?
Answers : If a variable is declared as final variable, then you can not change its value. It becomes constant.
Questions 24) What is static variable?
Answers : Static variables are shared by all instances of a class.



Java Operators interview questions with answers

Questions 1) What are operators and what are the various types of operators available in Java?
Answers : Operators are special symbols used in expressions.
The following are the types of operators:
Arithmetic operators,
Assignment operators,
Increment & Decrement operators,
Logical operators,
Biwise operators,
Comparison/Relational operators and
Conditional operators
Questions 2) The ++ operator is used for incrementing and the -- operator is used for
decrementing.
a)True
b)False
Answers : a.
Questions3) Comparison/Logical operators are used for testing and magnitude.
a)True
b)False
Answers : a.
Questions4) Character literals are stored as unicode characters.
a)True
b)False
Answers : a.
Questions5) What are the Logical operators?
Answers : OR(|), AND(&), XOR(^) AND NOT(~).
Questions6) What is the % operator?
Answers : % operator is the modulo operator or reminder operator. It returns the reminder of dividing the first operand by second operand.
7) What is the value of 111 % 13?
3
5
7
9
Answers : c.
Questions8) Is &&= a valid operator?
Answers : No.
Questions9) Can a double value be cast to a byte?
Answers : Yes
Questions 10) Can a byte object be cast to a double value ?
Answers : No. An object cannot be cast to a primitive value.
Questions 11) What are order of precedence and associativity?
Answers : Order of precedence the order in which operators are evaluated in expressions.
Associativity determines whether an expression is evaluated left-right or right-left.
Questions12) Which Java operator is right associativity?
Answers : = operator.
Questions 13) What is the difference between prefix and postfix of -- and ++ operators?
Answers : The prefix form returns the increment or decrement operation and returns the value of the increment or decrement operation.
The postfix form returns the current value of all of the expression and then
performs the increment or decrement operation on that value.
Questions14) What is the result of expression 5.45 + "3,2"?
The double value 8.6
The string ""8.6"
The long value 8.
The String "5.453.2"
Answers : d
Questions 15) What are the values of x and y ?
x = 5; y = ++x;
Answers : x = 6; y = 6
Questions 16) What are the values of x and z?
x = 5; z = x++;
Answers : x = 6; z = 5


Java Control Statements inteview questions and answers

Questions 1) What are the programming constructs?
Answers : a) Sequential
b) Selection -- if and switch statements
c) Iteration -- for loop, while loop and do-while loop
2) class conditional {
public static void main(String args[]) {
int i = 20;
int j = 55;
int z = 0;
z = i < j ? i : j; // ternary operator
System.out.println("The value assigned is " + z);
}
}
What is output of the above program?
Answers : The value assigned is 20
Questions3) The switch statement does not require a break.
a)True
b)False
Answers : b.
Questions4) The conditional operator is otherwise known as the ternary operator.
a)True
b)False
Answers : a.
Questions5) The while loop repeats a set of code while the condition is false.
a)True
b)False
Answers : b.
Questions6) The do-while loop repeats a set of code atleast once before the condition is tested.
a)True
b)False
Answers : a.
Questions 7) What are difference between break and continue?
Answers : The break keyword halts the execution of the current loop and forces control out of the loop.
The continue is similar to break, except that instead of halting the execution of the loop, it starts the next iteration.

Questions8) The for loop repeats a set of statements a certain number of times until a condition is matched.
a)True
b)False
Answers : a.
Questions9) Can a for statement loop indefintely?
Answers : Yes.
Questions10) What is the difference between while statement and a do statement/
Answers : A while statement checks at the beginning of a loop to see whether the next loop iteration should occur.
A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

Interview questions on java Classes and Methods

Questions1) Which is used to get the value of the instance variables?
Answers : Dot notation.
Questions2) The new operator creates a single instance named class and returns a
reference to that object.
a)True
b)False
Answers : a.
Questions3) A class is a template for multiple objects with similar features.
a)True
b)False
Answers : a.
Questions 4) What is mean by garbage collection?
Answers : 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.
Questions5) What are methods and how are they defined?
Answers : 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.
Questions6) What is calling method?
Answers : Calling methods are similar to calling or referring to an instance variable. These methods are accessed using dot notation.
Ex: obj.methodname(param1,param2)
Questions7) Which method is used to determine the class of an object?
Answers : 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.
Questions 8) All the classes in java.lang package are automatically imported when
a program is compiled.
a)True
b)False
Answers : a.
Questions 9) How can class be imported to a program?
Answers : To import a class, the import keyword should be used as shown.;
import classname;
Questions 10) How can class be imported from a package to a program?
Answers : import java . packagename . classname (or) import java.package name.*;
Questions 11) What is a constructor?
Answers : A constructor is a special kind of method that determines how an object is
initialized when created.
Questions 12) Which keyword is used to create an instance of a class?
Answers : new.
Questions 13) Which method is used to garbage collect an object?
Answers : finalize ().
Questions 14) Constructors can be overloaded like regular methods.
a)True
b)False
Answers : a.
Questions 15) What is casting?
Answers : Casting is bused to convert the value of one type to another.



Questions 16) Casting between primitive types allows conversion of one primitive type to another.
a)True
b)False
Answers : a.
Questions 17) Casting occurs commonly between numeric types.
a)True
b)False
Answers : a.
Questions 18) Boolean values can be cast into any other primitive type.
a)True
b)False
Answers : b.
Questions 19) Casting does not affect the original object or value.
a)True
b)False
Answers : a.
Questions 20) Which cast must be used to convert a larger value into a smaller one?
Answers : Explicit cast.
Questions 21) Which cast must be used to cast an object to another class?
Answers : Specific cast.
Questions 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 with in objects
d.The use of pointers
Answers : a,b,c.
Questions 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
Answers : a,b,d.
Questions 24) Suppose a given instance variable has been declared private.
Can this instance variable be manipulated by methods out side its class?
a.yes
b.no
Answers : b.
Questions 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 accessablde 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
Answers : a,c.
Questions 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
Answers : b,d.
Questions 27) You would use the ____ operator to create a single instance of a named class.
a.new
b.dot
Answers : a.
Questions 28) Which of the following statements correctly describes the relation 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 seperate from the variables of other objects
d.The instance variables of each object are stored together with the variables of other objects
Answers : a,b,c.
Questions 29) If no input parameters are specified in a method declaration then the declaration will include __.
a.an empty set of parantheses
b.the term void
Answers : a.
Questions 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
Answers : a,b,c.
Questions 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
Answers : c.
Questions 32) The this reference is used in conjunction with ___methods.
a.static
b.non-static
Answers : b.
Questions 33) Which of the following operators are used in conjunction with the this and super references?
a.The new operator
b.The instanceof operator
c.The dot operator
Answers : c.
Questions 34) A constructor is automatically called when an object is instantiated
a. true
b. false
Answers : a.
Questions 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
Answers : c.
Questions 36) Each class in java can have a finalizer method
a. true
b.false
Answers : a.
Questions 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
Answers : b.
Questions 38) Because finalize () belongs to the java.lang.Object class, it is present in all ___.
a.objects
b.classes
c.methods
Answers : b.
Questions 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
Answers : a,c.
Questions 40) When you write finalize() method for your class, you are overriding a finalizer
inherited from a super class.
a.true
b.false
Answers : a.
Questions 41) Java memory management mechanism garbage collects objects which are no longer referenced
a true
b.false
Answers : a.
Questions 42) are objects referenced by a variable candidates for garbage collection when the variable goes out of scope?
a yes
b. no
Answers : a.
Questions 43) Java's garbage collector runs as a ___ priority thread waiting for __priority threads to relinquish the processor.
a.high
b.low
Answers : a,b.
Questions 44) The garbage collector will run immediately when the system is out of memory
a.true
b.false
Answers : a.
Questions 45) You can explicitly drop a object reference by setting the value of a variable whose data type is a reference type to ___
Answers : null
Questions 46) When might your program wish to run the garbage collecter?
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
Answers : a,b,d
Questions 47) For externalizable objects the class is solely responsible for the external format of its contents
a.true
b.false
Answers : a
Questions 48) When an object is stored, are all of the objects that are reachable from that object stored as well?
a.true
b.false
Answers : a
Questions 49) The default__ of objects protects private and trancient data, and supports the __ of the classes
a.evolution
b.encoding
Answers : b,a.
Questions 50) Which are keywords in Java?
a) NULL
b) sizeof
c) friend
d) extends
e) synchronized
Answers : d and e
Questions 51) When must the main class and the file name coincide?
Answers :When class is declared public.
Questions 52) What are different modifiers?
Answers : public, private, protected, default, static, trancient, volatile, final, abstract.
Questions 53) What are access modifiers?
Answers : public, private, protected, default.
Questions 54) What is meant by "Passing by value" and " Passing by reference"?
Answers : objects – pass by referrence
Methods - pass by value
Questions 55) Is a class a subclass of itself?
Answers : A class is a subclass itself.

Questions 56) What modifiers may be used with top-level class?
Answers : public, abstract, final.
Questions 57) What is an example of polymorphism?
Inner class
Anonymous classes
Method overloading
Method overriding
Answers : c


java Packages and interface interview questions

1) What are packages ? what is use of packages ?
Answers :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.*;" ?
Answers :"java.applet.Applet" will import only the class Applet from the package java.applet
Where as "java.applet.*" will import all the classes from java.applet package.
3) What do you understand by package access specifier?
Answers : 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 interface? What is use of interface?
Answers : 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 support the multiple inheritance.
5) Is it is necessary to implement all methods in an interface?
Answers : Yes. All the methods have to be implemented.
6) Which is the default access modifier for an interface method?
Answers : public.
7) Can we define a variable in an interface ?and what type it should be ?
Answers : Yes we can define a variable in an interface. They are implicitly final and static.
8) What is difference between interface and an abstract class?
Answers : All the methods declared inside an Interface are abstract. Where as 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
Answers : True
10) Java compiler stores the .class files in the path specified in CLASSPATH
environmental variable.
True/False
Answers : False

11) User-defined package can also be imported just like the standard packages.
True/False
Answers : True
12) When a program does not want to handle exception, the ______class is used.
Answers : Throws
13) The main subclass of the Exception class is _______ class.
Answers : RuntimeException
14) Only subclasses of ______class may be caught or thrown.
Answers : Throwable
15) Any user-defined exception class is a subclass of the _____ class.
Answers : Exception
16) The catch clause of the user-defined exception class should ______ its
Base class catch clause.
Answers : Exception
17) A _______ is used to separate the hierarchy of the class while declaring an
Import statement.
Answers : Package

18) All standard classes of Java are included within a package called _____.
Answers : java.lang
19) All the classes in a package can be simultaneously imported using ____.
Answers : *
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 inheritance in Java?
Answers : Using interfaces.
26) How to declare an interface example?
Answers : access class classname implements interface.
27) Can you achieve multiple interface through interface?
a)True
b) false
Answers : a.
28) Can variables be declared in an interface ? If so, what are the modifiers?
Answers : Yes. final and static are the modifiers can be declared in an interface.
29) What are the possible access modifiers when implementing interface methods?
Answers : public.
30) Can anonymous classes be implemented an interface?
Answers : Yes.
31) Interfaces can’t be extended.
a)True
b)False
Answers : b.
32) Name interfaces without a method?
Answers : Serializable, Cloneble & Remote.
33) Is it possible to use few methods of an interface in a class ? If so, how?
Answers : Yes. Declare the class as abstract.

Java Exception Handling interview questions

Questions 1) What is the difference between ‘throw’ and ‘throws’ ?And it’s application?
Answers : Exceptions that are thrown by java runtime systems can be handled by Try and catch blocks. With throw exception we can handle the exceptions thrown by the program itself. If a method is capable of causing an exception that it does not
handle, it must specify this behavior so the callers of the method can guard
against that exception.
Questions 2) What is the difference between ‘Exception’ and ‘error’ in java?
Answers : Exception and Error are the subclasses of the Throwable class. Exception class is used for exceptional conditions that user program should catch. With exception class we can subclass to create our own custom exception.
Error defines exceptions that are not excepted to be caught by you program. Example is Stack Overflow.
Questions 3) What is ‘Resource leak’?
Answers : Freeing up other resources that might have been allocated at the beginning of a method.
Questions 4)What is the ‘finally’ block?
Answers : Finally block will execute whether or not an exception is thrown. If an exception is thrown, the finally block will execute even if no catch statement match the exception. Any time a method is about to return to the caller from inside try/catch block, via an uncaught exception or an explicit return statement, the finally clause is also execute.
Questions5) Can we have catch block with out try block? If so when?
Answers : No. Try/Catch or Try/finally form a unit.
Questions 6) What is the difference between the following statements?
Catch (Exception e),
Catch (Error err),
Catch (Throwable t)
Answers :



Questions 7) What will happen to the Exception object after exception handling?
Answers : It will go for Garbage Collector. And frees the memory.
Questions 8) How many Exceptions we can define in ‘throws’ clause?
Answers : We can define multiple exceptions in throws clause.
Signature is..
type method-name (parameter-list) throws exception-list

Questions 9) The finally block is executed when an exception is thrown, even if no catch matches it.
True/False
Answers : True
Questions 10) The subclass exception should precede the base class exception when used within the catch clause.
True/False
Answers : True
Questions 11) Exceptions can be caught or rethrown to a calling method.
True/False
Answers : True
Questions 12) The statements following the throw keyword in a program are not executed.
True/False
Answers : True
Questions 13) The toString ( ) method in the user-defined exception class is overridden.
True/False
Answers : True


Jave MULTI THREADING interview questions with Answers

Questions 1) What are the two types of multitasking?
Answers : 1.process-based
2.Thread-based
Questions2) What are the two ways to create the thread?
Answers : 1.by implementing Runnable
2.by extending Thread
Questions 3) What is the signature of the constructor of a thread class?
Answers : Thread(Runnable threadob,String threadName)
Questions 4) What are all the methods available in the Runnable Interface?
Answers : run()
Questions 5) What is the data type for the method isAlive() and this method is
available in which class?
Answers : boolean, Thread
Questions 6) What are all the methods available in the Thread class?
Answers : 1.isAlive()
2.join()
3.resume()
4.suspend()
5.stop()
6.start()
7.sleep()
8.destroy()
7) What are all the methods used for Inter Thread communication and what is the class in which these methods are defined?
Answers :1. wait(),notify() & notifyall()
2. Object class
8) What is the mechanisam defind by java for the Resources to be used by only one Thread at a time?
Answers : Synchronisation
9) What is the procedure to own the moniter by many threads?
Answers : not possible
10) What is the unit for 1000 in the below statement?
ob.sleep(1000)
Answers : long milliseconds
11) What is the data type for the parameter of the sleep() method?
Answers : long
12) What are all the values for the following level?
max-priority
min-priority
normal-priority
Answers : 10,1,5
13) What is the method available for setting the priority?
Answers : setPriority()
14) What is the default thread at the time of starting the program?
Answers : main thread
15) The word synchronized can be used with only a method.
True/ False
Answers : False
16) Which priority Thread can prompt the lower primary Thread?
Answers : Higher Priority
17) How many threads at a time can access a monitor?
Answers : one
18) What are all the four states associated in the thread?
Answers : 1. new 2. runnable 3. blocked 4. dead
19) The suspend()method is used to teriminate a thread?
True /False
Answers : False
20) The run() method should necessary exists in clases created as subclass of thread?
True /False
Answers : True
21) When two threads are waiting on each other and can't proceed the programe is said to be in a deadlock?
True/False
Answers : True
22) Which method waits for the thread to die ?
Answers : join() method

23) Which of the following is true?
1) wait(),notify(),notifyall() are defined as final & can be called only from with in a synchronized method
2) Among wait(),notify(),notifyall() the wait() method only throws IOException
3) wait(),notify(),notifyall() & sleep() are methods of object class
1
2
3
1 & 2
1,2 & 3
Answers : D
24) Garbage collector thread belongs to which priority?
Answers : low-priority
25) What is meant by timeslicing or time sharing?
Answers : Timeslicing is the method of allocating CPU time to individual threads in a priority schedule.
26) What is meant by daemon thread? In java runtime, what is it's role?
Answers : Daemon thread is a low priority thread which runs intermittently in the background doing the garbage collection operation for the java runtime system.


Java Inheritance interview questions with answers

Questions 1) What is the difference between superclass & subclass?
Answers : A super class is a class that is inherited whereas subclass is a class that does the inheriting.
Questions 2) Which keyword is used to inherit a class?
Answers : extends
Questions 3) Subclasses methods can access superclass members/ attributes at all times?
True/False
Answers : False

Questions 4) When can subclasses not access superclass members?
Answers : When superclass is declared as private.
Questions 5) Which class does begin Java class hierarchy?
Answers : Object class
Questions 6) Object class is a superclass of all other classes?
True/False
Answers : True
Questions 7) Java supports multiple inheritance?
True/False
Answers : False
Questions 8) What is inheritance?
Answers : Deriving an object from an existing class. In the other words, Inheritance is the process of inheriting all the features from a class
Questions 9) What are the advantages of inheritance?
Answers : Reusability of code and accessibility of variables and methods of the superclass by subclasses.
Questions10) Which method is used to call the constructors of the superclass from the subclass?
Answers : super(argument)
Questions 11) Which is used to execute any method of the superclass from the subclass?
Answers : super.method-name(arguments)
Questions 12) Which methods are used to destroy the objects created by the constructor methods?
Answers : finalize()
Questions 13) What are abstract classes?
Answers : Abstract classes are those for which instances can’t be created.
Questions 14) What must a class do to implement an interface?
Answers : It must provide all of the methods in the interface and identify the interface in its implements clause.
Questions 15) Which methods in the Object class are declared as final?
Answers : getClass(), notify(), notifyAll(), and wait()
Questions 16) Final methods can be overridden.
True/False
Answers : False
Questions 17) Declaration of methods as final results in faster execution of the program?
True/False
Answers : True
Questions 18) Final variables should be declared in the beginning?
True/False
Answers : True
Questions 19) Can we declare variable inside a method as final variables? Why?
Answers : Cannot because, local variable cannot be declared as final variables.
Questions20) Can an abstract class may be final?
Answers : An abstract class may not be declared as final.
Questions 21) Does a class inherit the constructors of it's super class?
Answers : A class does not inherit constructors from any of it's super classes.
Questions 22) What restrictions are placed on method overloading?
Answers : Two methods may not have the same name and argument list but different return types.
Questions 23) What restrictions are placed on method overriding?
Answers : Overridden methods must have the same name , argument list , and return type. The overriding method may not limit the access of the method it overridees.The overriding method may not throw any exceptions that may not be thrown by the overridden method.
Questions 24) What modifiers may be used with an inner class that is a member of an outer class?
Answers : a (non-local) inner class may be declared as public, protected, private, static, final or abstract.
Questions 25) How this() is used with constructors?
Answers : this() is used to invoke a constructor of the same class
Questions 26) How super() used with constructors?
Answers : super() is used to invoke a super class constructor
Questions 27) Which of the following statements correctly describes an interface?
a)It's a concrete class
b)It's a superclass
c)It's a type of abstract class
Answers : c
28) An interface contains __ methods
a)Non-abstract
b)Implemented
c)unimplemented
Answers :c
STRING HANDLING
Which package does define String and StringBuffer classes?
Answers : java.lang package.
Which method can be used to obtain the length of the String?
Answers : length( ) method.
How do you concatenate Strings?
Answers : By using " + " operator.
Which method can be used to compare two strings for equality?
Answers : equals( ) method.
Which method can be used to perform a comparison between strings that ignores case differences?
Answers : equalsIgnoreCase( ) method.
What is the use of valueOf( ) method?
Answers : valueOf( ) method converts data from its internal format into a human-readable form.
What are the uses of toLowerCase( ) and toUpperCase( ) methods?
Answers : The method toLowerCase( ) converts all the characters in a string from uppercase to
lowercase.
The method toUpperCase( ) converts all the characters in a string from lowercase to
uppercase.
Which method can be used to find out the total allocated capacity of a StrinBuffer?
Answers : capacity( ) method.
Which method can be used to set the length of the buffer within a StringBuffer object?
Answers : setLength( ).
What is the difference between String and StringBuffer?
Answers : String objects are constants, whereas StringBuffer objects are not.
String class supports constant strings, whereas StringBuffer class supports growable, modifiable strings.
What are wrapper classes?
Answers : Wrapper classes are classes that allow primitive types to be accessed as objects.
Which of the following is not a wrapper class?
String
Integer
Boolean
Character
Answers : a.
What is the output of the following program?
public class Question {
public static void main(String args[]) {
String s1 = "abc";
String s2 = "def";
String s3 = s1.concat(s2.toUpperCase( ) );
System.out.println(s1+s2+s3);
}
}
abcdefabcdef
abcabcDEFDEF
abcdefabcDEF
None of the above
Answers : c.
Which of the following methods are methods of the String class?
delete( )
append( )
reverse( )
replace( )
Answers : d.
Which of the following methods cause the String object referenced by s to be changed?
s.concat( )
s.toUpperCase( )
s.replace( )
s.valueOf( )
Answers : a and b.
String is a wrapper class?
True
False
Answers : b.
17) If you run the code below, what gets printed out?
String s=new String("Bicycle");
int iBegin=1;
char iEnd=3;
System.out.println(s.substring(iBegin,iEnd));
Bic
ic
c) icy
d) error: no method matching substring(int,char)
Answers : b.
18) Given the following declarations
String s1=new String("Hello")
String s2=new String("there");
String s3=new String();
Which of the following are legal operations?
s3=s1 + s2;
s3=s1 - s2;
c) s3=s1 & s2
d) s3=s1 && s2
Answers : a.
19) Which of the following statements are true?
The String class is implemented as a char array, elements are addressed using the stringname[] convention
b) Strings are a primitive type in Java that overloads the + operator for concatenation
c) Strings are a primitive type in Java and the StringBuffer is used as the matching wrapper type
d) The size of a string can be retrieved using the length property.
Answers : b.



EXPLORING JAVA.LANG
java.lang package is automatically imported into all programs.
True
False
Answers : a
What are the interfaces defined by java.lang?
Answers : Cloneable, Comparable and Runnable.
What are the constants defined by both Flaot and Double classes?
Answers : MAX_VALUE,
MIN_VALUE,
NaN,
POSITIVE_INFINITY,
NEGATIVE_INFINITY and
TYPE.
What are the constants defined by Byte, Short, Integer and Long?
Answers : MAX_VALUE,
MIN_VALUE and
TYPE.
What are the constants defined by both Float and Double classes?
Answers : MAX_RADIX,
MIN_RADIX,
MAX_VALUE,
MIN_VALUE and
TYPE.
What is the purpose of the Runtime class?
Answers : The purpose of the Runtime class is to provide access to the Java runtime system.
What is the purpose of the System class?
Answers : The purpose of the System class is to provide access to system resources.
Which class is extended by all other classes?
Answers : Object class is extended by all other classes.
Which class can be used to obtain design information about an object?
Answers : The Class class can be used to obtain information about an object’s design.
Which method is used to calculate the absolute value of a number?
Answers : abs( ) method.
What are E and PI?
Answers : E is the base of the natural logarithm and PI is the mathematical value pi.
Which of the following classes is used to perform basic console I/O?
System
SecurityManager
Math
Runtime
Answers : a.
Which of the following are true?
The Class class is the superclass of the Object class.
The Object class is final.
The Class class can be used to load other classes.
The ClassLoader class can be used to load other classes.
Answers : c and d.
Which of the following methods are methods of the Math class?
absolute( )
log( )
cosine( )
sine( )
Answers : b.
Which of the following are true about the Error and Exception classes?
Both classes extend Throwable.
The Error class is final and the Exception class is not.
The Exception class is final and the Error is not.
Both classes implement Throwable.
Answers : a.
Which of the following are true?
The Void class extends the Class class.
The Float class extends the Double class.
The System class extends the Runtime class.
The Integer class extends the Number class.
Answers : d.



17) Which of the following will output -4.0
System.out.println(Math.floor(-4.7));
System.out.println(Math.round(-4.7));
System.out.println(Math.ceil(-4.7));
d) System.out.println(Math.Min(-4.7));
Answers : c.
18) Which of the following are valid statements
a) public class MyCalc extends Math
b) Math.max(s);
c) Math.round(9.99,1);
d) Math.mod(4,10);
e) None of the above.
Answers : e.
19) What will happen if you attempt to compile and run the following code?
Integer ten=new Integer(10);
Long nine=new Long (9);
System.out.println(ten + nine);
int i=1;
System.out.println(i + ten);
19 followed by 20
19 followed by 11
Error: Can't convert java lang Integer
d) 10 followed by 1
Answers : c.

INPUT / OUTPUT : EXPLORING JAVA.IO
What is meant by Stream and what are the types of Streams and classes of the Streams?
Answers : A Stream is an abstraction that either produces or consumes information.
There are two types of Streams. They are:
Byte Streams : Byte Streams provide a convenient means for handling input and output of bytes.
Character Streams : Character Streams provide a convenient means for handling input and output of characters.
Byte Stream classes : Byte Streams are defined by using two abstract classes. They are:InputStream and OutputStream.
Character Stream classes : Character Streams are defined by using two abstract classes. They are : Reader and Writer.
Which of the following statements are true?
UTF characters are all 8-bits.
UTF characters are all 16-bits.
UTF characters are all 24-bits.
Unicode characters are all 16-bits.
Bytecode characters are all 16-bits.
Answers : d.
Which of the following statements are true?
When you construct an instance of File, if you do not use the filenaming semantics of the local machine, the constructor will throw an IOException.
When you construct an instance of File, if the corresponding file does not exist on the local file system, one will be created.
When an instance of File is garbage collected, the corresponding file on the local file system is deleted.
None of the above.
Answers : a,b and c.
The File class contains a method that changes the current working directory.
True
False
Answers : b.
It is possible to use the File class to list the contents of the current working directory.
True
False
Answers : a.
Readers have methods that can read and return floats and doubles.
True
False
Answers : b.
You execute the code below in an empty directory. What is the result?
File f1 = new File("dirname");
File f2 = new File(f1, "filename");
A new directory called dirname is created in the current working directory.
A new directory called dirname is created in the current working directory. A new file called filename is created in directory dirname.
A new directory called dirname and a new file called filename are created, both in the current working directory.
A new file called filename is created in the current working directory.
No directory is created, and no file is created.
Answers : e.
What is the difference between the Reader/Writer class hierarchy and the
InputStream/OutputStream class hierarchy?
Answers : The Reader/Writer class hierarchy is character-oriented and the InputStream/OutputStream class hierarchy is byte-oriented.
What is an I/O filter?
Answers : An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
What is the purpose of the File class?
Answers : The File class is used to create objects that provide access to the files and directories of a local file system.
What interface must an object implement before it can be written to a stream as an object?
Answers : An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.
What is the difference between the File and RandomAccessFile classes?
Answers : The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.
What class allows you to read objects directly from a stream?
Answers : The ObjectInputStream class supports the reading of objects from input streams.
What value does read( ) return when it has reached the end of a file?
Answers : The read( ) method returns – 1 when it has reached the end of a file.
What value does readLine( ) return when it has reached the end of a file?
Answers : The readLine( ) method returns null when it has reached the end of a file.
How many bits are used to represent Unicode, ASCII, UTF-16 and UTF-8 characters?
Answers : Unicode requires 16-bits and ASCII requires 8-bits. Although the ASCII character set uses only 1-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.
Which of the following are true?
The InputStream and OutputStream classes are byte-oriented.
The ObjectInputStream and ObjectOutputStream do not support serialized object input and output.
The Reader and Writer classes are character-oriented.
The Reader and Writer classes are the preferred solution to serialized object output.
Answers : a and c.
Which of the following are true about I/O filters?
Filters are supported on input, but not on output.
Filters are supported by the InputStream/OutputStream class hierarchy, but not by the Reader/Writer class hierarchy.
Filters read from one stream and write to another.
A filter may alter data that is read from one stream and written to another.
Answers : c and d.
Which of the following are true?
Any Unicode character is represented using 16-bits.
7-bits are needed to represent any ASCII character.
UTF-8 characters are represented using only 8-bits.
UTF-16 characters are represented using only 16-bits.
Answers : a and b.
Which of the following are true?
The Serializable interface is used to identify objects that may be written to an output stream.
The Externalizable interface is implemented by classes that control the way in which their objects are serialized.
The Serializable interface extends the Externalizable interface.
The Externalizable interface extends the Serializable interface.
Answers : a, b and d.
Which of the following are true about the File class?
A File object can be used to change the current working directory.
A File object can be used to access the files in the current directory.
When a File object is created, a corresponding directory or file is created in the local file system.
File objects are used to access files and directories on the local file system.
File objects can be garbage collected.
When a File object is garbage collected, the corresponding file or directory is deleted.
Answers : b, d and e.
How do you create a Reader object from an InputStream object?
Use the static createReader( ) method of InputStream class.
Use the static createReader( ) method of Reader class.
Create an InputStreamReader object, passing the InputStream object as an argument to the InputStreamReader constructor.
Create an OutputStreamReader object, passing the InputStream object as an argument to the OutputStreamReader constructor.
Answers : c.
Which of the following are true?
Writer classes can be used to write characters to output streams using different character encodings.
Writer classes can be used to write Unicode characters to output streams.
Writer classes have methods that support the writing of the values of any Java primitive type to output streams.
Writer classes have methods that support the writing of objects to output streams.
Answers : a and b.
The isFile( ) method returns a boolean value depending on whether the file object is a file or a directory.
True.
False.
Answers : a.
Reading or writing can be done even after closing the input/output source.
True.
False.
Answers : b.

The ________ method helps in clearing the buffer.
Answers : flush( ).
The System.err method is used to print error message.
True.
False.
Answers : a.
What is meant by StreamTokenizer?
Answers : StreamTokenizer breaks up InputStream into tokens that are delimited by sets of characters.
It has the constructor : StreamTokenizer(Reader inStream).
Here inStream must be some form of Reader.
What is Serialization and deserialization?
Answers : Serialization is the process of writing the state of an object to a byte stream.
Deserialization is the process of restoring these objects.
30) Which of the following can you perform using the File class?
a) Change the current directory
b) Return the name of the parent directory
c) Delete a file
d) Find if a file contains text or binary information
Answers : b and c.
31)How can you change the current working directory using an instance of the File class called FileName?
FileName.chdir("DirName").
FileName.cd("DirName").
FileName.cwd("DirName").
The File class does not support directly changing the current directory.
Answers : d.


EVENT HANDLING
The event delegation model, introduced in release 1.1 of the JDK, is fully compatible with the
event model.
True
False
Answers : b.
A component subclass that has executed enableEvents( ) to enable processing of a certain kind of event cannot also use an adapter as a listener for the same kind of event.
True
False
Answers : b.
What is the highest-level event class of the event-delegation model?
Answers : The java.util.eventObject class is the highest-level class in the event-delegation hierarchy.
What interface is extended by AWT event listeners?
Answers : All AWT event listeners extend the java.util.EventListener interface.
What class is the top of the AWT event hierarchy?
Answers : The java.awt.AWTEvent class is the highest-level class in the AWT event class hierarchy.
What event results from the clicking of a button?
Answers : The ActionEvent event is generated as the result of the clicking of a button.
What is the relationship between an event-listener interface and an event-adapter class?
Answers : An event-listener interface defines the methods that must be implemented by an event
handler for a particular kind of event.
An event adapter provides a default implementation of an event-listener interface.
In which package are most of the AWT events that support the event-delegation model defined?
Answers : 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.
What is the advantage of the event-delegation model over the earlier event-inheritance model?
Answers : The event-delegation has two advantages over the event-inheritance model. They are :
It enables event handling by objects other than the ones that generate the events. This
allows a clean separation between a component’s design and its use.
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.
What is the purpose of the enableEvents( ) method?
Answers :The enableEvents( ) method is used to enable an event for a particular object.
Which of the following are true?
The event-inheritance model has replaced the event-delegation model.
The event-inheritance model is more efficient than the event-delegation model.
The event-delegation model uses event listeners to define the methods of event-handling classes.
The event-delegation model uses the handleEvent( ) method to support event handling.
Answers : c.
Which of the following is the highest class in the event-delegation model?
java.util.EventListener
java.util.EventObject
java.awt.AWTEvent
java.awt.event.AWTEvent
Answers : b.
When two or more objects are added as listeners for the same event, which listener is first invoked to handle the event?
The first object that was added as listener.
The last object that was added as listener.
There is no way to determine which listener will be invoked first.
It is impossible to have more than one listener for a given event.
Answers : c.
Which of the following components generate action events?
Buttons
Labels
Check boxes
Windows
Answers : a.
Which of the following are true?
A TextField object may generate an ActionEvent.
A TextArea object may generate an ActionEvent.
A Button object may generate an ActionEvent.
A MenuItem object may generate an ActionEvent.
Answers : a,c and d.
Which of the following are true?
The MouseListener interface defines methods for handling mouse clicks.
The MouseMotionListener interface defines methods for handling mouse clicks.
The MouseClickListener interface defines methods for handling mouse clicks.
The ActionListener interface defi