Core Java

Front 1

What is the Collections API?

Back 1

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

Front 2

What is the List interface?

Back 2

The List interface provides support for ordered collections of objects

Front 3

What is the Vector class?

Back 3

The Vector class provides the capability to implement a growable array of objects.

Front 4

What is an Iterator interface?

Back 4

The Iterator interface is used to step through the elements of a Collection .

Front 5

Which java.util classes and interfaces support event handling?

Back 5

The EventObject class and the EventListener interface support event processing.

Front 6

What is the GregorianCalendar class?

Back 6

The GregorianCalendar provides support for traditional Western calendars

Front 7

What is the Map interface?

Back 7

The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

Front 8

What is the Collection interface?

Back 8

The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates.

Front 9

What is the Set interface?

Back 9

The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

Front 10

What is the typical use of Hashtable?

Back 10

Whenever a program wants to store a key value pair, one can use Hashtable.

Front 11

I am trying to store an object using a key in a Hashtable. And some other object already exists in that location, then what will happen? The existing object will be overwritten? Or the new object will be stored elsewhere?

Back 11

The existing object will be overwritten and thus it will be lost.

Front 12

What is the difference between the size and capacity of a Vector?

Back 12

he size is the number of elements actually stored in the vector, while capacity is the maximum number of elements it can store at a given instance of time.

Front 13

Can a vector contain heterogenous objects?

Back 13

Yes a Vector can contain heterogenous objects. Because a Vector stores everything in terms of Object.

Front 14

an a ArrayList contain heterogenous objects?

Back 14

Yes a ArrayList can contain heterogenous objects. Because a ArrayList stores everything in terms of Object.

Front 15

What is an enumeration?

Back 15

An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It is a construct which collection classes return when you request a collection of all the objects stored in the collection. It allows sequential access to all the elements stored in the collection.

Front 16

Considering the basic properties of Vector and ArrayList, where will you use Vector and where will you use ArrayList?

Back 16

he basic difference between a Vector and an ArrayList is that, vector is synchronized while ArrayList is not. Thus whenever there is a possibility of multiple threads accessing the same instance, one should use Vector. While if not multiple threads are going to access the same instance then use ArrayList. Non synchronized data structure will give better performance than the synchronized one.

Front 17

What is the difference between an Interface and an Abstract class?

Back 17

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

Front 18

What is the purpose of garbage collection in Java, and when is it used?

Back 18

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Front 19

Describe synchronization in respect to multithreading.

Back 19

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

Front 20

Explain different way of using thread?

Back 20

The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.

Front 21

What are pass by reference and passby value?

Back 21

Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.

Front 22

What is HashMap and Map?

Back 22

Map is Interface and Hashmap is class that implements that.

Front 23

Difference between HashMap and HashTable?

Back 23

The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is non synchronized and Hashtable is synchronized.

Front 24

What is the difference between a constructor and a method?

Back 24

A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

Front 25

What is an Iterators?

Back 25

Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

Front 26

State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.

Back 26

A: public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.

Front 27

What is an abstract class?

Back 27

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

Front 28

What is static in java?

Back 28

tatic means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

Front 29

What is final?

Back 29

A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

Front 30

What if the main method is declared as private?

Back 30

The program compiles properly but at runtime it will give "Main method not public." message.

Front 31

What if the static modifier is removed from the signature of the main method?

Back 31

Program compiles. But at runtime throws an error "NoSuchMethodError".

Front 32

What if I write static public void instead of public static void?

Back 32

Program compiles and runs properly.

Front 33

What if I do not provide the String array as the argument to the method?

Back 33

Program compiles but throws a runtime error "NoSuchMethodError".

Front 34

What is the first argument of the String array in main method?

Back 34

The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.

Front 35

If I do not provide any arguments on the command line, then the String array of Main method will be empty of null?

Back 35

It is empty. But not null.

Front 36

How can one prove that the array is not null but empty?

Back 36

Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.

Front 37

Can I have multiple main methods in the same class?

Back 37

No the program fails to compile. The compiler says that the main method is already defined in the class.

Front 38

Can an application have multiple classes having main method?

Back 38

Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.

Front 39

What are Checked and UnChecked Exception?

Back 39

A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.
Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method•
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the
exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method• Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

Front 40

What is Overriding?

Back 40

When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.

Front 41

What are different types of inner classes?

Back 41

Nested top-level classes, Member classes, Local classes, Anonymous classes

Front 42

Member Classes?

Back 42

Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.

Front 43

Local CLasses?

Back 43

Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a
more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.

Front 44

Anonymous classes?

Back 44

Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Front 45

Nested top-level classes?

Back 45

Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class.
Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety.

Front 46

Can a top level class be private or protected?

Back 46

No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.

Front 47

What type of parameter passing does Java support?

Back 47

In Java the arguments are always passed by value .

Front 48

Primitive data types are passed by reference or pass by value?

Back 48

Primitive data types are passed by value.

Front 49

Objects are passed by value or by reference?

Back 49

Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .

Front 50

What is serialization?

Back 50

Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

Front 51

How do I serialize an object to a file?

Back 51

The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.

Front 52

Which methods of Serializable interface should I implement?

Back 52

he serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.

Front 53

How can I customize the seralization process? i.e. how can one have a control over the serialization process?

Back 53

Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.

Front 54

What is the common usage of serialization?

Back 54

Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.

Front 55

What is Externalizable interface?

Back 55

Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

Front 56

What happens to the object references included in the object?

Back 56

The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.

Front 57

What one should take care of while serializing the object?

Back 57

One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.

Front 58

What happens to the static fields of a class during serialization?

Back 58

A: There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields

Front 59

What are wrapper classes?

Back 59

A: Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.

Front 60

Why do we need wrapper classes?

Back 60

A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.

Front 61

What are runtime exceptions?

Back 61

A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

Front 62

What is the difference between error and an exception?

Back 62

A: An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).

Front 63

If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?

Back 63

A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

Front 64

What happens to an unhandled exception?

Back 64

One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

Front 65

How does an exception permeate through the code?

Back 65

n unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

Front 66

What are the different ways to handle exceptions?

Back 66

There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.

Front 67

What is the basic difference between the 2 approaches to exception handling...1> try catch block and 2> specifying the candidate exceptions in the throws clause?
When should you use which approach?

Back 67

A: In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

Front 68

Is it necessary that each try block must be followed by a catch block?

Back 68

t is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

Front 69

If I write return at the end of the try block, will the finally block still execute?

Back 69

A: Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.

Front 70

If I write System.exit (0); at the end of the try block, will the finally block still execute?

Back 70

A: No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

Front 71

How are Observer and Observable used?

Back 71

A: 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.

Front 72

What is synchronization and why is it important?

Back 72

A: 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.

Front 73

Does garbage collection guarantee that a program will not run out of memory?

Back 73

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

Front 74

What is the difference between preemptive scheduling and time slicing?

Back 74

A: 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.

Front 75

When a thread is created and started, what is its initial state?

Back 75

A: A thread is in the ready state after it has been created and started.

Front 76

What is the purpose of finalization?

Back 76

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

Front 77

How are this() and super() used with constructors?

Back 77

A: Othis() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

Front 78

What are synchronized methods and synchronized statements?

Back 78

A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Front 79

What is daemon thread and which method is used to create the daemon thread?

Back 79

A: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.

Front 80

What are the steps in the JDBC connection?

Back 80

A: While making a JDBC connection we go through the following steps :

Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();

Front 81

How does a try statement determine which catch clause should be used to handle an exception?

Back 81

A: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.

Front 82

Can an unreachable object become reachable again?

Back 82

A: An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

Front 83

What modifiers are allowed for methods in an Interface?

Back 83

Only public and abstract modifiers are allowed for methods in interfaces.

Front 84

What are some alternatives to inheritance?

Back 84

A: 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).

Front 85

What does it mean that a method or field is "static"?

Back 85

A: 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.

Front 86

What is the difference between preemptive scheduling and time slicing?

Back 86

A: 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.

Front 87

What is the catch or declare rule for method declarations?

Back 87

A: 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.

Front 88

What is transient variable?

Back 88

Answer: Transient variable can't be serialize. For example if a variable is
declared as transient in a Serializable class and the class is written to an
ObjectStream, the value of the variable can't be written to the stream instead when
the class is retrieved from the ObjectStream the value of the variable becomes
null.

Front 89

Is Iterator a Class or Interface? What is its use?

Back 89

Iterator is an interface which is used to step through the elements of a
Collection.

Front 90

Question: What is similarities/difference between an Abstract class and
Interface?

Back 90

Differences are as follows:
• Interfaces provide a form of multiple inheritance. A class can extend only
one other class.
• Interfaces are limited to public methods and constants with no
implementation. Abstract classes can have a partial implementation, protected
parts, static methods, etc.
• A Class may implement several interfaces. But in case of abstract class, a
class may extend only one abstract class.
• Interfaces are slow as it requires extra indirection to to find
corresponding method in in the actual class. Abstract classes are fast.
Similarities:
• Neither Abstract classes or Interface can be instantiated.

Front 91

Explain the Encapsulation principle.

Back 91

Answer: Encapsulation is a process of binding or wrapping the data and the codes
that operates on the data into a single entity. This keeps the data safe from
outside interface and misuse. One way to think about encapsulation is as a
protective wrapper that prevents code and data from being arbitrarily accessed by
other code defined outside the wrapper.

Front 92

Question: Explain the Inheritance principle.

Back 92

Answer: Inheritance is the process by which one object acquires the properties of
another object.

Front 93

Explain the Polymorphism principle.

Back 93

Answer: The meaning of Polymorphism is something like one name many forms.
Polymorphism enables one entity to be used as as general category for different
types of actions. The specific action is determined by the exact nature of the
situation. The concept of polymorphism can be explained as "one interface, multiple
methods".

Front 94

Question: Explain the different forms of Polymorphism.

Back 94

From a practical programming viewpoint, polymorphism exists in three
distinct forms in Java:
• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface

Front 95

Can a private method of a superclass be declared within a
subclass?

Back 95

Sure. A private field or method or inner class belongs to its declared
class and hides from its subclasses. There is no way for private stuff to have a
runtime overloading or overriding (polymorphism) features.

Front 96

What is the difference between final, finally and finalize?

Back 96

o final - declare constant
o finally - handles exception
o finalize - helps in garbage collection

Front 97

Where and how can you use a private constructor.

Back 97

Private constructor can be used if you do not want any other class to
instanstiate the object , the instantiation is done from a static public method,
this method is used when dealing with the factory method pattern when the designer
wants only one controller (fatory method ) to create the object.

Front 98

In System.out.println(),what is System,out and println,pls
explain?

Back 98

System is a predefined final class,out is a PrintStream object and println
is a built-in overloaded method in the out object.

Front 99

What is meant by "Abstract Interface"?

Back 99

First, an interface is abstract. That means you cannot have any
implementation in an interface. All the methods declared in an interface are
abstract methods or signatures of the methods.

Front 100

arsers? DOM vs SAX parser

Back 100

parsers are fundamental xml components, a bridge between XML documents and
applications that process that XML. The parser is responsible for handling xml
syntax, checking the contents of the document against constraints established in a
DTD or Schema.

Front 101

What are three ways in which a thread can enter the waiting
state?

Back 101

A thread can enter the waiting state byinvoking its sleep() method, by
blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by
invoking an object's wait() method. It can also enter the waiting state by invoking
its (deprecated) suspend() method.

Front 102

Can a lock be acquired on a class?

Back 102

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

Front 103

What is thread?

Back 103

A thread is an independent path of execution in a system.

Front 104

What is multithreading?

Back 104

Multithreading means various threads that run in a system.

Front 105

How does multithreading take place on a computer with a single
CPU?

Back 105

The operating system's task scheduler allocates execution time to multiple
tasks. By quickly switching between executing tasks, it creates the impression that
tasks execute sequentially.

Front 106

How to create multithread in a program?

Back 106

You have two ways to do so. First, making your class "extends" Thread
class. Second, making your class "implements" Runnable interface. Put jobs in a
run() method and call start() method to start the thread.

Front 107

Can Java object be locked down for exclusive use by a given
thread?

Back 107

Yes. You can lock an object by putting it in a "synchronized" block. The
locked object is inaccessible to any thread other than the one that explicitly
claimed it

Front 108

What invokes a thread's run() method?

Back 108

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

Front 109

What is the purpose of the wait(), notify(), and notifyAll() methods?

Back 109

The wait(),notify(), and notifyAll() methods are used to provide an
efficient way for threads to communicate each other.

Front 110

What are the high-level thread states?

Back 110

The high-level thread states are ready, running, waiting, and dead.

Front 111

What is the difference between yielding and sleeping?

Back 111

hen 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.

Front 112

What is an Object and how do you allocate memory to it?

Back 112

Object is an instance of a class and it is a software unit that combines a
structured set of data with a set of operations for inspecting and manipulating
that data. When an object is created using new operator, memory is allocated to it.

Front 113

What is the difference between constructor and method?

Back 113

Constructor will be automatically invoked when an object is created whereas
method has to be called explicitly.

Front 114

What are methods and how are they defined?

Back 114

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.

Front 115

What are Transient and Volatile Modifiers?

Back 115

ransient: The transient modifier applies to variables only and it is not
stored as part of its object’s Persistent state. Transient variables are not
serialized. Volatile: Volatile modifier applies to variables only and it tells the
compiler that the variable modified by volatile can be changed unexpectedly by
other parts of the program.

Front 116

What is method overloading and method overriding?

Back 116

Method overloading: When a method in a class having the same method name with
different arguments is said to be method overloading. Method overriding : When a
method in a class having the same method name with same arguments is said to be
method overriding.

Front 117

What is difference between overloading and overriding?

Back 117

a) In overloading, there is a relationship between methods available in the
same class whereas in overriding, there is relationship between a superclass method
and subclass method. b) Overloading does not block inheritance from the superclass
whereas overriding blocks inheritance from the superclass. c) In overloading,
separate methods share the same name whereas in overriding, subclass method
replaces the superclass. d) Overloading must have different method signatures
whereas overriding must have same signature.

Front 118

What modifiers may be used with top-level class?

Back 118

public, abstract and final can be used for top-level class.

Front 119

What are inner class and anonymous class?

Back 119

nner class : classes defined in other classes, including those defined in
methods are called inner classes. An inner class can have any accessibility including private.

Anonymous class : Anonymous class is a class defined inside a
method without a name and is instantiated and declared in the same place and cannot
have explicit constructors.

Front 120

What is interface and its use?

Back 120

Interface is similar to a class which may
contain method’s signature only but not bodies and it is a formal set of method and
constant declarations that must be defined by the class that implements it.
Interfaces are useful for: a)Declaring methods that one or more classes are
expected to implement b)Capturing similarities between unrelated classes without
forcing a class relationship. c)Determining an object’s programming interface
without revealing the actual body of the class.

Front 121

What is a cloneable interface and how many methods does it contain?

Back 121

It is not having any method because it is a TAGGED or MARKER interface.

Front 122

What is the difference between abstract class and interface?

Back 122

a) 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.
b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods.
c) Abstract class must have subclasses whereas interface can’t have subclasses.

Front 123

Can you have an inner class inside a method and what variables can you
access?

Back 123

Yes, we can have an inner class inside a method and final variables can be accessed.

Front 124

What is the difference between String and String Buffer?

Back 124

a) String objects are constants and immutable whereas StringBuffer objects are not.
b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.

Front 125

What is the difference between Array and vector?

Back 125

Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.

Front 126

What is the difference between exception and error?

Back 126

The exception class
defines mild error conditions that your program encounters. Exceptions can occur
when trying to open the file, which does not exist, the network connection is
disrupted, operands being manipulated are out of prescribed ranges, the class file
you are interested in loading is missing. The error class defines serious error
conditions that you should not attempt to recover from. In most cases it is
advisable to let the program terminate when such an error is encountered.

Front 127

What is the difference between process and thread?-

Back 127

Process is a program in
execution whereas thread is a separate path of execution in a program.

Front 128

What is multithreading and what are the methods for inter-thread
communication and what is the class in which these methods are defined?-

Back 128

Multithreading is the mechanism in which more than one thread run independent of
each other within the process. wait (), notify () and notifyAll() methods can be
used for inter-thread communication and these methods are in Object class. wait() :
When a thread executes a call to wait() method, it surrenders the object lock and
enters into a waiting state. notify() or notifyAll() : To remove a thread from the
waiting state, some other thread must make a call to notify() or notifyAll() method
on the same object.

Front 129

What is the class and interface in java to create thread and which is the
most advantageous method?

Back 129

hread class and Runnable interface can be used to
create threads and using Runnable interface is the most advantageous method to
create threads because we need not extend thread class here.

Front 130

When you will synchronize a piece of your code?-

Back 130

When you expect your code
will be accessed by different threads and these threads may change a particular
data causing data corruption.

Front 131

What is deadlock?-

Back 131

When two threads are waiting each other and can’t
precede the program is said to be deadlock.

Front 132

What is daemon thread and which method is used to create the daemon
thread?-

Back 132

Daemon thread is a low priority thread which runs intermittently in the
back ground doing the garbage collection operation for the java runtime system.
setDaemon method is used to create a daemon thread.

Front 133

What are Vector, Hashtable, LinkedList and Enumeration?-

Back 133

Vector : The
Vector class provides the capability to implement a growable array of objects.
Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable
indexes and stores objects in a dictionary using hash codes as the object’s keys.
Hash codes are integer values that identify objects. LinkedList: Removing or
inserting elements in the middle of an array can be done using LinkedList. A
LinkedList stores each object in a separate link whereas an array stores object
references in consecutive locations. Enumeration: An object that implements the
Enumeration interface generates a series of elements, one at a time. It has two
methods, namely hasMoreElements() and nextElement(). HasMoreElemnts() tests if this
enumeration has more elements and nextElement method returns successive elements of
the series.

Front 134

What is the difference between set and list?

Back 134

Set stores elements in an
unordered way but does not contain duplicate elements, whereas list stores elements
in an ordered way but may contain duplicate elements.

Front 135

What is a stream and what are the types of Streams and classes of the
Streams?-

Back 135

A Stream is an abstraction that either produces or consumes information.
There are two types of Streams and they are: Byte Streams: Provide a convenient
means for handling input and output of bytes. Character Streams: Provide a
convenient means for handling input & output of characters. Byte Streams classes:
Are defined by using two abstract classes, namely InputStream and OutputStream.
Character Streams classes: Are defined by using two abstract classes, namely Reader
and Writer.

Front 136

What JDBC drivers are available?-

Back 136

) JDBC-ODBC Bridge driver b) Native API
Partly-Java driver c) JDBC-Net Pure Java driver d) Native-Protocol Pure Java driver

Front 137

What is the difference between JDBC and ODBC

Back 137

a) OBDC is for Microsoft and
JDBC is for Java applications. b) ODBC can’t be directly used with Java because it
uses a C interface. c) ODBC makes use of pointers which have been removed totally
from Java. d) ODBC mixes simple and advanced features together and has complex
options for simple queries. But JDBC is designed to keep things simple while
allowing advanced capabilities when required. e) ODBC requires manual installation
of the ODBC driver manager and driver on all client machines. JDBC drivers are
written in Java and JDBC code is automatically installable, secure, and portable on
all platforms. f) JDBC API is a natural Java interface and is built on ODBC. JDBC
retains some of the basic features of ODBC.

Front 138

What are the types of statements in JDBC?

Back 138

Statement: to be used
createStatement() method for executing single SQL statement PreparedStatement — To
be used preparedStatement() method for executing same SQL statement over and over.
CallableStatement — To be used prepareCall() method for multiple SQL statements
over and over.

Front 139

What is stored procedure?

Back 139

Stored procedure is a group of SQL statements
that forms a logical unit and performs a particular task. Stored Procedures are
used to encapsulate a set of operations or queries to execute on database. Stored
procedures can be compiled and executed with different parameters and results and
may have any combination of input/output parameters.

Front 140

How to create and call stored procedures?

Back 140

To create stored procedures:
Create procedure procedurename (specify in, out and in out parameters) BEGIN Any
multiple SQL statement; END; To call stored procedures: CallableStatement csmt =
con. prepareCall(”{call procedure name(?,?)}”); csmt. registerOutParameter(column
no. , data type); csmt. setInt(column no. , column name) csmt. execute();

Front 141

What is servlet?

Back 141

Servlets are modules that extend
request/response-oriented servers, such as java-enabled web servers. For example, a
servlet might be responsible for taking data in an HTML order-entry form and
applying the business logic used to update a company’s order database.

Front 142

What are the classes and interfaces for servlets?

Back 142

There are two packages
in servlets and they are javax. servlet and

Front 143

What is the difference between doPost and doGet methods?-

Back 143

a) doGet() method is used to get information, while doPost() method is used for posting information.
b) doGet() requests can’t send large amount of information and is limited to
240-255 characters. However, doPost()requests passes all of its data, of unlimited
length.
c) A doGet() request is appended to the request URL in a query string and
this allows the exchange is visible to the client, whereas a doPost() request
passes directly over the socket connection as part of its HTTP request body and the
exchange are invisible to the client.

Front 144

What is the life cycle of a servlet?-

Back 144

Each Servlet has the same life cycle:
a) A server loads and initializes the servlet by init () method. b) The servlet
handles zero or more client’s requests through service() method. c) The server
removes the servlet through destroy() method.

Front 145

What are implicit objects? List them?

Back 145

ertain objects that are available for the use in JSP documents without

being declared first. These objects are parsed by the JSP engine and inserted into

the generated servlet. The implicit objects re listed below
• request
• response
• pageContext
• session
• application
• out
• config
• page
• exception

Front 146

Difference between forward and sendRedirect?

Back 146

When you invoke a forward request, the request is sent to another resource
on the server, without the client being informed that a different resource is going
to process the request. This process occurs completly with in the web container.
When a sendRedirtect method is invoked, it causes the web container to return to
the browser indicating that a new URL should be requested. Because the browser
issues a completly new request any object that are stored as request attributes
before the redirect occurs will be lost. This extra round trip a redirect is slower
than forward.

Front 147

What are the different scope valiues for the <jsp:useBean>?

Back 147

A: The different scope values for <jsp:useBean> are
1. page
2. request
3.session
4.application