Q).What is casting ?
A).Casting means converting one type to another. There are mainly two types of casting. Casting between primitive types and casting between object references. Casting between primitive numeric types is used to convert larger data types to smaller data types. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

Q).What is the difference between == and equals ?
A).The equals method can be considered to perform a deep comparison of the value of an object, whereas the == operator performs a shallow comparison.  If we are not overriding the equals method both will give the same result. == will is used to compare the object references. It is used to check whether two objects are points to the same reference.

Q).What are the different ways in which a thread can enter into waiting state?
A).There are three ways for a thread to enter into waiting state. By invoking 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.
   
Q).What are synchronized methods and synchronized statements?
A).Synchronized methods are methods that are declared with the keyword synchronized. A thread executes a synchronized method only after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. It is a block of code declared with synchronized keyword. A synchronized statement can be executed only after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q).What is hashCode?
A).The hashcode of a Java Object is simply a number (32-bit signed int) that allows an object to be managed by a hash-based data structure. A hashcode should be, equal for equal object (this is mandatory!) ,  fast to compute based on all or most of the internal state of an object, use all or most of the space of 32-bit integers in a fairly uniform way , and likely to be different even for objects that are very similar. If you are overriding hashCode you need to override equals method also.

Q).What do you mean by immutable ? How to create an immutable object ?
A).Immutability means an object cannot be modified after it has been initialized.  There will not be any setter methods in an immutable class. And normally these classes will be final.

Q).Can we compile a java program without main?    
A).Yes, we can. In order to compile a java program, we don't require any main method. But to execute a java program we must have a main in it (unless it is an applet or servlet). Because main is the starting point of a java program.

Q).What is daemon thread?    
A).Theards which are running on the background are called deamon threads. daemon thread is a thread which doesn't give any chance to run other threads once it enters into the run state it doesn't give any chance to run other threads. Normally it will run forever, but when all other non-daemon threads are dead, daemon thread will be killed by JVM.

Q).What is volatile variable?    
A).A volatile variable is not allowed to have a local copy of a variable that is different from the value currently held in "main" memory. Volatile modifier requests the JVM to always access the shared copy of the variable so the its most current value is always read.

Q).What is sandbox?    
 A).A sandbox is a security mechanism for safely running programs. The sandbox typically provides a tightly-controlled set of resources for guest programs to run in, such as scratch space on disk and memory.
   
Q).What is the difference between throw and throws clause?    
A).throw is used to throw an exception manually, where as throws is used in the case of checked exceptions, to tell the compiler that we haven't  handled the exception, so that the exception will be handled by the calling function.

Q).What are the different inner classes types?    
A).There are mainly four types available. They are Member classes, Nested top-level classes,  Local classes,  Anonymous classes.

Q).Can a top level class be private or protected?    
A).No. A top level class can not be private or protected. It can have either "public" or no modifier.

Q).What is the use of assert keyword    
A).Assert keyword validates certain expressions. It replaces the if block effectively and throws an AssertionError on failure. The assert keyword should be used only for critical arguments (means without that the method does nothing).

Q).What are the methods in Object?    
 A).clone, equals, wait, finalize, getClass, hashCode, notify, notifyAll, toString .


   
   
   

Read more...

71.What Are the different Collection Views That Maps Provide?
A).Maps Provide Three Collection Views.

    Key Set - allow a map's contents to be viewed as a set of keys.
    Values Collection - allow a map's contents to be viewed as a set of values.
    Entry Set - allow a map's contents to be viewed as a set of key-value mappings.


72.What is a KeySet View ?
A).KeySet is a set returned by the keySet() method of the Map Interface, It is a set that contains all the keys present in the Map.

73.What is a Values Collection View ?
A).Values Collection View is a collection returned by the values() method of the Map Interface, It contains all the objects present as values in the map.

74.What is an EntrySet View ?
A).Entry Set view is a set that is returned by the entrySet() method in the map and contains Objects of type Map. Entry each of which has both Key and Value.

75.How do you sort an ArrayList (or any list) of user-defined objects ?
A).Create an implementation of the java.lang.Comparable interface that knows how to order your objects and pass it to java.util.Collections.sort(List, Comparator).

76.What is the Comparable interface ?
A).The Comparable interface is used to sort collections and arrays of objects using the Collections.sort() and java.utils.Arrays.sort() methods respectively. The objects of the class implementing the Comparable interface can be ordered.

The Comparable interface in the generic form is written as follows:

    interface Comparable<T>

where T is the name of the type parameter.

All classes implementing the Comparable interface must implement the compareTo() method that has the return type as an integer. The signature of the compareTo() method is as follows:

      int i = object1.compareTo(object2)

    If object1 < object2: The value of i returned will be negative.
    If object1 > object2: The value of i returned will be positive.
    If object1 = object2: The value of i returned will be zero.


77.What are the differences between the Comparable and Comparator interfaces ?
A).Comparable     Comparator

It uses the compareTo() method.

int objectOne.compareTo(objectTwo).
    t uses the compare() method.

int compare(ObjOne, ObjTwo)
It is necessary to modify the class whose instance is going to be sorted.     A separate class can be created in order to sort the instances.
Only one sort sequence can be created.     Many sort sequences can be created.
It is frequently used by the API classes.     It used by third-party classes to sort instances.

Read more...

54.Difference between ArrayList and Vector ?
A).ArrayList vs    Vector
ArrayList is NOT synchronized by default.     Vector List is synchronized by default.
ArrayList can use only Iterator to access the elements.     Vector list can use Iterator and Enumeration Interface to access the elements.
The ArrayList increases its array size by 50 percent if it runs out of room.     A Vector defaults to doubling the size of its array if it runs out of room
ArrayList has no default size.     While vector has a default size of 10.

55.How to obtain Array from an ArrayList ?

Array can be obtained from an ArrayList using toArray() method on ArrayList.

    List arrayList = new ArrayList();
      arrayList.add(…

    Object  a[] = arrayList.toArray();


56.Why insertion and deletion in ArrayList is slow compared to LinkedList ?
A).    ArrayList internally uses and array to store the elements, when that array gets filled by inserting elements a new array of roughly 1.5 times the size of the original array is created and all the data of old array is copied to new array.
    During deletion, all elements present in the array after the deleted elements have to be moved one step back to fill the space created by deletion. In linked list data is stored in nodes that have reference to the previous node and the next node so adding element is simple as creating the node an updating the next pointer on the last node and the previous pointer on the new node. Deletion in linked list is fast because it involves only updating the next pointer in the node before the deleted node and updating the previous pointer in the node after the deleted node.


57.Why are Iterators returned by ArrayList called Fail Fast ?
A).Because, if list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

58.How do you decide when to use ArrayList and When to use LinkedList?
A).If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList offers the optimal collection. If, however, you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList offers the better implementation.

59.What is the Set interface ?
 A). The Set interface provides methods for accessing the elements of a finite mathematical set
    Sets do not allow duplicate elements
    Contains no methods other than those inherited from Collection
    It adds the restriction that duplicate elements are prohibited
    Two Set objects are equal if they contain the same elements

60.What are the main Implementations of the Set interface ?
A).The main implementations of the List interface are as follows:

    HashSet
    TreeSet
    LinkedHashSet
    EnumSet

61.What is a HashSet ?
A). A HashSet is an unsorted, unordered Set.
    It uses the hashcode of the object being inserted (so the more efficient your hashcode() implementation the better access performance you’ll get).
    Use this class when you want a collection with no duplicates and you don’t care about order when you iterate through it.

62.What is a TreeSet ?
A).TreeSet is a Set implementation that keeps the elements in sorted order. The elements are sorted according to the natural order of elements or by the comparator provided at creation time.

63.What is an EnumSet ?
A).An EnumSet is a specialized set for use with enum types, all of the elements in the EnumSet type that is specified, explicitly or implicitly, when the set is created.

64.Difference between HashSet and TreeSet ?
A).HashSet vs    TreeSet
HashSet is under set interface i.e. it  does not guarantee for either sorted order or sequence order.     TreeSet is under set i.e. it provides elements in a sorted  order (acceding order).
We can add any type of elements to hash set.     We can add only similar types
of elements to tree set.


65.What is a Map ?
A).A map is an object that stores associations between keys and values (key/value pairs).
    Given a key, you can find its value. Both keys  and  values are objects.
    The keys must be unique, but the values may be duplicated.
    Some maps can accept a null key and null values, others cannot.


66.What are the main Implementations of the Map interface ?
A).The main implementations of the List interface are as follows:

    HashMap
    HashTable
    TreeMap
    EnumMap


67.What is a TreeMap ?
A).TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

68.How do you decide when to use HashMap and when to use TreeMap ?
A).For inserting, deleting, and locating elements in a Map, the HashMap offers the best alternative. If, however, you need to traverse the keys in a sorted order, then TreeMap is your better alternative. Depending upon the size of your collection, it may be faster to add elements to a HashMap, then convert the map to a TreeMap for sorted key traversal.

69.Difference between HashMap and Hashtable ?
A).HashMap vs     Hashtable
HashMap lets you have null values as well as one null key.     HashTable  does not allows null values as key and value.
The iterator in the HashMap is fail-safe (If you change the map while iterating, you’ll know).     The enumerator for the Hashtable is not fail-safe.
HashMap is unsynchronized.     Hashtable is synchronized.

Note: Only one NULL is allowed as a key in HashMap. HashMap does not allow multiple keys to be NULL. Nevertheless, it can have multiple NULL values.


70.How does a Hashtable internally maintain the key-value pairs?
A).TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

Read more...

40).What is final modifier?
A).The final modifier keyword makes that the programmer cannot change the value anymore. The actual meaning depends on whether it is applied to a class, a variable, or a method.

    final Classes- A final class cannot have subclasses.
    final Variables- A final variable cannot be changed once it is initialized.
    final Methods- A final method cannot be overridden by subclasses.


41.What are the uses of final method?
A).There are two reasons for marking a method as final:

    Disallowing subclasses to change the meaning of the method.
    Increasing efficiency by allowing the compiler to turn calls to the method into inline Java code.


42.What is static block?
A).Static block which exactly executed exactly once when the class is first loaded into JVM. Before going to the main method the static block will execute.

43.What are static variables?
A).Variables that have only one copy per class are known as static variables. They are not attached to a particular instance of a class but rather belong to a class as a whole. They are declared by using the static keyword as a modifier.

         static type  varIdentifier;

where, the name of the variable is varIdentifier and its data type is specified by type.
Note: Static variables that are not explicitly initialized in the code are automatically initialized with a default value. The default value depends on the data type of the variables.

44.What is the difference between static and non-static variables?
A).A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

45.What are static methods?
A).Methods declared with the keyword static as modifier are called static methods or class methods. They are so called because they affect a class as a whole, not a particular instance of the class. Static methods are always invoked without reference to a particular instance of a class.
Note:The use of a static method suffers from the following restrictions:

    A static method can only call other static methods.
    A static method must only access static data.
    A static method cannot reference to the current object using keywords super or this

.What is an Iterator ?
A).The Iterator interface is used to step through the elements of a Collection.
    Iterators let you process each element of a Collection.
    Iterators are a generic way to go through all the elements of a Collection no matter how it is organized.
    Iterator is an Interface implemented a different way for every Collection.


47.How do you traverse through a collection using its Iterator?
A).To use an iterator to traverse through the contents of a collection, follow these steps:

    Obtain an iterator to the start of the collection by calling the collection’s iterator() method.
    Set up a loop that makes a call to hasNext(). Have the loop iterate as long as hasNext() returns true.
    Within the loop, obtain each element by calling next().


48.How do you remove elements during Iteration?
A).Iterator also has a method remove() when remove is called, the current element in the iteration is deleted.

49.What is the difference between Enumeration and Iterator?

Enumeration vs    Iterator

Enumeration doesn't have a remove() method
   

Iterator has a remove() method

Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects
   

Can be abstract, final, native, static, or synchronized

Note: So Enumeration is used whenever we want to make Collection objects as Read-only.

50.How is ListIterator?
A).ListIterator is just like Iterator, except it allows us to access the collection in either the forward or backward direction and lets us modify an element

51.What is the List interface?
A).The List interface provides support for ordered collections of objects.
    Lists may contain duplicate elements.


52.What are the main implementations of the List interface ?
A).The main implementations of the List interface are as follows :

    ArrayList : Resizable-array implementation of the List interface. The best all-around implementation of the List interface.
    Vector : Synchronized resizable-array implementation of the List interface with additional "legacy methods."
    LinkedList : Doubly-linked list implementation of the List interface. May provide better performance than the ArrayList implementation if elements are frequently inserted or deleted within the list. Useful for queues and double-ended queues (deques).


53.What are the advantages of ArrayList over arrays ?
A).Some of the advantages ArrayList has over arrays are:

    It can grow dynamically
    It provides more powerful insertion and search mechanisms than arrays.

Read more...

31.What is Constructor?
A).A constructor is a special method whose task is to initialize the object of its class.
    It is special because its name is the same as the class name.
    They do not have return types, not even void and therefore they cannot return values.
    They cannot be inherited, though a derived class can call the base class constructor.
    Constructor is invoked whenever an object of its associated class is created.


32.How does the Java default constructor be provided?
A).If a class defined by the code does not have any constructor, compiler will automatically provide one no-parameter-constructor (default-constructor) for the class in the byte code. The access modifier (public/private/etc.) of the default constructor is the same as the class itself.

33.Can constructor be inherited?
A).No, constructor cannot be inherited, though a derived class can call the base class constructor.

34.What are the differences between Contructors and Methods?
A).      Constructors vs    Methods

Purpose
   

Create an instance of a class
   

Group Java statements

Modifiers
   

Cannot be abstract, final, native, static, or synchronized
   

Can be abstract, final, native, static, or synchronized

Return Type
   

No return type, not even void
   

void or a valid return type

Name
   

Same name as the class (first letter is capitalized by convention) -- usually a noun
   

Any name except the class. Method names begin with a lowercase letter by convention -- usually the name of an action

this
   

Refers to another constructor in the same class. If used, it must be the first line of the constructor
   

Refers to an instance of the owning class. Cannot be used by static methods.

super
    Calls the constructor of the parent class. If used, must be the first line of the constructor     Calls an overridden method in the parent class

Inheritance
    Constructors are not inherited     Methods are inherited

35.How are this() and super() used with constructors?
A). Constructors use this to refer to another constructor in the same class with a different parameter list.
    Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.


36.What are the differences between Class Methods and Instance Methods?
A).
Class Methods vs    Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the class     Instance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword.
Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are unaware of instance members.     Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class.
Class methods are methods which are declared as static. The method can be called without creating an  instance of the class.     Instance methods are not declared as static.

37.How are this() and super() used with constructors?
A).Constructors use this to refer to another constructor in the same class with a different parameter list.
    Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.


38.What are Access Specifiers?
A).One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of data in a class and making this class available only through methods. Java allows you to control access to classes, methods, and fields via so-called access specifiers..

39.What are Access Specifiers available in Java?
A).Java offers four access specifiers, listed below in decreasing accessibility:

    Public- public classes, methods, and fields can be accessed from everywhere.
    Protected- protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package.
    Default(no specifier)- If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.
    Private- private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses.

Read more...

18.How do you prevent a method from being overridden?
A).To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy.

                           public final void exampleMethod() {
                          //  Method statements
                          }


19.What is an Interface?
A).An interface is a description of a set of methods that conforming implementing classes must have.
Note:

    You can’t mark an interface as final.
    Interface variables must be static.
    An Interface cannot extend anything but another interfaces.


20.Can we instantiate an interface?
A).You can’t instantiate an interface directly, but you can instantiate a class that implements an interface.

21.Can we create an object for an interface?
A).Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfill all the methods defined in it.

22.Do interfaces have member variables?
A).Interfaces may have member variables, but these are implicitly public, static, and final- in other words, interfaces can declare only constants, not instance variables that are available to all implementations and may be used as key references for method arguments for example.

23.What modifiers are allowed for methods in an Interface?
A).Only public and abstract modifiers are allowed for methods in interfaces.

24.What is a marker interface?
A).Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface and Cloneable are typical marker interfaces. These do not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.

25.What is an abstract class?
A).Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation.
Note:
    If even a single method is abstract, the whole class must be declared abstract.
    Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
    You can’t mark a class as both abstract and final.


26.Can we instantiate an abstract class?
A).An abstract class can never be instantiated. Its sole purpose is to be extended (subclassed).

27.What are the differences between Interface and Abstract class?
A).Abstract Class VS    Interfaces
An abstract class can provide complete, default code and/or just the details that have to be overridden.     An interface cannot provide any code at all,just the signature.
In case of abstract class, a class may extend only one abstract class.     A Class may implement several interfaces.
An abstract class can have non-abstract methods.     All methods of an Interface are abstract.
An abstract class can have instance variables.     An Interface cannot have instance variables.
An abstract class can have any visibility: public, private, protected.     An Interface visibility must be public (or) none.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.     If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
An abstract class can contain constructors .     An Interface cannot contain constructors .
Abstract classes are fast.     Interfaces are slow as it requires extra indirection to find corresponding method in the actual class.

28.When should I use abstract classes and when should I use interfaces?
A).Use Interfaces when…
    You see that something in your design will change frequently.
    If various implementations only share method signatures then it is better to use Interfaces.
    you need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.

Use Abstract Class when…

    If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
    When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.
    Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.


29.When you declare a method as abstract, can other nonabstract methods access it?
A).Yes, other nonabstract methods can access a method that you declare as abstract.

30.Can there be an abstract class with no abstract methods in it?
A).Yes, there can be an abstract class without abstract methods.

Read more...

1.What are the principle concepts of OOPS?
A).There are four principle concepts upon which object oriented design and programming rest. They are:
    Abstraction
    Polymorphism
    Inheritance
    Encapsulation
    (i.e. easily remembered as A-PIE).

2.What is Abstraction?
A).Abstraction refers to the act of representing essential features without including the background details or explanations.

3.What is Encapsulation?
A).Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object.

4.What is the difference between abstraction and encapsulation?
A).Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented.
    Abstraction solves the problem in the design side while Encapsulation is the Implementation.
    Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.


5.What is Inheritance?
A).Inheritance is the process by which objects of one class acquire the properties of objects of another class.
    A class that is inherited is called a superclass.
    The class that does the inheriting is called a subclass.
    Inheritance is done by using the keyword extends.
    The two most common reasons to use inheritance are:
        To promote code reuse
        To use polymorphism


6.What is Polymorphism?
A).Polymorphism is briefly described as "one interface, many implementations." Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts - specifically, to allow an entity such as a variable, a function, or an object to have more than one form.

7.How does Java implement polymorphism?
A).(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java).
Polymorphism manifests itself in Java in the form of multiple methods having the same name.
    In some cases, multiple methods have the same name, but different    formal argument lists (overloaded methods).
    In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).


8.Explain the different forms of Polymorphism.
A).There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface.
Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:
    Method overloading
    Method overriding through inheritance
    Method overriding through the Java interface


9.What is runtime polymorphism or dynamic method dispatch?
A).In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.

10.What is Dynamic Binding?
A).Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding (also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time. It is associated with polymorphism and inheritance.

11.What is method overloading?
A).Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type.
Note:
    Overloaded methods MUST change the argument list
    Overloaded methods CAN change the return type
    Overloaded methods CAN change the access modifier
    Overloaded methods CAN declare new or broader checked exceptions
    A method can be overloaded in the same class or in a subclass


12.What is method overriding?
A).Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type.
Note:
    The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
    You cannot override a method marked final
    You cannot override a method marked static

13.What are the differences between method overloading and method overriding?
A).         Overloaded Method                  Overridden Method
Arguments     Must change                         Must not change


Return type  Can change                         Can’t change except for
                                                     covariant returns

   
Exceptions    Can change                         Can reduce or
                                             eliminate. Must not throw new or broader checked exceptions


Access           Can change                    Must not make more
                                             restrictive (can be less restrictive)
                                       
   

Invocation   Reference type determines      Object type determines which
              which overloaded version      method is selected. Happens                                 at runtime.

14.Can overloaded methods be override too?
A).Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.

15.Is it possible to override the main method?
A).NO, because main is a static method. A static method can't be overridden in Java.

16.How to invoke a superclass version of an Overridden method?
A).To invoke a superclass method that has been overridden in a subclass, you must either call the method directly through a superclass instance, or use the super prefix in the subclass itself. From the point of the view of the subclass, the super prefix provides an explicit reference to the superclass' implementation of the method.

     // From subclass
        super.overriddenMethod();


17.What is super?
A).super is a keyword which is used to access the method or member variables from the superclass. If a method hides one of the member variables in its superclass, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its superclass, the method can invoke the overridden method through the use of the super keyword.
Note: You can only go back one level.
    In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.

Read more...

Q).What are different type of exceptions in Java?
A).There are two types of exceptions in java. Checked exceptions and Unchecked exceptions. Any exception that is is derived from Throwable and Exception is called checked exception except RuntimeException and its sub classes. The compiler will check whether the exception is caught or not at compile time. We need to catch the checked exception or declare in the throws clause.  Any exception that is derived from Error and RuntimeException is called unchecked exception. We don't need to explicitly catch a unchecked exception.

Q).Can we catch an error in our java program ?
A).Yes. We can . We can catch anything that is derived from Throwable. Since Error is a sub class of Throwable we can catch an error also.

Q).What is thread priority?
A).Thread Priority is an integer value that identifies the relative order in which it should be executed with respect to others. The thread priority values ranging from 1- 10 and the default value is 5. But if a thread have higher priority doesn't means that it will execute first. The thread scheduling depends on the OS.
   
Q).How many times may an object's finalize() method be invoked by the garbage collector?
A).Only once.

Q).What must a class do to implement an interface?
A).It must identify the interface in its implements clause. Also it must provide definition for all the methods in the interface otherwise it must be declared abstract.
       
Q).What is the difference between notify and notifyAll method ?
A).notify wakes up a single thread that is waiting for object's monitor. If any threads are waiting on this object, one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. notifyAll Wakes up all threads that are waiting on this object's monitor. A thread waits on an object's monitor by calling one of the wait methods.

Q).What does wait method do ?
A).It causes current thread to wait until either another thread invokes notify or notifyAll method of the current object, or a specified amount of time has elapsed.

Q).What are the different states of a thread ?
A).The different thread states are ready, running, waiting and dead.
   
Q).What is the difference between String and StringBuffer class ?
A).Strings are immutable (constant), their values cannot be changed after they are created. StringBuffer supports mutable objects.

Q).Which is the base class for all classes ?
A). java.lang.Object.

Read more...

Q).what is a the difference between System.err and System.out
A).We can redirect System.out to another file but we cannot redirect System.err stream.

Q).What are  the differences between an abstract class and an interface?
A).An abstract class can have concrete method, which is not allowed in an interface. Abstract class can have private or protected methods and variables and only public methods and variables are allowed in interface. We can implement more than one interface , but we can extend only one abstract class. Interfaces provides loose coupling where as abstract class provides tight coupling.

Q).What is the difference between synchronized block and synchronized method ?
A).Synchronized blocks place locks for the specified block where as synchronized methods place locks for the entire method.

Q).How can you force garbage collection in java?
A).You cannot force Garbage Collection, but you can request for it by calling the method System.gc().  But it doesn't mean that Garbage Collection will start immediately. The garbage collection is a low priority thread of JVM.

Q).How can you call a constructor from another constructor ?
A).By using this() reference.

Q).How can you call the constructor of super class ?
A).By using super() syntax.

Q).What's the difference between normal methods and constructors?
A).Constructors must have the same name of the class and can not have a return type. They are called only once,  while regular methods can be called whenever required. We cannot explicitly call a constructor.

Q).What is the use of packages in java ?
A).Packages are a way to organize files in java when a project consists of more than one module. It helps in resolving name conflicts when different modules have classes with the same names.

Q).What must be the order of catch blocks when catching more than one exception?
A).The sub classes must come first. Otherwise it will give a compile time error.

Q).How can we call a method or variable of the super class from child class ?
A).We can use super.method() or super.variable syntax for this purpose.

Q).If you are overriding equals() method of a class, what other methods you might need to override ?
A).HashCode

Q).How can you create your own exception ?
A).Our class must extend either Exception or its sub class.

Q).What is serialization ?
A).Serialization is the process of saving the state of an object.

Q).What is de-serialization?
A).De-serialization is the process of restoring the state of an object.

Read more...

Q).Why threads block or enters to waiting state on I/O?
A).Threads enters to waiting state or block on I/O because other threads can execute while the I/O operations are performed.

Q).What are transient variables in java?
A).Transient variables are variable that cannot be serialized.
   
Q).What is synchronization
A).Synchronization is the ability to control the access of multiple threads to shared resources. Synchronization stops multithreading. With synchronization , at  a time only one thread will be able to access a shared resource.

Q).What is List interface ?
A).List is an ordered collection of objects.
       
Q).What is a Vector
A).Vector is a grow able array of objects.

Q).What is the difference between yield() and sleep()?
A).When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not ready state.
       
Q).What are Wrapper Classes ?
A).They are wrappers to primitive data types. They allow us to access primitives as objects.
       
Q).Can we call finalize() method ?
A).Yes.  Nobody will stop us to call any method , if it is accessible in our class. But a garbage collector cannot call an object's finalize method if that object is reachable.

Q).What is the initial state of a thread when it is created and started?
A).The thread is in ready state.

Q).Can we declare an anonymous class as both extending a class and implementing an interface?
A).No. An anonymous class can extend a class or implement an interface, but it cannot be declared to do both

Q).What is the use of the finally block?
A).Finally is the block of code that executes always. The code in finally block will execute even if an exception is occurred. finally will not execute when the user calls System.exit().

Q).    What is an abstract method ?
A).    An abstract method is a method that don't have a body. It is declared with modifier abstract.

Read more...

Q).Why do threads block on I/O? 
A).Threads block on i/o (that is enters the waiting state) so that other threads may execute while the I/O operation is performed.

Q).What is synchronization and why is it important?
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.

Q).Is null a keyword?
A).The null value is not a keyword.

Q).Which characters may be used as the second character of an identifier,but not as the first character of an identifier? 
A).The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.

Q).What modifiers may be used with an inner class that is a member of an outer class? 
A).A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

Q).How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters? 
A).Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.

Q).What are wrapped classes? 
A).Wrapped classes are classes that allow primitive types to be accessed as objects.

Q).What restrictions are placed on the location of a package statement within a source code file? 
A).A package statement must appear as the first line in a source code file (excluding blank lines and comments).

Q).What is the difference between preemptive scheduling and time slicing? 
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.

Q).What is a native method?
A).A native method is a method that is implemented in a language other than Java.

Q).What are order of precedence and associativity, and how are they used?
A).Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left.

Q).What is the catch or declare rule for method declarations? 
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.

Q).Can an anonymous class be declared as implementing an interface and extending a class? 
A).An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

Q).What is the range of the char type? 
A).The range of the char type is 0 to 2^16 - 1.

Read more...

Q).Can there be an abstract class with no abstract methods in it?
A).Yes

Q).Can an Interface be final? 
A).No

Q).Can an Interface have an inner class?
A).Yes.

public interface abc
 {
  static int i=0; void dd();
  class a1
  {
   a1()
   {
    int j;
    System.out.println("inside");
   };
   public static void main(String a1[])
   {
    System.out.println("in interfia");
   }
  }
 }

Q).Can we define private and protected modifiers for variables in interfaces? 
A).No

Q).What is Externalizable? 
A).Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

Q).What modifiers are allowed for methods in an Interface? 
A).Only public and abstract modifiers are allowed for methods in interfaces.

Q).What is a local, member and a class variable? 
A).Variables declared within a method are “local” variables. Variables declared within the class i.e not within any methods are “member” variables (global variables). Variables declared within the class i.e not within any methods and are defined as “static” are class variables.

Q).What are the different identifier states of a Thread?
A).The different identifiers of a Thread are: R - Running or runnable thread, S - Suspended thread, CW - Thread waiting on a condition variable, MW - Thread waiting on a monitor lock, MS - Thread suspended waiting on a monitor lock.

Q).What are some alternatives to inheritance? 
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).

Q).Why isn’t there operator overloading? 
A).Because C++ has proven by example that operator overloading makes code almost impossible to maintain. In fact there very nearly wasn’t even method overloading in Java, but it was thought that this was too useful for some very basic methods like print(). Note that some of the classes like DataOutputStream have unoverloaded methods like writeInt() and writeByte().

Q).What does it mean that a method or field is “static”? 
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.

Q).How do I convert a numeric IP address like 192.18.97.39 into a hostname like java.sun.com?
A).String hostname = InetAddress.getByName("192.18.97.39").getHostName.

Read more...