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Ans. 1. Overriding method can not be more restrictive than the overridden method. reason : in case of polymorphism , at object creation jvm look for actual runtime object. jvm does not look for reference type and while calling methods it look for overridden method. If by means subclass were allowed to change the access modifier on the overriding method, then suddenly at runtime when the JVM invokes the true objects version of the method rather than the reference types version then it will be problematic 2. In case of subclass and superclass define in different package, we can override only those method which have public or protected access. 3. We can not override any private method because private methods can not be inherited and if method can not be inherited then method can not be overridden. | ||||
Sample Code for Overriding | ||||
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Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 3 feedback ). Among first few questions in many interviews. | ||||
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Ans. Abstract classes can have both abstract methods ( method declarations ) as well as concrete methods ( inherited to the derived classes ) whereas Interfaces can only have abstract methods ( method declarations ). A class can extend single abstract class whereas it can implement multiple interfaces. | ||||
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Basic and Very Frequently asked. | ||||
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Ans. Polymorphism means the condition of occurring in several different forms. Polymorphism in Java is achieved in two manners 1. Static polymorphism is the polymorphic resolution identified at compile time and is achieved through function overloading whereas 2. Dynamic polymorphism is the polymorphic resolution identified at runtime and is achieved through method overriding. | ||||
Sample Code for overloading Sample Code for overriding | ||||
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Frequently asked. | ||||
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Ans. ArrayList are implemented in memory as arrays and hence allows fast retrieval through indices but are costly if new elements are to be inserted in between other elements. LinkedList allows for constant-time insertions or removals using iterators, but only sequential access of elements 1. Retrieval - If Elements are to be retrieved sequentially only, Linked List is preferred. 2. Insertion - If new Elements are to be inserted in between other elements , Linked List is preferred. 3. Search - Binary Search and other optimized way of searching is not possible on Linked List. 4. Sorting - Initial sorting could be pain but lateral addition of elements in a sorted list is good with linked list. 5. Adding Elements - If sufficiently large elements needs to be added very frequently ,Linked List is preferable as elements don't need consecutive memory location. | ||||
Sample Code for ArrayList Sample Code for LinkedList | ||||
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Ans. Lighter and faster than XML as on-the-wire data format Object Representation - Information is presented in object notations and hence better understandable. Easy to parse and conversion to objects for information consumption. Support multiple data types - JSON supports string, number, array, boolean whereas XML data are all string. | ||||
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Frequently asked in all types of companies especially Indian Services companies. Frequently asked in CTS (Based on 2 feedback) | ||||
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Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. The value received from hashcode() is used as bucket number for storing elements. This bucket number is the address of the element inside the set/map. when you do contains() then it will take the hashcode of the element, then look for the bucket where hashcode points to and if more than 1 element is found in the same bucket (multiple objects can have the same hashcode) then it uses the equals() method to evaluate if object are equal, and then decide if contain() is true or false, or decide if element could be added in the set or not. | ||||
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Recently asked in TCS , Tech Mahindra, HCL , Accenture and Fidelity. | ||||
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Ans. The DispatcherServlet configured in web.xml file receives the request. The DispatcherServlet finds the appropriate Controller with the help of HandlerMapping and then invokes associated Controller. Then the Controller executes the logic business logic and then returns ModeAndView object to the DispatcherServlet. The DispatcherServlet determines the view from the ModelAndView object. Then the DispatcherServlet passes the model object to the View. The View is rendered and the Dispatcher Servlet sends the output to the Servlet container. Finally Servlet Container sends the result back to the user. | ||||
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Not frequently asked as it was introduced with Java 8. | ||||
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Ans. Final variable means a variable that has been declared final and hence cannot be de referenced after initialization. Effective final means a variable that has not been declared final but haven't been reassigned the value after initialization. First is the regulation that restricts the reassignment and will raise a compilation error if we try to do so. Second is the outcome without the restriction. Effective Final is the eventual treatment of the variable that is required for many features. For eq - Java 8 requires that local variables referenced from a lambda expression must be final or effectively final.It means all local referenced from lambda expressions must be such that their value shouldn't be changed after initialization whether declared final or not. | ||||
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Ans. abstract and final | ||||
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Frequently asked in Infosys India | ||||
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Ans. String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference. | ||||
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Very Frequently asked to fresh graduates and less experienced. Favorite question in Walk in drives. Frequently asked in Indian Services companies. | ||||
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Ans. Overloading - Similar Signature but different definition , like function overloading. Overriding - Overriding the Definition of base class in the derived class. | ||||
Sample Code for overloading Sample Code for overriding | ||||
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Ans. There are different verticals in which Java Classes can be classified. 1. Access - Public , Private , default or Protected. 2. Packaging - System, library or User Defined 3. Structure - Outer or Inner 4. Object Derivation - Abstract Class or Concrete Class. 5. Object Creation - Normal, Singleton , Doubleton , Immutable or Enum. 6. Functionality - String , Util , Stream etc. | ||||
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Ans. Car Engine is an example of encapsulation and abstraction. You ignite the car using an interface called starter and least bothered about how the tire actually moves (This is abstraction). The engine encapsulates the complete process to itself only and doesn't allow you to start the other components like the radiator etc ( this is excapsulation ) | ||||
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Ans. 1. Set lazy=false in the hibernate config file. 2. Set @Basic(fetch=FetchType.EAGER) at the mapping. 3. Make sure that we are accessing the dependent objects before closing the session. 4. Force initialization using Hibernate.initialize 5. Using Fetch Join in HQL. | ||||
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Very frequently asked across all types of companies. | ||||
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Ans. Hashtable is synchronized whereas HashMap is not.HashMap allows null values whereas Hashtable doesnt allow null values. | ||||
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Advanced level question usually asked in High end product companies. Have been asked in Google and Amazon (Based on 1 Feedback) | ||||
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Ans. The Java runtime environment deletes objects when it determines that they are no longer being used. This process is known as garbage collection. The Java runtime environment supports a garbage collector that periodically frees the memory used by objects that are no longer needed. The Java garbage collector is a mark-sweep garbage collector that scans Java dynamic memory areas for objects, marking those that are referenced. After all possible paths to objects are investigated, those objects that are not marked (i.e. are not referenced) are known to be garbage and are collected. | ||||
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Frequently asked in Cognizant (CTS) | ||||
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Ans. Storing the state of an object in a file or other medium is called serialization. Classes can communicate only if they are built together ( as they need Byte code for communication ). What if we need to enable communication between different applications ( i.e they have been built independently or even they reside at different locations ), We need a mechanism that will transfer the Bean state to a Medium than can be transferred to the receiving application. | ||||
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Frequently asked question in companies using hibernate. | ||||
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Ans. 1. First level cache is enabled by default whereas Second level cache needs to be enabled explicitly. 2. First level Cache came with Hibernate 1.0 whereas Second level cache came with Hibernate 3.0. 3. First level Cache is Session specific whereas Second level cache is shared by sessions that is why First level cache is considered local and second level cache is considered global. | ||||
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Ans. Advantages Can do meta object operations ( like validating something before creating objects , keep count of number of objects ) Can do operations which have nothing to do with objects but still you want them to be tied to Class. Disadvantages Commonly used to static variables sometime leads to problems due to access by different objects. Are not tied to objects so doesn't reflect pure Object Oriented approach. Needs to be synchronized so as to avoid update conflicts by mutiple objects and threads. Some limitation in testing as not all frameworks have facility to mock them. Powermock has but Mockito doesnt | ||||
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Ans. Inner join is the intersection of two tables on the condition defined by the where clause i.e will get records from both tables matched by a column. Outer join is the union of two tables i.e will get all records from both tables and will put null in the columns where related records are not present. Left Outer join is the left union of two tables i.e all records from the table on the left and values from the right table for related records else null for the columns from right table. Right Outer join is the right union of two tables i.e all records from the table on the right and values from the left table for related records else null for the columns from left table. | ||||
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Ans. String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment or hacker over internet. Once a String constant is created in Java , it stays in string constant pool until garbage collected and hence stays there much longer than what's needed. Any unauthorized access to string Pool pose a threat of exposing these values. | ||||
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Ans. Its a method which cannot be overridden. Compiler throws an error if we try to override a method which has been declared final in the parent class. | ||||
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Ans. There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference | ||||
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Ans. clone() - Creates and returns a copy of this object. equals() - Indicates whether some other object is "equal to" this one. finalize() - Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. getClass() - Returns the runtime class of an object. hashCode() - Returns a hash code value for the object. toString() - Returns a string representation of the object. notify(), notifyAll(), and wait() - Play a part in synchronizing the activities of independently running threads in a program. | ||||
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Ans. No, because both this and super should be the first statement. | ||||
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Ans. Derived object carries the body of its class as well as the body of the parent class. Its body ( member elements ) is initialized using its own class constructor whereas the body ( member elements ) carried from the parent class are initialized using super class constructor. So In order to initialize the elements of the parent class before its own elements are even initialized, super is called. | ||||
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Ans. Synchronized | ||||
Sample Code for Synchronized | ||||
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Ans. All members of a family share a common last name and thats how outsiders recognize them, but to recognize individual team members , we need an overloaded name and hence we use First Name along with Last Name to uniquely identify. Some members of a family have a Nick Name as the real name is very long or hard to call. So the Nick name overrides the Original Name in certain circumstances. | ||||
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Usually asked in relation to casting and ClassCastException. | ||||
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Ans. The operator instanceOf is used to verify if the specified object is the instance of specified class or interface. Syntax if(x instanceOf ABC) where x is an object reference and ABC could be a class name or interface name. The above statement will be true if x holds an object that is an instance of ABC or any of the child class of ABC or if x holds an object that implements ABC. instanceOf operator is used to verify in case of downcasting. For ex - DerivedClass extends BaseClass x is the reference of BaseClass but holds DerivedClass object ( Polymorphism ) There is an operation that is defined in Derived Class, let's say derivedClassMethod() We cannot call derivedClassMethod() directly using x as x is reference of BaseClass and not DerivedClass and hence can only access methods that are defined in BaseClass and overridden in derived class. Though we can cast it to DerivedClass as following ((DerivedClass)x).derivedClassMethod(); But it may throw ClassCastException in case x doesn't hold an instance of DerivedClass at that point. So before casting it to DerivedClass we may like to make sure that it is an instance of DerivedClass and hence won't throw ClassCastException. So we make a check for it if(x instanceOf DerivedClass) { ((DerivedClass)x).derivedClassMethod(); } | ||||
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Ans. They are different the way their elements are stored in memory. TreeMap stores the Keys in order whereas HashMap stores the key value pairs randomly. | ||||
Sample Code for treemap Sample Code for hashmap | ||||
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