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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. Frequently asked in HCL Technologies, TCS and Accenture. | ||||
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Ans. final - constant variable, objects cannot be de-referenced, restricting method overriding, restricting class sub classing. finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of what happens within the try block. Use the finally block to close files or to release other system resources like database connections, statements etc. finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state. | ||||
Sample Code for final Sample Code for finally Sample Code for finalize | ||||
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Very frequently asked to Fresh graduates. | ||||
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Ans. 1.Abstraction solves the problem at design level while encapsulation solves the problem at implementation level 2.Abstraction is used for hiding the unwanted data and giving relevant data. while Encapsulation means hiding the code and data into a single unit to protect the data from outside world. 3. Abstraction lets you focus on what the object does instead of how it does it while Encapsulation means hiding the internal details or mechanics of how an object does something. 4.For example: Outer Look of a Television, like it has a display screen and channel buttons to change channel it explains Abstraction but Inner Implementation detail of a Television how CRT and Display Screen are connect with each other using different circuits , it explains Encapsulation. | ||||
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Ans. Yes, we can substitute outer classes wherever we need to have inner classes but Inner classes have advantage in certain cases and hence preferred - Ease - Why to implement a class outside if its objects are only intended to be part of an outer object. Its easy to define the class within another class if the use is only local. Protection - Making a call an outer exposes a threat of it being used by any of the class. Why should it be made an outer class if its object should only occur as part of other objects. For example - You may like to have an class address whose object should have a reference to city and by design thats the only use of city you have in your application. Making Address and City as outer class exposes City to any of the Class. Making it an inner class of Address will make sure that its accessed using object of Address. | ||||
Sample Code for inner class | ||||
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Ans. It means that the type of variables are checked at compile time in Java.The main advantage here is that all kinds of checking can be done by the compiler and hence will reduce bugs. | ||||
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Ans. Declaration is intimation to the compiler about the nature of Data a reference is going to hold. For example - List myList; Instantiation is reservation of memory. For example myList = new ArrayList(); Initialization or construction is setting the default values for member elements. For example myList = new ArrayList(mySet); ** Example 2nd is both for instantiation as well as initialization. The only difference is that 2nd will initialized the member elements to their default values whereas 3rd will initialized it with the elements from set. | ||||
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Ans. Underlying data structure for ArrayList is Array whereas LinkedList is the linked list and hence have following differences - 1. ArrayList needs continuous memory locations and hence need to be moved to a bigger space if new elements are to be added to a filled array which is not required for LinkedList. 2. Removal and Insertion at specific place in ArrayList requires moving all elements and hence leads to O(n) insertions and removal whereas its constant O(1) for LinkedList. 3. Random access using index in ArrayList is faster than LinkedList which requires traversing the complete list through references. 4. Though Linear Search takes Similar Time for both, Binary Search using LinkedList requires creating new Model called Binary Search Tree which is slower but offers constant time insertion and deletion. 5. For a set of integers you want to sort using quicksort, it's probably faster to use an array; for a set of large structures you want to sort using selection sort, a linked list will be faster. | ||||
Sample Code for ArrayList Sample Code for LinkedList | ||||
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Frequently asked in Infosys and HCL Technologies ( Based on 2 feedback ) | ||||
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Ans. Using new operator - new xyzClass() Using factory methods - xyzFactory.getInstance( ) Using newInstance( ) method - (Class.forName(xyzClass))emp.newInstance( ) By cloning an already available object - (xyzClass)obj1.clone( ) | ||||
Sample Code for object initialization using clone Sample Code for object initialization using getInstance | ||||
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Frequently asked question in companies using Hibernate. | ||||
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Ans. It's a feature to lazily initialize dependencies , relationship and associations from the Database. Any related references marked as @OneToMany or @ManyToMany are loaded lazily i.e when they are accessed and not when the parent is loaded. | ||||
Sample Code for Lazy Initialization | ||||
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Ans. "this" keyword is a reference to the current object and can be used for following - 1. Passing itself to another method. 2. Referring to the instance variable when local variable has the same name. 3. Calling another constructor in constructor chaining. | ||||
Sample Code for this keyword | ||||
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Ans. By putting code within static method. With Java 6 and earlier versions, even static block can be used. | ||||
Sample Code for static block | ||||
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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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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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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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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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