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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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 Discuss Correct / Improve  instanceOf instanceOf operator Why we need instanceOf operator use of instanceOf operator  Asked in 2 Companies  Intermediate  frequent | ||||
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| Ans. It in Java is used to indicate that a field should not be serialized. | ||||
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| Discuss Correct / Improve java oops serialization transient java keywords Asked in 39 Companies intermediate frequent | ||||
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| Frequently asked to fresh graduates. | ||||
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| Ans. ACID stands for Atomicity, Consistency, Isolation, Durability is a set of properties of database transactions. Atomicity means all or nothing. i.e parts of a transaction shouldn't commit if any one of them fails. Either the whole transaction should succeed or it should be complete rollback. Consistency means that any transaction should lead database from one stabe state to another. Isolation means that the execution of transaction results in a system state that would be obtained if transactions were executed serially. Durability means that when a transaction is committed it forms the permanent state of database. | ||||
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| Discuss Correct / Improve database acid Asked in 7 Companies Intermediate | ||||
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| Ans. Cookie and Session are used to store the user information. Cookie stores user information on client side and Session does it on server side. Primarily, Cookies and Session are used for authentication, user preferences, and carrying information across multiple requests. Session is meant for the same purpose as the cookie does. Session does it on server side and Cookie does it on client side. One more thing that quite differentiates between Cookie and Session. Cookie is used only for storing the textual information. Session can be used to store both textual information and objects. | ||||
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| Discuss Correct / Improve session session management java cookies authentication web application ebay intermediate | ||||
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| Ans. 1. Static class is a class which cannot be instantiated and all its members are static whereas Singleton is the class that only permit creation of single object and then the object is reused. 2. As there is no object in Static class, it cannot participate in runtime Polymorphism. 3. As Static class doesnt allow creating objects and hence it cannot be serialized. 4. Static class body is initialized eagerly at application load time whereas Singleton object can be initiated eagerly using static blocks or lazily on first need. 5. Its not recommended to use pure static class as it fails to use many OOPs concepts. | ||||
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| Discuss Correct / Improve Static Class Singleton Static Class vs Singleton Asked in 3 Companies Intermediate frequent | ||||
| Very frequently asked in companies using SOA. | ||||
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| Ans. REST or Representational State Transfer is a flexible architecture style for creating web services that recommends the following guidelines - 1. http for client server communication, 2. XML / JSON as formatiing language , 3. Simple URI as address for the services and, 4. stateless communication. | ||||
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| Discuss Correct / Improve java web services rest java j2ee architecture Asked in 14 Companies intermediate frequent | ||||
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| Ans. New operator in Java creates objects. Constructor is the later step in object creation. Constructor's job is to initialize the members after the object has reserved memory for itself. | ||||
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| Discuss Correct / Improve java constructor object creation intermediate rare | ||||
| Frequently asked question for intermediate developers. Frequently asked in HCL Technologies and EPAM. | ||||
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| Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldnt be cached. | ||||
 Sample Code for volatile | ||||
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| Discuss Correct / Improve java oops synchronization volatile java keywords Asked in 42 Companies intermediate frequent | ||||
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| Ans. Yes, but the overloaded main methods without single String[] argument doesn't get any special status by the JVM. They are just another methods that needs to be called explicitly. | ||||
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| Discuss Correct / Improve java main method overloading yes-no Asked in 6 Companies intermediate frequent | ||||
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| Ans. Modularity - First sign of good code is whether it has been segregated into methods and classes appropriately. I dont mind it in excess because I believe that is forward looking strategy as applications tends to expand and eventually become hard to read. Self Explanatory - Variables and methods should be named in a way that the code should be self explanatory even without comments. Use of Constant variables to explain use of literal. Proper Code Reuse - If there is anything being reused , it should be moved to parent classes / methods. Proper composition calls - Composed hierarchy should not be access in just single line. One or two levels is ok but having multiple levels make it hard to read and debug. | ||||
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| Discuss Correct / Improve code review clean code Asked in 1 Companies intermediate frequent | ||||
| Frequently asked to fresh graduates and less experienced developers. | ||||
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| Ans. 1. Multithreading provides better interaction with the user by distribution of task 2. Threads in Java appear to run concurrently, so it provides simulation for simultaneous activities.The processor runs each thread for a short time and switches among the threads to simulate sim-ultaneous execution (context-switching) and it make appears that each thread has its own processor.By using this feature, users can make it appear as if multiple tasks are occurring simultaneously when, in fact, each is running for only a brief time before the context is switched to the next thread. 3. We can do other things while waiting for slow I/O operations.In the java.iopackage, the class InputStreamhas a method, read(), that blocks until a byte is read from the stream or until an IOExceptionis thrown. The thread that executes this method cannot do anything elsewhile awaiting the arrival of another byte on the stream. | ||||
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| Discuss Correct / Improve java threads multi threading concurrency multithreading Asked in 5 Companies intermediate frequent | ||||
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| Ans. We can make a class immutable by 1. Making all methods and variables as private. 2. Setting variables within constructor. Public Class ImmutableClass{ private int member; ImmutableClass(int var){ member=var; } } and then we can initialize the object of the class as ImmutableClass immutableObject = new ImmutableClass(5); Now all members being private , you cant change the state of the object. | ||||
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| Discuss Correct / Improve java oops immutable immutability immutable immutability class technical lead Asked in 5 Companies intermediate frequent | ||||
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| Ans. Iterators in java are used to iterate over the Collection objects. Fail-Fast iterators immediately throw ConcurrentModificationException if there is any addition, removal or updation of any element. Fail-Safe iterators don't throw any exception if a collection is structurally modified while iterating over it. This is because, they operate on the clone of the collection and not on the original collection. | ||||
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| Discuss Correct / Improve ail fast fail saf Asked in 9 Companies intermediate | ||||
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| Ans. 1. Query Optimization ( Query Rewriting , Prepared Statements ) 2. Restructuring Indexes. 3. DB Caching Tuning ( if using ORM ) 4. Identifying the problems ( if any ) with the ORM Strategy ( If using ORM ) | ||||
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| Discuss Correct / Improve java db database hibernate orm at&t overstock.com performance improvement architecture technical lead architect intermediate | ||||
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| Ans. 1. Objects are serialized and not classes and hence Static variables are ignored. 2. Transient is an explicit declaration to ignore the variable during serialization and hence transient instance variables are ignored too. 3. Base class instance variables if the base class hasn't been declared serializable. | ||||
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| Discuss Correct / Improve serialization Asked in 1 Companies intermediate frequent | ||||
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| Ans. They are loaded at runtime when the respective Class is loaded. | ||||
| Sample Code for static variable | ||||
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| Discuss Correct / Improve java oops static static variable memory intermediate | ||||
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| Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object. | ||||
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| Discuss Correct / Improve serialization java oops static static variables Asked in 1 Companies intermediate rare | ||||
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| Very frequently asked in phone and walk in interviews. | ||||
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| Ans. An interface without any method declaration is called as marker interface. there are 3 in-built interfaces in JVM i.e. serializable, clonable, remote | ||||
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| Discuss Correct / Improve java oops interfaces marker interface serializable clonable Asked in 21 Companies intermediate frequent | ||||
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| Ans. As we only downcast class in the hierarchy, The ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance. | ||||
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| Discuss Correct / Improve java oops class casting classcastexception exception error Asked in 4 Companies intermediate frequent | ||||
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| Ans. The purpose of comparator interface is to compare objects of the same class to identify the sorting order. Sorted Collection Classes ( TreeSet, TreeMap ) have been designed such to look for this method to identify the sorting order, that is why class need to implement Comparator interface to qualify its objects to be part of Sorted Collections. | ||||
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| Discuss Correct / Improve java collections treemap treeset comparator Asked in 2 Companies Intermediate | ||||
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| Ans. This is the exception that is thrown when we try to modify the non concurrent collection class while iterating through it. | ||||
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| Discuss Correct / Improve java collections concurrentmodificationexception exception concurrency Asked in 14 Companies intermediate | ||||
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| Ans. JSON.stringify() turns an object into a JSON text and stores that JSON text in a string. So If we stringfy above notation , it will become {"name":"xyz","gender":"male";"age":30} | ||||
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| Ans. One can import the same package or same class multiple times. Neither compiler nor JVM complains wil complain about it. And the JVM will internally load the class only once no matter how many times you import the same class. | ||||
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| Discuss Correct / Improve java import jvm advanced Asked in 1 Companies intermediate rare | ||||
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| Ans. No. Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior. A static method is not associated with any instance of a class so the concept of overriding for runtime polymorphism using static methods is not applicable. | ||||
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| Discuss Correct / Improve java oops static static methods java keywords yes-no intermediate frequent | ||||
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| Ans. Class , Heap , Stack , Register , Native Method Stack. | ||||
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| Discuss Correct / Improve java jvm memory management memory advanced architecture technical architect Asked in 1 Companies intermediate frequent | ||||
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| Ans. No Abstract methods can only be declared protected or public. | ||||
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| Discuss Correct / Improve java abstract oops access specifier private yes-no intermediate rare | ||||
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| Ans. It is used to sort collections and arrays of objects using the collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered. | ||||
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| Discuss Correct / Improve java collections comparable interface Asked in 7 Companies intermediate frequent | ||||
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| Ans. Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. | ||||
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| Discuss Correct / Improve java serialization externalizable interface Asked in 2 Companies intermediate rare | ||||
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| Ans. new operator is used to statically create an instance of object. newInstance() is used to create an object dynamically ( like if the class name needs to be picked from configuration file ). If you know what class needs to be initialized , new is the optimized way of instantiating Class. | ||||
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| Discuss Correct / Improve java oops object instantiation object creation class.forname newinstance new operator difference between advanced intermediate | ||||
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| Ans. 1. Collection should have an index for random access. 2. Collection should have ordered elements. | ||||
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| Discuss Correct / Improve java collections search algorithm search binary search at&t intermediate | ||||
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Which of the following is true ?
