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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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Like Discuss Correct / Improve  java   constructor   object creation intermediate   rare | ||||
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Ans. FALSE. == operator compares object references, a and b are references to two different objects, hence the FALSE. .equals method is used to compare string object content. | ||||
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Like Discuss Correct / Improve  string   string class   java   ==   object references   coding Asked in 2 Companies basic   frequent | ||||
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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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Like Discuss Correct / Improve  java   oops   synchronization   volatile   java keywords Asked in 42 Companies intermediate   frequent | ||||
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Ans. If we don't declare the list to be of specific type, it treats it as list of objects. int 1 is auto boxed to Integer and "1" is String and hence both are objects. | ||||
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Like Discuss Correct / Improve  java   collections   arraylist   list   autoboxing   wrapper classes expert   rare | ||||
Advanced level question. Recently asked in few Indian service companies ( Based on 3 inputs ) | ||||
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Ans. Bootstrap - Loads JDK internal classes, java.* packages. Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE System - Loads classes from system classpath. | ||||
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Like Discuss Correct / Improve  java   jvm   memory management   class loaders   bootstrap   extensions   system  classloaders   advanced   technical lead   technical architect Asked in 7 Companies | ||||
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Ans. A DOM (Document Object Model) parser creates a tree structure in memory from an input document whereas A SAX (Simple API for XML) parser does not create any internal structure. A SAX parser serves the client application always only with pieces of the document at any given time whereas A DOM parser always serves the client application with the entire document no matter how much is actually needed by the client. A SAX parser, however, is much more space efficient in case of a big input document whereas DOM parser is rich in functionality. Use a DOM Parser if you need to refer to different document areas before giving back the information. Use SAX is you just need unrelated nuclear information from different areas. Xerces, Crimson are SAX Parsers whereas XercesDOM, SunDOM, OracleDOM are DOM parsers. | ||||
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Like Discuss Correct / Improve  java   xml   parsers   sax   dom parser   difference   architecture   technical lead   technical architect  markup language   sax vs dom Asked in 14 Companies   frequent | ||||
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Ans. 1)The overriding methods can throw any runtime Exception , here in the case of runtime exception overriding method (subclass method) should not worry about exception being thrown by superclass method. 2)If superclass method does not throw any exception then while overriding, the subclass method can not throw any new checked exception but it can throw any runtime exception 3) Different exceptions in java follow some hierarchy tree(inheritance). In this case , if superclass method throws any checked exception , then while overriding the method in subclass we can not throw any new checked exception or any checked exception which are higher in hierarchy than the exception thrown in superclass method | ||||
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Like Discuss Correct / Improve  java   overriding   exceptions   inheritence   inheritance  object oriented programming (oops)  oops concepts   oops | ||||
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Ans. All exceptions must be a child of Throwable. If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class. You want to write a runtime exception, you need to extend the RuntimeException class. | ||||
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Frequently asked to fresh graduates and less experienced. | ||||
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Ans. Inheritance means a object inheriting reusable properties of the base class. Compositions means that an abject holds other objects. In Inheritance there is only one object in memory ( derived object ) whereas in Composition , parent object holds references of all composed objects. From Design perspective - Inheritance is "is a" relationship among objects whereas Composition is "has a" relationship among objects. | ||||
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Ans. NullPointerException at line: "File file = new File("/folder", name);" | ||||
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Like Discuss Correct / Improve  java   io   file   fileio   coding   code   file handling | ||||
Not frequently asked as it was introduced with Java 8. | ||||
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Ans. StringJoiner is a util method to construct a string with desired delimiter. This has been introduced with wef from Java 8. Sample Code StringJoiner strJoiner = new StringJoiner("."); | ||||
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Ans. public or default | ||||
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Ans. Methods , Variables , Initialization Blocks and nested Classes | ||||
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Ans. We can override main method | ||||
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Ans. In first case we are trying to initialize Inner class object using the instance of Outer Class whereas in second case we are trying to initialize the Inner class object directly using the Outer class name. In second case , Inner class is "static inner class" as we cannot access "non static inner class" using Classname alone. In first case, the inner class could be either "static inner class" or "non static inner class". | ||||
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Like Discuss Correct / Improve  inner classes  inner class   static inner class   new operator | ||||
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Ans. 1. Regulation / Enforcement / Policy / Contract , It's primary use 2. Use as a collection of utility methods(since java 8 through default methods) 3. Collecting constants together, As Interface are lighter, it makes sense to use them instead of classes if no getters and setters required. 4. Creation of Custom Annotations 5. Special services like marker interface | ||||
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Frequently asked in high end product companies. Frequently asked in Deloitte. | ||||
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Ans. https://medium.com/javarevisited/internal-working-of-hashmap-in-java-97aeac3c7beb#:~:text=HashMap%20internally%20uses%20HashTable%20implementation,the%20entries%20into%20the%20map. | ||||
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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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Like Discuss Correct / Improve  code review  clean code Asked in 1 Companies intermediate   frequent | ||||
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Ans. We can overload the main method by specifying different argument types. For example - 2 main methods with different arguments is perfectly legal public static void main(); public static void main(String[] args); The following are not legal as compiler will complain of duplicate methods public static void main(String[] args); public static void main(String[] args); Even The following are not legal as we cannot overload on return types public static String main(String[] args); public static void main(String[] args); | ||||
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Ans. Interfaces don't have member elements and method definitions that could cause diamond problem. With Java 8, Interfaces have default method definitions. This could have created diamond problem but Java introduced a compile time check for "duplicate default methods" in case same method is derived from multiple interfaces and no definition is overridden by the class. | ||||
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Ans. boolean - java.lang.Boolean byte - java.lang.Byte char - java.lang.Character double - java.lang.Double float - java.lang.Float int - java.lang.Integer long - java.lang.Long short - java.lang.Short void - java.lang.Void | ||||
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Ans. java.lang.StringBuffer. | ||||
Sample Code for StringBuffer | ||||
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Ans. There can be two different elements with the same hashcode. When two elements have the same hashcode then Java uses the equals to further differentation. So there can be one or two objects depending on the content of the objects. | ||||
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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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Ans. They are wrappers to primitive data types. They allow us to access primitives as objects. | ||||
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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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Like Discuss Correct / Improve  session   session management   java   cookies   authentication   web application   ebay intermediate | ||||
Very frequently asked. Usually asked in different variants like Diff between StringBuffer , String Builder ; Difference between StringBuilder and String class; Choice between these classes etc. | ||||
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Ans. A string buffer implements a mutable sequence of characters. A string buffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls. The String class represents character strings. All string literals in Java programs, such as "abc" are constant and implemented as instances of this class; their values cannot be changed after they are created. | ||||
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Ans. The JDBC is a pure Java API used to execute SQL statements. It provides a set of classes and interfaces that can be used by developers to write database applications. The steps needed to execute a SQL query using JDBC: 1. Open a connection to the database. 2. Execute a SQL statement. 3. Process th results. 4. Close the connection to the database. | ||||
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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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Ans. Yes.we cannot access them directly but we can access them using object reference.Static methods belong to a class and not objects whereas non static members are tied to an instance. Accessing instance variables without the instance handler would mean an ambiguity regarding which instance the method is referring to and hence its prohibited. | ||||
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