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Interview Questions and Answers - Order By Rating | ||||
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Ans. dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. This is used to achieve runtime polymorphism in java. | ||||
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Like Discuss Correct / Improve  overriding  runtime polymorphism  object oriented programming (oops)  oops concepts | ||||
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Ans. Java identifies the method to be called at runtime by the object that is being referenced. | ||||
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Like Discuss Correct / Improve  polymorphism  object oriented programming (oops)  oops concepts  overloading  overriding basic | ||||
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Ans. runtime polymorphism or method overriding doesn't require method name and signature to be different whereas compile time polymorphism or method overloading requires method name to be same but the signature to be different. | ||||
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Like Discuss Correct / Improve  polymorphism  object oriented programming (oops)  oops concepts  overloading  overriding basic | ||||
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Ans. This looks like the case for optimistic locking wherein hibernate suspects that the information in table was updated by some other transaction after the entity was loaded by current transaction. One way is to have synchronized entity state and don't detach the entity. Other could be to merge the entity with the table record rather than just directly persisting the entity. | ||||
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Like Discuss Correct / Improve  StaleObjectStateException  Optimistic locking | ||||
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Ans. Access Control List or ACL is the list of permissions attached to an object in the File System. | ||||
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Like Discuss Correct / Improve  acl  file system  file security  object security  operating system | ||||
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Ans. Composition and Inheritance are the different types of Associations for Classes. Composition is a has-a association between classes. Inheritance is a is-a association between classes. | ||||
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Like Discuss Correct / Improve  composition  object oriented programming (oops)  oops concepts  inheritance  object oriented programming (oops)  oops concepts  association  relationship between objects | ||||
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Ans. 1. Security and Safety - They can be shared across multiple threads as they are thread safe. Moreover, it protects then from bad state due to interception by the other code segment. One such problem due to mutability and access by alternate code segment could be the change of hash code and then the impact on its search with hash collections. 2. Reuse - In some cases they can be reused as only one copy would exist and hence it can be relied upon. For example - String Pool | ||||
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Like Discuss Correct / Improve  immutable  immutability objects Asked in 1 Companies Intermediate   frequent | ||||
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Ans. wait and notify are declared final in object class and hence cannot be overridden. | ||||
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Like Discuss Correct / Improve  object class methods  wait  notify | ||||
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Ans. Class is a template using which objects are created in memory. It's kind of a mold using which objects with body / state are made. | ||||
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Like Discuss Correct / Improve  class  object  class vs object Asked in 4 Companies basic   frequent | ||||
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Ans. Shallow copy is one of the way for object cloning, other being deep copy. | ||||
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Like Discuss Correct / Improve  cloning  shallow copy  object cloning basic | ||||
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Ans. No, Java doesn't have a sizeOf operator. In C / C++ , its required to determine how much memory allocation is required which is not the case with Java. Java handles memory allocation and deallocation intrinsically. | ||||
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Like Discuss Correct / Improve  sizeOf  size of object  sizeOf operator Asked in 1 Companies   rare | ||||
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Ans. Null | ||||
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Like Discuss Correct / Improve  object reference  null Asked in 1 Companies basic | ||||
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Ans. class A { void test() { System.out.println("test() method"); } } class B { void test() { System.out.println("test() method"); } } Suppose if Java allows multiple inheritance like this, class C extends A, B { } A and B test() methods are inheriting to C class. So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity. | ||||
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Like Discuss Correct / Improve  multiple inheritance  object oriented programming (oops)  oops concepts  diamond problem Asked in 20 Companies basic   frequent | ||||
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Ans. Yes, we can do that. Compiler wont complain. But using object reference we can only access methods which have been defined for object class i.e clone(), equals(), hashCode(), toString() etc. We cannot access methods defined in String class or in any class in hierarchy between String and Object. For example - we cannot do obj.append("abc") as it will now give compile time error. | ||||
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Like Discuss Correct / Improve  object initialization  object declaration   runtime polymorphism  object oriented programming (oops)  oops concepts | ||||
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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Like Discuss Correct / Improve  polymorphism  object oriented programming (oops)  oops concepts  oops concepts Asked in 108 Companies Basic   frequent | ||||
Try 2 Question(s) Test | ||||
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Ans. For objects or references, == operator check if the reference on left and right points to the same object. For primitive types or variables, == operator check if the variable on left and right holds the same value. | ||||
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Like Discuss Correct / Improve  ==  object equality  operator Basic | ||||
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Ans. = is the assignment operator that assigns the result of the expression on the right to the variable on the left, whereas == is the operator to check object equality to see if the reference on left and right are pointing to the same object. For primitive types, its used to check if both variables holds the same value. | ||||
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Like Discuss Correct / Improve  =  ==  assignment operator  object equality  difference between Basic | ||||
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Ans. Multiple Inheritance refers to the concept of a class inheriting multiple classes. Example - Class C extends Class A ,Class B. This is not allowed in Java. Multilevel Inheritance refers to the concept of Inheritance in a chain. Example - Class B extends Class A, Class C extends Class B. This is permitted in Java. | ||||
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Like Discuss Correct / Improve  inheritance  object oriented programming (oops)  oops concepts   multiple inheritance  object oriented programming (oops)  oops concepts   multi level inheritance  object oriented programming (oops)  oops concepts Basic   frequent | ||||
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Ans. No, If both Parent and Derived are outer classes. public class Vehicle { private static String manufacturingDate = "2016"; } public class Car extends Vehicle{ public static void main(String args[]){ System.out.println(manufacturingDate); // error - The field Vehicle.manufacturingDate is not visible } } Yes, If derived is the inner class of Parent. public class Vehicle { private static String manufacturingDate = "2016"; static public class Car extends Vehicle{ public static void main(String args[]){ System.out.println(manufacturingDate); // no problem } } } | ||||
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Like Discuss Correct / Improve  private members   private methods   private variables   inheritance  object oriented programming (oops)  oops concepts   members visibility   inner classes  nexted classes basic   frequent | ||||
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Ans. It make sense only if we intend to modify either of the object and would like to preserve original state in other. Otherwise we can reuse the original object by making it singleton. | ||||
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Like Discuss Correct / Improve  clone  clone objects  prototype design pattern   immutable  immutability  immutability  cloning | ||||
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Ans. Copy Constructor is used for creating a duplicate of another object. Duplicate object will have the same state of the copied object but will have independent values in different memory locations. Copy Constructor can be useful to create duplicates of immutable objects as the Original cannot be tampered. Moreover It can be useful if base copies are required for individual requests in threading. | ||||
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Like Discuss Correct / Improve  copy constructor  clone  cloning object   immutable  immutability  immutability | ||||
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Ans. Default methods results in multiple inheritance of behavior and not of state. In case we try to implement multiple interfaces with default method having same name and signature, and don't override it in implementation class, it will throw an error. For example - interface MyInterface { public void default myMethod(){ } } interface MyInterface2 { public void default myMethod(){ } } class MyClass implements MyInterface,MyInterface2 { } This code will compilation error "Duplicate Default Method" if we specify the definition of myMethod() in myClass, compiler won't complain and there is no conflict and MyClass can use overridden definition. But if we don't override myMethod() in MyClass, Java would be in conflict as to what definition should be carried to MyClass and hence throws compilation error. | ||||
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Like Discuss Correct / Improve  default methods  java 8  multiple inheritance  object oriented programming (oops)  oops concepts  diamond problem   interfaces | ||||
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Ans. Multi Level Inheritance is multi level hierarchy of classes. Here is the example - http://www.buggybread.com/2015/09/java-se-class-diagram-classes-that_603.html Class RoleList extends ArrayList which in turn extends AbstractList which in turn extends AbstractCollection. | ||||
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Like Discuss Correct / Improve  multilevel inheritance  object oriented programming (oops)  oops concepts  inheritance  object oriented programming (oops)  oops concepts  oops concepts Asked in 3 Companies basic | ||||
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Ans. VO or Value objects are the objects that are supposed to hold some value. For Example , Integer, Float , Money etc. | ||||
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Ans. POJO is Plain Java Object that holds only Member Elements , getters are setters and minimal processing on that. The primary purpose of such object is to either transfer information or keeping state for a while. They are not intended to provide any processing or transformation. | ||||
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Like Discuss Correct / Improve  POJO   Plain Java Objects | ||||
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Ans. DAO is Data Access Object which is used within Persistence Layer to make Database Transaction. For example , the class holding the Database connection and CRUD methods. | ||||
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Like Discuss Correct / Improve  DAO  Data Access Object basic   frequent | ||||
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Ans. DTO is Data Transfer Object i.e Pojo which is supposed to move between different layers of software architecture for transferring information. Example could be the object passed from client in case of web service call or the object passed to Persister for Database Persistence. | ||||
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Like Discuss Correct / Improve  DTO   Data Transfer Object basic   frequent | ||||
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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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Like Discuss Correct / Improve  encapsulation  object oriented programming (oops)  oops concepts  abstraction  oops  oops features  java features Asked in 13 Companies | ||||
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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 | ||||
Very frequently asked. Among first few questions in almost all interviews. Among Top 5 frequently asked questions. Frequently asked in Indian service companies (HCL,TCS,Infosys,Capgemini etc based on multiple feedback ) and Epam Systems | ||||
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Ans. "equals" is the method of object class which is supposed to be overridden to check object equality, whereas "==" operator evaluate to see if the object handlers on the left and right are pointing to the same object in memory. x.equals(y) means the references x and y are holding objects that are equal. x==y means that the references x and y have same object. Sample code: String x = new String("str"); String y = new String("str"); System.out.println(x == y); // prints false System.out.println(x.equals(y)); // prints true | ||||
Sample Code for equals | ||||
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Like Discuss Correct / Improve  java   string comparison   string   object class   ==   equals   object equality  operator   == vs equals   equals vs == Asked in 294 Companies basic   frequent | ||||
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