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Ans. Inner Class is a class that is nested within another class whereas sub class is a class that extends or inherit another class. Inner class can only be accessed using reference of outer class ( Class name , if inner class is static or object reference, if inner class is non static ) whereas Sub class is accessed directly. In terms of memory, inner class is stored just like another class having it's own body whereas sub class carries the body of parent class as well as it's own fields in memory. | ||||
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Like Discuss Correct / Improve  inner classes  nested classes  inner class vs sub class  nested class vs sub class Basic | ||||
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Ans. Simple Inner Class, Local Inner Class, Anonymous Inner Class, Static Nested Inner Class. | ||||
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Like Discuss Correct / Improve  java   inner classes   classes Asked in 6 Companies basic   frequent | ||||
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Frequently asked in Indian Service Companies. Tech Mahindra ( Based on 3 inputs ) and Infosys ( 3 inputs ) | ||||
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Ans. List - Members are stored in sequence in memory and can be accessed through index. Set - There is no relevance of sequence and index. Sets doesn't contain duplicates whereas multiset can have duplicates. Map - Contains Key , Value pairs. | ||||
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Ans. Java doesn't support multiple inheritance. Interfaces does't facilitate inheritance and hence implementation of multiple interfaces doesn't make multiple inheritance. | ||||
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Frequently asked at HCL Technologies ( Based on 3 feedback ) | ||||
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Ans. Checked exceptions are the exceptions for which compiler throws an errors if they are not checked whereas unchecked exceptions are caught during run time only and hence can't be checked. | ||||
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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. | ||||
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Ans. Final variable is a variable constant that cannot be changed after initialization. | ||||
Sample Code for final variable | ||||
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Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated. | ||||
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Ans. Vectors are synchronized whereas Array lists are not. | ||||
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Very Frequently asked. Usually asked along with String Class related questions. | ||||
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Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created. | ||||
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Frequently asked to fresh graduates and less experienced. | ||||
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Ans. Inheritance. | ||||
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Like Discuss Correct / Improve  inheritance  object oriented programming (oops)  oops concepts   java   oops   composition  object oriented programming (oops)  oops concepts Asked in 1 Companies basic   frequent | ||||
Frequently asked to fresh graduates. | ||||
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Ans. Process is a program in execution whereas thread is a separate path of execution in a program. | ||||
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Ans. Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes | ||||
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Very frequently asked. Usually difference between String,StringBuffer and StringBuilder is asked in different variations. | ||||
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Ans. StringBuffer is synchronized whereas StringBuilder is not. | ||||
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Ans. It is the process of examining / modifying the behaviour of an object at runtime. | ||||
Sample Code for reflection | ||||
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Ans. Both belong to the class as a whole and not to the individual objects. Static methods are explicitly called for execution whereas Static block gets executed when the Class gets loaded by the JVM. | ||||
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Very Frequently asked. | ||||
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Ans. throw is used to re throw an exception.throws is used to declare that the method throws the respective exceptions.try block is used to identify if the respective block has thrown any exception.catch is used to catch the exception that has been thrown by the respective try block. | ||||
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Ans. GET is supposed to get information from the server. Client sends the minimal information so that Server can respond with the response body on basis of request. For example - You want to get complete employment record for employee id 123 POST is supposed to send the information for submission. Payload or a Body is usually sent so that it can be persisted on the server. For example - Sending the complete information of an employee ( id, name , dept etc ) to the server for persisting it. | ||||
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Like Discuss Correct / Improve  get vs post   web service   rest Asked in 11 Companies basic   frequent | ||||
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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 | ||||
Usually asked to entry level software developers. | ||||
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Ans. public static void main(String[] args) { int num1 = 1; int num2 = 2; num1 = num1^num2; num2 = num1^num2; num1 = num1^num2; System.out.print("num1 = " + num1 +", num2 = "+num2); } | ||||
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Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements. | ||||
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Ans. JSON is "JavaScript Object Notation", primarily used for client-server or server-server communication. Its a much lighter and readable alternative to XML. JSON is language independent and is easily parse-able in all programming languages. | ||||
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Ans. An API is a kind of technical contract which defines functionality that two parties must provide: a service provider (often called an implementation) and an application. an API simply defines services that a service provider (i.e., the implementation) makes available to applications. | ||||
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Ans. URL is Uniform Resource Locator which is representation of HTTP address. | ||||
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Like Discuss Correct / Improve  j2ee   http   internet   url basic   rare | ||||
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Ans. HTTP or Hypertext Transfer Protocol is internet protocol for tranmission of hypertext ( text with meta data ) over internet. | ||||
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Ans. For top level class we can only use "public" and "default". We can use private with inner class. | ||||
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Ans. Threads in Java can be implement either by Extending Thread class or implementing runnable interface. | ||||
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Ans. A Method that cannot be overriden in the sub class. | ||||
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Very Frequently asked.Favorite question in Walk in drive for many Indian service companies. | ||||
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Ans. A Class that cannot be sub classed. | ||||
Sample Code for final class | ||||
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Very frequently asked. Usually asked with questions related to String. Frequently asked at CTS / Cognizant | ||||
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Ans. Object that can't be changed after instantiation. | ||||
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Ans. IndexOutofBoundException , NoClassDefFoundException , OutOfMemoryException , IllegalArgumentException, NullPointerException, IllegalStateException | ||||
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