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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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Like Discuss Correct / Improve  abstract class   interfaces Asked in 2 Companies Basic | ||||
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Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as code reuse through inheritance ( extending abstract class ) Comparing to concrete class they have an advantage of providing interface which a concrete class doesn't provide. Comparing to interfaces they have an advantage of providing code reuse through inheritance which interfaces dont provide. | ||||
Sample Code for abstract class | ||||
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Like Discuss Correct / Improve  abstract class  importance of abstract class Basic   frequent | ||||
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Ans. Because Abstract class is incomplete i.e there are abstract methods that needs to be defined by extending classes. | ||||
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Like Discuss Correct / Improve  abstract class | ||||
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Ans. The objective of an abstract class is to provide an interface as well as code reuse. Though we cannot instantiate an instance of abstract class but we can reuse the code by extending such a class. Moreover abstract class provides interfacing to outside components just like interfaces. | ||||
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Ans. Classes having only static methods in java is called util classes. As they don't have specific object state, they are basically intend to have shared code and state through static elements and methods. They save resources by sharing code. | ||||
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Like Discuss Correct / Improve  util classes | ||||
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Ans. OuterClass.class AND OuterClass$InnerClass.class | ||||
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Like Discuss Correct / Improve  class files  compilation  compile  inner classes   nested classes | ||||
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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. It depends on the type of inner class To access non static inner class method new OuterClass().new InnerClass().getMethod(); To access static method of static inner class new OuterClass.InnerClass().getMethod(); | ||||
Sample Code for inner class | ||||
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Ans. PATH, CLASSPATH and JAVA_HOME | ||||
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Like Discuss Correct / Improve  environment variables  path  classpath  java_home Asked in 3 Companies basic   frequent | ||||
Very frequently asked. Usually asked in this format or as difference with interfaces / concrete classes. | ||||
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Ans. Abstract class is the class that is not supposed to be instantiated. The purpose of the class to only have extension to the derived class. | ||||
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Ans. B ctor B ctor B ctor A ctor A ctor | ||||
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Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as inheritance ( extending abstract class ). So they should be used in place of interfaces in case there is some code ( methods ) or object body ( member elements ) that can be reused with inheritance. | ||||
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Ans. Collections in java is a framework of classes that provides an abstracted data structure to store and manipulate the group of objects. Each class is unique in the way it stores , search , sort and modify the elements. | ||||
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Like Discuss Correct / Improve  collections  collection classes Asked in 1 Companies Basic   frequent | ||||
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Ans. toString() is an overloaded method of String class that is used to convert many data types to String, Boolean being one of them. toString(Boolean bool) | ||||
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Like Discuss Correct / Improve  String  String class  Boolean Asked in 1 Companies Basic | ||||
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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 | ||||
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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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Like Discuss Correct / Improve  Static Class  Singleton  Static Class vs Singleton Asked in 3 Companies Intermediate   frequent | ||||
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Ans. String class is immutable as well as final. Because of these properties , String objects offer many benefits 1. String Pool - When a string is created and if it exists in the pool, the reference of the existing string will be returned instead of creating a new object. If string is not immutable, changing the string with one reference will lead to the wrong value for the other references. Example - String str1 = "String1"; String str2 = "String1"; // It doesnt create a new String and rather reuses the string literal from pool // Now both str1 and str2 pointing to same string object in pool, changing str1 will change it for str2 too 2. To Cache its Hashcode - If string is not immutable, One can change its hashcode and hence its not fit to be cached. 3. Security - String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment. | ||||
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Ans. It means that the respective folder is not looked upon by Java to look for java or resource files and hence not being included as part of build.It needs to be added to Java Build Path. | ||||
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Like Discuss Correct / Improve  build path.build class path | ||||
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Ans. println prints an additional end of line whereas printf doesnt. | ||||
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Like Discuss Correct / Improve  System class  Streams  Input Output  println  printf | ||||
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Ans. System is a class within java.lang package that contains several useful class fields and methods. It cannot be instantiated and hence can use only statically.even in this case this has been used statically i.e with class name itself and without creating an instance. out is the static reference of Printstream declared as following in the System Class - public final static PrintStream out = null; println is the method of PrintStream class. | ||||
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Like Discuss Correct / Improve  System class  Streams  Input Output  println Asked in 1 Companies basic   frequent | ||||
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Ans. Yes, we can define an inner class within interface.Inside the interface, the inner class is implicitly public static. So the following is legit in Java public interface BuggyBreadInterface { void doSomething(); public class BuggyBreadClass implements BuggyBreadInterface{ void doSomething(){ System.out.println("Do something"); } } } | ||||
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Ans. A trigger is a special kind of stored procedure that automatically gets executed upon an event in the database server. | ||||
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Ans. Combiners are used to increase the efficiency of a Map Reduce program. They are used to aggregate intermediate map output locally on individual mapper outputs. Combiners can help you reduce the amount of data that needs to be transferred across to the reducers. | ||||
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Ans. The simplest way is to ignore them if Maven enforcer plugin is complaining about it but it may lead to runtime problems later. We can do the dependency:tree to see from where these duplicate ones are coming and hence can exclude the duplicate one. | ||||
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Like Discuss Correct / Improve  maven   ban duplicate classes enforcer plugin | ||||
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Ans. Yes , we have been using this plugin with our projects and its purpose is to warn and stop the Build if there are duplicates of the same package and class are being carried either directly or through transitive dependencies. the duplicate could be coming through different types of dependencies or through different versions of the same dependency. Its purpose is to make sure that there is only one copy thats being used at compile time and runtime and hence shouldnt later result in runtime problems. | ||||
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Ans. public class BuggyBread { | ||||
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Ans. Classpath is the parameter for JVM to look for java classes ( .class files ) that are to be looad by class loader | ||||
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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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Ans. It is a class level variable that is shared among the objects of that class. | ||||
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Like Discuss Correct / Improve  static   static variables   class level variables Asked in 1 Companies basic   frequent | ||||
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