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Ans. java.lang.StringBuffer. | ||||
Sample Code for StringBuffer | ||||
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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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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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Ans. "this" keyword is a reference to the current object and can be used for following - 1. Passing itself to another method. 2. Referring to the instance variable when local variable has the same name. 3. Calling another constructor in constructor chaining. | ||||
Sample Code for this keyword | ||||
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Ans. One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializable Exception. | ||||
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Ans. 1. Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data. 2. The instance variables of the objects are initialized to their default values. 3. The constructor for the most derived class is invoked. The first thing a constructor does is call the constructor for its superclasses. This process continues until the constructor for java.lang.Object is called,as java.lang.Object is the base class for all objects in java. 4. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last. | ||||
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Ans. clone() - Creates and returns a copy of this object. equals() - Indicates whether some other object is "equal to" this one. finalize() - Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. getClass() - Returns the runtime class of an object. hashCode() - Returns a hash code value for the object. toString() - Returns a string representation of the object. notify(), notifyAll(), and wait() - Play a part in synchronizing the activities of independently running threads in a program. | ||||
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Ans. 1.Abstraction solves the problem at design level while encapsulation solves the problem at implementation level 2.Abstraction is used for hiding the unwanted data and giving relevant data. while Encapsulation means hiding the code and data into a single unit to protect the data from outside world. 3. Abstraction lets you focus on what the object does instead of how it does it while Encapsulation means hiding the internal details or mechanics of how an object does something. 4.For example: Outer Look of a Television, like it has a display screen and channel buttons to change channel it explains Abstraction but Inner Implementation detail of a Television how CRT and Display Screen are connect with each other using different circuits , it explains Encapsulation. | ||||
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Ans. Only declaring variables as final makes them immutable. Making objects final means that the object handler cannot be used to target some other object but the object is still mutable. | ||||
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Ans. Though It's often confused with each other, Object Creation ( Instantiation ) and Initialization ( Construction ) are different things in Java. Construction follows object creation. Object Creation is the process to create the object in memory and returning its handler. Java provides New keyword for object creation. Initialization is the process of setting the initial / default values to the members. Constructor is used for this purpose. If we don't provide any constructor, Java provides one default implementation to set the default values according to the member data types. | ||||
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Ans. Inheritance. | ||||
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Ans. 1.this is a reference to the current object in which this keyword is used whereas super is a reference used to access members specific to the parent Class. 2.this is primarily used for accessing member variables if local variables have same name, for constructor chaining and for passing itself to some method whereas super is primarily used to initialize base class members within derived class constructor. | ||||
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Ans. Class.getInstance doesn't call the constructor whereas if we create an object using new operator , we need to have a matching constructor or copiler should provide a default constructor. | ||||
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Ans. Using new operator - new xyzClass() Using factory methods - xyzFactory.getInstance( ) Using newInstance( ) method - (Class.forName(xyzClass))emp.newInstance( ) By cloning an already available object - (xyzClass)obj1.clone( ) | ||||
Sample Code for object initialization using clone Sample Code for object initialization using getInstance | ||||
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Ans. 1. Overriding method can not be more restrictive than the overridden method. reason : in case of polymorphism , at object creation jvm look for actual runtime object. jvm does not look for reference type and while calling methods it look for overridden method. If by means subclass were allowed to change the access modifier on the overriding method, then suddenly at runtime when the JVM invokes the true objects version of the method rather than the reference types version then it will be problematic 2. In case of subclass and superclass define in different package, we can override only those method which have public or protected access. 3. We can not override any private method because private methods can not be inherited and if method can not be inherited then method can not be overridden. | ||||
Sample Code for Overriding | ||||
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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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Ans. Object is a run time entity whose state is stored in fields and behavior is shown via methods. Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication. | ||||
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Ans. The default value of the boolean type is false. | ||||
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Ans. Java expects the superclass ( Object Class ) constructor to be called while creation of any object. So super constructor is called in case there are no instance variables to initialize. | ||||
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Ans. Combine the data of our application and its manipulation at one place. Encapsulation Allow the state of an object to be accessed and modified through behaviors. Reduce the coupling of modules and increase the cohesion inside them. | ||||
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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. 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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Ans. Yes. | ||||
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Ans. Yes but as Hibernate creates the Proxy Classes inherited from the Entity Classes to communicate with Database for lazy initialization. Declaring entity classes as final will prohibit communication with database lazily and hence will be a performance hit. | ||||
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Ans. No. Enums are final by design. | ||||
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Ans. Yes, we can substitute outer classes wherever we need to have inner classes but Inner classes have advantage in certain cases and hence preferred - Ease - Why to implement a class outside if its objects are only intended to be part of an outer object. Its easy to define the class within another class if the use is only local. Protection - Making a call an outer exposes a threat of it being used by any of the class. Why should it be made an outer class if its object should only occur as part of other objects. For example - You may like to have an class address whose object should have a reference to city and by design thats the only use of city you have in your application. Making Address and City as outer class exposes City to any of the Class. Making it an inner class of Address will make sure that its accessed using object of Address. | ||||
Sample Code for inner class | ||||
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Ans. Composition - has-a relationship between objects. Inheritance - is-a relationship between classes. Composition - Composing object holds a reference to composed objects and hence relationship is loosely bound. Inheritance - Derived object carries the base class definition in itself and hence its tightly bound. Composition - Used in Dependency Injection Inheritance - Used in Runtime Polymorphism Composition - Single class objects can be composed within multiple classes. Inheritance - Single class can only inherit 1 Class. Composition - Its the relationship between objects. Inheritance - Its the relationship between classes. | ||||
Sample Code for inheritance Sample Code for composition | ||||
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Ans. Derived object carries the body of its class as well as the body of the parent class. Its body ( member elements ) is initialized using its own class constructor whereas the body ( member elements ) carried from the parent class are initialized using super class constructor. So In order to initialize the elements of the parent class before its own elements are even initialized, super is called. | ||||
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Ans. Abstraction is provided in Java by following ways - Coding to the ( Interfaces / Abstract Classes ) or contracts By Encapsulating details within classes and exposing the minimal Door ( few public methods ) | ||||
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Ans. No. This will result in compilation error as Polymorphism cannot be performed on Object types. | ||||
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