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| Ans. You can put related classes together as a single logical group. Nested classes can access all class members of the enclosing class, which might be useful in certain cases. Nested classes are sometimes useful for specific purposes. For example, anonymous inner classes are useful for writing simpler event-handling code with AWT/Swing. | ||||
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 Discuss Correct / Improve  java nested classes inner classes oops classes | ||||
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| Ans. The accessibility (public, protected, etc.) of the inner class is defined by the outer class. Just like top-level classes, an inner class can extend a class or can implement interfaces. Similarly, an inner class can be extended by other classes, and an inner interface can be implemented or extended by other classes or interfaces. An inner class can be declared final or abstract.Inner classes can have inner classes, but you will have a hard time reading or understanding such complex nesting of classes. | ||||
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| Ans. You can create a non-static local class inside a body of code. Interfaces cannot have local classes, and you cannot create local interfaces. Local classes are accessible only from the body of the code in which the class is defined. The local classes are completely inaccessible outside the body of the code in which the class is defined. You can extend a class or implement interfaces while defining a local class. A local class can access all the variables available in the body of the code in which it is defined. You can pass only final variables to a local inner class. | ||||
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| Ans. Anonymous classes are defined in the new expression itself, so you cannot create multiple objects of an anonymous class. You cannot explicitly extend a class or explicitly implement interfaces when defining an anonymous class. An anonymous inner class is always created as part of a statement; don't forget to close the statement after the class definition with a curly brace. This is a rare case in Java, a curly brace followed by a semicolon. Anonymous inner classes have no name, and their type must be either a subclass of the named type or an implementer of the named interface | ||||
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| Ans. Arrays are of fixed size and have to reserve memory prior to use. Hence if we don't know size in advance arrays are not recommended to use. Arrays can store only homogeneous elements. Arrays store its values in contentious memory location. Not suitable if the content is too large and needs to be distributed in memory. There is no underlying data structure for arrays and no ready made method support for arrays, for every requriment we need to code explicitly | ||||
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| Ans. Cloneable is a declaration that the class implementing it allows cloning or bitwise copy of it's object state. It is not having any method because it is a MARKER interface. | ||||
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| Ans. A Stream is an abstraction that either produces or consumes information. There are two types of Streams : Byte Streams: Provide a convenient means for handling input and output of bytes. Character Streams: Provide a convenient means for handling input & output of characters. Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream. Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer. | ||||
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| Ans. Collections are re-sizable in nature. We can increase or decrease the size as per recruitment. Collections can hold both homogeneous and heterogeneous data's. Every collection follows some standard data structures. Collection provides many useful built in methods for traversing,sorting and search. | ||||
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| Ans. Collections can only hold objects, It can't hold primitive data types. Collections have performance overheads as they deal with objects and offer dynamic memory expansion. This dynamic expansion could be a bigger overhead if the collection class needs consecutive memory location like Vectors. Collections doesn't allow modification while traversal as it may lead to concurrentModificationException. | ||||
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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. Generalization or UpCasting is a phenomenon where a sub class is prompted to a super class, and hence becomes more general. Generalization needs widening or up-casting. Specialization or DownCasting is a phenomenon where a super class is narrowed down to a sub class. Specialization needs narrowing or down-casting. | ||||
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| Ans. Yes, We can call garbage collector directly but it doesn't guarantees that the gc will start executing immediately. This gc( ) method appears in both Runtime and System classes of java.lang package. | ||||
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| Advanced level question usually asked to senior developers , leads and architects. | ||||
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| Ans. Volatile is an instruction that the variables can be accessed by multiple threads and hence shouldn't be cached. As volatile variables are never cached and hence their retrieval cannot be optimized. | ||||
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| Ans. Open ended Questions. | ||||
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| Ans. Open ended Questions. | ||||
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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. co-variant return type states that return type of overriding method can be subtype of the return type declared in method of superclass. it has been introduced since jdk 1.5 | ||||
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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. Java is a portable-language because without any modification we can use Java byte-code in any platform(which supports Java). So this byte-code is portable and we can use in any other major platforms. | ||||
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| Ans. Java was initially found in 1991 by James Gosling, Sun Micro Systems. At first it was called as "Oak". In 1995 then it was later renamed to "Java". java is a originally a platform independent language. Currently Oracle, America owns Java. | ||||
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| Ans. You can find JVM - 32 bit or 64 bit by using System.getProperty() from Java program. | ||||
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| Ans. No. Every Class only needs to have one constructor - With parameters or without parameters. Compiler provides a default non parameterized constructor if no constructors is defined. | ||||
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| Ans. throw is used to explicitly throw an exception especially custom exceptions, whereas throws is used to declare that the method can throw an exception. We cannot throw multiple exceptions using throw statement but we can declare that a method can throw multiple exceptions using throws and comma separator. | ||||
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| Ans. No. Even though "this" would mean a reference to current object id the method gets called using object reference but "this" would mean an ambiguity if the same static method gets called using Class name. | ||||
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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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| Ans. Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX/Linux like HP-Unix, Sun Solaris, Redhat Linux, Ubuntu, CentOS, etc | ||||
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| Ans. Java uses Just-In-Time compiler to enable high performance. Just-In-Time compiler is a program that turns Java bytecode into instructions that can be sent directly to the processor. | ||||
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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. A class is a blue print or Mold using which individual objects are created. A class can contain fields and methods to describe the behavior of an object. | ||||
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| Ans. Postfix operators i.e () [] . is at the highest precedence. | ||||
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Which of following can be nested into another ?
