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Ans. It's a technique to allow multiple clients to make use of a cached set of shared and reusable connection objects providing access to a database or other resource.
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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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LikeDiscussCorrect / Improve java collections collections classes disadvantages of collections over arrays collections vs arrays
Q1214. Can we call constructor explicitly ?
Ans. Yes.
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Ans. Both bitwise right shift operator ( >> ) and bitwise zero fill right shift operator ( >>> ) are used to shift the bits towards right. The difference is that >> will protect the sign bit whereas the >>> operator will not protect the sign bit. It always fills 0 in the sign bit.
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Q1218. What is the difference between System.out ,System.err and System.in?
Ans. System.out and System.err both represent the monitor by default and hence can be used to send data or results to the monitor. But System.out is used to display normal messages and results whereas System.err is used to display error messages and System.in represents InputStream object, which by default represents standard input device, i.e., keyboard.
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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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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. 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.
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. 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. IDE stands of Integrated Development Environment. Few Java IDE's are WSAD ( Websphhere Application Developer ) , RAD ( Rational Application Developer ) , Eclipse and Netbeans.
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Sample Code for object initialization using clone
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