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Ans. Its a method which cannot be overridden. Compiler throws an error if we try to override a method which has been declared final in the parent class.
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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. String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.
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Ans. 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 doesn't 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 it's 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. 1. Stack Segment - Contains primitives, Class / Interface names and references.
2. Heap Segment - Contains all created objects in runtime, objects only plus their object attributes (instance variables), Static variables are also stored in heap.
3. Code Segment - The segment where the actual compiled Java bytecodes resides when loaded
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Ans. Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection
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Ans. There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference
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Very frequently asked. Usually asked in different variants like Diff between StringBuffer , String Builder ; Difference between StringBuilder and String class; Choice between these classes etc.
Q112. What is the difference between StringBuffer and String class ?
Ans. A string buffer implements a mutable sequence of characters. A string buffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls.
The String class represents character strings. All string literals in Java programs, such as "abc" are constant and implemented as instances of this class; their values cannot be changed after they are created.
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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. 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. Storing the state of an object in a file or other medium is called serialization.
Classes can communicate only if they are built together ( as they need Byte code for communication ). What if we need to enable communication between different applications ( i.e they have been built independently or even they reside at different locations ), We need a mechanism that will transfer the Bean state to a Medium than can be transferred to the receiving application.
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Ans. An interface without any method declaration is called as marker interface. there are 3 in-built interfaces in JVM i.e. serializable, clonable, remote
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Ans. If the Object value will not change, use String Class because a String object is immutable.
If the Object value can change and will only be modified from a single thread, use StringBuilder because StringBuilder is unsynchronized(means faster).
If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).
Ans. Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection.
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Ans. final - constant variable, objects cannot be de-referenced, restricting method overriding, restricting class sub classing.
finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of what happens within the try block. Use the finally block to close files or to release other system resources like database connections, statements etc.
finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state.
Which of the following is wrong for final instance variables ?
They cannot be changed after initialization
They can be initialized directly within static method
They can be declared and initialized together at the same place
They can be initialized within constructor
Which of the following is false for final ?
Final methods cannot be overriden
Final methods cannot be overloaded
Final classes cannot be subclassed
Final class cannot be abstract
Which of the following methods are used by Java Garbage Collection Mechanism ?
final
finally
finalize
All of the above
In which case finally won't get executed ?
in case of exception
in case of normal execution
in case of return statement before end of try block
in case of force program termination
Q129. When do you get ClassCastException?
or
What is ClassCastException ?
Ans. As we only downcast class in the hierarchy, The ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance.
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Ans. Comparable interface is used for single sequence sorting i.e.sorting the objects based on single data member where as comparator interface is used to sort the object based on multiple data members.
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Which of the following is wrong for final instance variables ?
