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Frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q11. What are the methods of Object Class ?Core Java
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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 Q12. Why every object constructor automatically call super() in Object before its own constructors?Core Java
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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 Q13. Describe what happens when an object is created in Java ?Core Java
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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 Q14. Does Constructor creates the object ?Core Java
Ans. New operator in Java creates objects. Constructor is the later step in object creation. Constructor's job is to initialize the members after the object has reserved memory for itself.

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 Q15. What will this code print ?

String a = new String ("TEST");
String b = new String ("TEST");
if(a == b) {
System.out.println ("TRUE");
} else {
System.out.println ("FALSE");
}		Core Java
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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 Q16. How compiler handles the exceptions in overriding ?Core Java
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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Frequently asked to fresh graduates and less experienced.
  Q17. Difference between Composition and Inheritance ?Core Java
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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 Q18. Which String class does not override the equals() and hashCode() methods, inheriting them directly from class Object?Core Java
Ans. java.lang.StringBuffer.

  Sample Code for StringBuffer

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 Q19. There are two objects a and b with same hashcode. I am inserting these two objects inside a hashmap.

hMap.put(a,a);
hMap.put(b,b);

where a.hashCode()==b.hashCode()

Now tell me how many objects will be there inside the hashmap?		Core Java
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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 Q20. Why Java is not considered pure OOP's language ?Core Java
Ans. There are 2 reasons for it.

1. Usage of Primitive types - Though Java provides classes for the primitive data types but as the usage of primitives is permissible, its considered unpure OOP's language.

2. Usage of Static members - Static members belong to the class and not objects and hence not considered fit for pure OOP's programming.

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 Q21. What one should take care of, while serializing the object?
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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 Q22. Difference between object instantiation and construction ?Core Java
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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Frequently asked to fresh graduates and less experienced.
 Q23. Which of the following is tightly bound ? Inheritance or Composition ?Core Java
Ans. Inheritance.

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 Q24. Difference Between this() and super() ?Core Java
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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  Q25. Why java doesn't support multiple Inheritance ?

or

Explain Java Diamond Problem.		Core Java
Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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 Q26. What is the use of Object class ?Core Java
Ans. 1. Default definition for some of the methods , like equals, hashcode etc that gets carried to all objects even if you don't define anything. But that default definition is kind of like assigning 0 to integer, it just provide a safe state , nothing much for comparison.

2. Places where you have no idea about what object you may receive and just want to perform out of 8 basic methods of object class, something like printing their string representation ( defined by their toString method ), or equality ( using their equals method )

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 Q27. Does Declaring an object "final" makes it immutable ?Core Java
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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 Q28. What are the features of encapsulation ?Core Java
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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 Q29. Difference between new operator and Class.forName().newInstance() ?
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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 Q30. Will this code compile fine ?

ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(new File("newFile.txt")));		Core Java
Ans. Yes.

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 Q31. How is Abstraction implemented in Java ?Core Java
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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 Q32. If everything is an object , Cant we declare every object as

Object obj = new String();		Core Java
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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 Q33. Can we override wait() or notify() methods?Core Java
Ans. wait and notify are declared final in object class and hence cannot be overridden.

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 Q34. How does java identifies which method to be called in method overriding or runtime polymorphism, when both methods share the same name and signature ? Core Java
Ans. Java identifies the method to be called at runtime by the object that is being referenced.

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 Q35. Can we have multiple inheritance in java using abstract class ?Core Java
Ans. No

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 Q36. Does use of Final class enforces composition over Inheritance in Java ? Core Java
Ans. Yes, to a certain extent. But the objective for Final class could be beyond just enforcing composition as certain classes might have been created without inheritance or composition in mind.

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 Q37. What could be the reasons for restricting inheritance using final class ?Core Java
Ans. 1. Enforcing composition over inheritance
2. Restricting overriding of certain methods
3. Final methods are faster than regular instance methods
4. Enforcing Immutability

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 Q38. Difference between Class#getInstance() and new operator ?
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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 Q39. What is an Object ?Core Java
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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 Q40. Variable of the boolean type is automatically initialized as?Core Java
Ans. The default value of the boolean type is false.

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