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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.
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. 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. 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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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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LikeDiscussCorrect / Improve  final class   reasons for final class   restricting inheritance  object oriented programming (oops)  oops concepts expert
Q12. What will happen if class implement two interface having common method?
Ans. That would not be a problem as both are specifying the contract that implement class has to follow. If class C implement interface A & interface B then Class C thing I need to implement print() because of interface A then again Class think I need to implement print() again because of interface B, it sees that there is already a method called test() implemented so it's satisfied.
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Q15. Shouldn't we make a class with all static members is its just expected to be executed as a standalone program with just one thread. Moreover Lets assume that there is no runtime Polymorphism required and there is no need for serialization ?
Ans. Still No in case we are making use of inheritance. we may have problem wherein we have program flow moving across common inherited method and specific methods of the derived class. call made to another static method in the parent class will only access the static class of the Parent class irrespective of the call from any of the derived class.
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Ans. There was no multiple inheritance in java before version 8 as implementing multiple interfaces cannot be termed as inheritance.
With Java 8 , came the concept of default methods wherein the class implementing the interface carries the default implementation from the interface and hence facilitating multiple inheritance.
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Ans. Multi Level Inheritance is multi level hierarchy of classes. Here is the example - http://www.buggybread.com/2015/09/java-se-class-diagram-classes-that_603.html
Class RoleList extends ArrayList which in turn extends AbstractList which in turn extends AbstractCollection.
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Ans. Default methods results in multiple inheritance of behavior and not of state. In case we try to implement multiple interfaces with default method having same name and signature, and don't override it in implementation class, it will throw an error.
For example -
interface MyInterface {
public void default myMethod(){
}
}
interface MyInterface2 {
public void default myMethod(){
}
}
class MyClass implements MyInterface,MyInterface2 {
}
This code will compilation error "Duplicate Default Method"
if we specify the definition of myMethod() in myClass, compiler won't complain and there is no conflict and MyClass can use overridden definition. But if we don't override myMethod() in MyClass, Java would be in conflict as to what definition should be carried to MyClass and hence throws compilation error.
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Ans. No, If both Parent and Derived are outer classes.
public class Vehicle {
private static String manufacturingDate = "2016";
}
public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // error - The field Vehicle.manufacturingDate is not visible
}
}
Yes, If derived is the inner class of Parent.
public class Vehicle {
private static String manufacturingDate = "2016";
static public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // no problem
}
}
}
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Ans. Multiple Inheritance refers to the concept of a class inheriting multiple classes. Example - Class C extends Class A ,Class B. This is not allowed in Java.
Multilevel Inheritance refers to the concept of Inheritance in a chain. Example - Class B extends Class A, Class C extends Class B. This is permitted in Java.
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Q28. If class A implements X interface, and Class B implements Y interface, and X is the parent of Y, Which of the following initialization is correct ?