Java - Interview Questions and Answers on Composition

Q1.  Difference between Compositions and Inheritance ?

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.  

Q2.  Can we compose the Parent Class object like this ?

class BuggyBread1 extends BuggyBread2 {
private BuggyBread2 buggybread2; 
public static void main(String[] args){
buggybread2 = new BuggyBread2();

Ans. Yes.

Q3.  What are the difference between composition and inheritance in Java?

Ans. Composition - has-a relationship between classes.
Inheritance - is-a relationship between classes.

Composition - Composing object holds a reference to composing classes 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.