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Java object oriented programming - OOPS

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rithustutorials

Object oriented programming in Java. Abstraction , Encapsulation, Polymorphism and Inheritance. What is Coupling and Cohesion and more.

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Java  Interview  Questions  –  www.JavaInterview.in   1     OOPS   Following  picture  show  the  topics  we  would  cover  in  this  article.     What  is  the  super  class  of  every  class  in  Java?   Every class in java is a sub class of the class Object. When we create a class we inherit all the methods and properties of Object class. Let’s look at a simple example:   String  str  =  "Testing";   System.out.println(str.toString());   System.out.println(str.hashCode());   System.out.println(str.clone());     if(str  instanceof  Object){          System.out.println("I  extend  Object");//Will  be  printed   }   In the above example, toString, hashCode and clone methods for String class are inherited from Object class and overridden. Can  super  class  reference  variable  can  hold  an  object  of  sub  class?   Yes. Look at the example below:   Actor reference variables actor1, actor2 hold the reference of objects of sub classes of Animal, Comedian and Hero. Since object is super class of all classes, an Object reference variable can also hold an instance of any class.
2   Java  Interview  Questions  –  www.JavaInterview.in       //Object  is  super  class  of  all  java  classes   Object  object  =  new  Hero();       public  class  Actor  {          public  void  act(){                  System.out.println("Act");          };   }     //IS-­‐A  relationship.  Hero  is-­‐a  Actor   public  class  Hero  extends  Actor  {          public  void  fight(){                  System.out.println("fight");          };   }     //IS-­‐A  relationship.  Comedian  is-­‐a  Actor   public  class  Comedian  extends  Actor  {          public  void  performComedy(){                  System.out.println("Comedy");          };   }     Actor  actor1  =  new  Comedian();   Actor  actor2  =  new  Hero();   Is  Multiple  Inheritance  allowed  in  Java?   Multiple Inheritance results in a number of complexities. Java does not support Multiple Inheritance.   class  Dog  extends  Animal,  Pet  {  //COMPILER  ERROR   }   However, we can create an Inheritance Chain class  Pet  extends  Animal  {   }     class  Dog  extends  Pet  {   }   What  is  Polymorphism?   Refer  to  this  video(https://www.youtube.com/watch?v=t8PTatUXtpI)  for  a  clear  explanation  of   polymorphism.   Polymorphism  is  defined  as  “Same  Code”  giving  “Different  Behavior”.  Let’s  look  at  an  example.     Let’s  define  an  Animal  class  with  a  method  shout.   public  class  Animal  {          public  String  shout()  {                  return  "Don't  Know!";          }  
Java  Interview  Questions  –  www.JavaInterview.in   3     }     Let’s  create  two  new  sub  classes  of  Animal  overriding  the  existing  shout  method  in  Animal.   class  Cat  extends  Animal  {          public  String  shout()  {                  return  "Meow  Meow";          }   }     class  Dog  extends  Animal  {          public  String  shout()  {                  return  "BOW  BOW";          }            public  void  run(){                            }   }     Look  at  the  code  below.  An  instance  of  Animal  class  is  created.  shout  method  is  called.     Animal  animal1  =  new  Animal();                   System.out.println(                  animal1.shout());  //Don't  Know!     Look  at  the  code  below.  An  instance  of  Dog  class  is  created  and  store  in  a  reference  variable  of  type   Animal.   Animal  animal2  =  new  Dog();     //Reference  variable  type  =>  Animal   //Object  referred  to  =>  Dog   //Dog's  bark  method  is  called.   System.out.println(                  animal2.shout());  //BOW  BOW     When  shout  method  is  called  on  animal2,  it  invokes  the  shout  method  in  Dog  class  (type  of  the  object   pointed  to  by  reference  variable  animal2).   Even  though  dog  has  a  method  run,  it  cannot  be  invoked  using  super  class  reference  variable.   //animal2.run();//COMPILE  ERROR   What  is  the  use  of  instanceof  Operator  in  Java?   instanceof operator checks if an object is of a particular type. Let us consider the following class and interface declarations: class  SuperClass  {   };     class  SubClass  extends  SuperClass  {   };  
4   Java  Interview  Questions  –  www.JavaInterview.in       Java  Interview  Questions  –  www.JavaInterview.in   At  http://www.JavaInterview.in,  we  want  you  to  clear  java  interview  with  ease.  So,  in  addition  to   focussing  on  Core  and  Advanced  Java  we  also  focus  on  topics  like  Code  Reviews,  Performance,     Design  Patterns,  Spring  and  Struts.   We  have  created  more  than  20  videos  to  help  you  understand  these  topics  and  become  an  expert  at   them.  Visit  our  website  http://www.JavaInterview.in  for  complete  list  of  videos.    Other  than  the   videos,  we  answer  the  top  200  frequently  asked  interview  questions  on  our  website.   With  more  900K  video  views  (Apr  2015),  we  are  the  most  popular  channel  on  Java  Interview   Questions  on  YouTube.   Register  here  for  more  updates  :     https://feedburner.google.com/fb/a/mailverify?uri=RithusTutorials   POPULAR  VIDEOS   Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:  https://www.youtube.com/watch?v=njZ48YVkei0   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:  https://www.youtube.com/watch?v=xyXuo0y-xoU   Java  Interview  :  A  Guide  for  Experienced:  https://www.youtube.com/watch?v=0xcgzUdTO5M Collections  Interview  Questions  1:  https://www.youtube.com/watch?v=GnR4hCvEIJQ Collections  Interview  Questions  2:  https://www.youtube.com/watch?v=6dKGpOKAQqs Collections  Interview  Questions  3:  https://www.youtube.com/watch?v=_JTIYhnLemA Collections  Interview  Questions  4:  https://www.youtube.com/watch?v=ZNhT_Z8_q9s Collections  Interview  Questions  5:  https://www.youtube.com/watch?v=W5c8uXi4qTw   interface  Interface  {   };     class  SuperClassImplementingInteface  implements  Interface  {   };     class  SubClass2  extends  SuperClassImplementingInteface  {   };     class  SomeOtherClass  {   };     Let’s  consider  the  code  below.    We  create  a  few  instances  of  the  classes  declared  above.  
Java  Interview  Questions  –  www.JavaInterview.in   5     SubClass  subClass  =  new  SubClass();   Object  subClassObj  =  new  SubClass();     SubClass2  subClass2  =  new  SubClass2();   SomeOtherClass  someOtherClass  =  new  SomeOtherClass();     Let’s  now  run  instanceof  operator  on  the  different  instances  created  earlier.   System.out.println(subClass  instanceof  SubClass);//true   System.out.println(subClass  instanceof  SuperClass);//true   System.out.println(subClassObj  instanceof  SuperClass);//true     System.out.println(subClass2                    instanceof  SuperClassImplementingInteface);//true     instanceof  can  be  used  with  interfaces  as  well.  Since  Super  Class  implements  the  interface,  below  code   prints  true.   System.out.println(subClass2                    instanceof  Interface);//true     If  the  type  compared  is  unrelated  to  the  object,  a  compilation  error  occurs.   //System.out.println(subClass     //                        instanceof  SomeOtherClass);//Compiler  Error     Object  referred  by  subClassObj(SubClass)-­‐  NOT  of  type  SomeOtherClass   System.out.println(subClassObj  instanceof  SomeOtherClass);//false   What  is  an  Abstract  Class?   An   abstract   class   (Video   Link   -­‐   https://www.youtube.com/watch?v=j3GLUcdlz1w )   is   a   class   that  cannot  be  instantiated,  but  must  be  inherited  from.  An  abstract  class  may  be  fully  implemented,   but  is  more  usually  partially  implemented  or  not  implemented  at  all,  thereby  encapsulating  common   functionality  for  inherited  classes. In   code   below   ”AbstractClassExample   ex   =   new   AbstractClassExample();”   gives   a   compilation   error   because  AbstractClassExample  is  declared  with  keyword  abstract.     public  abstract  class  AbstractClassExample  {          public  static  void  main(String[]  args)  {                  //An  abstract  class  cannot  be  instantiated                  //Below  line  gives  compilation  error  if  uncommented                  //AbstractClassExample  ex  =  new  AbstractClassExample();          }   }   How  do  you  define  an  abstract  method?   An Abstract method does not contain body. An abstract method does not have any implementation. The implementation of an abstract method should be provided in an over-riding method in a sub class.        //Abstract  Class  can  contain  0  or  more  abstract  methods          //Abstract  method  does  not  have  a  body  
6   Java  Interview  Questions  –  www.JavaInterview.in              abstract  void  abstractMethod1();          abstract  void  abstractMethod2();     Abstract  method  can  be  declared  only  in  Abstract  Class.  In  the  example  below,  abstractMethod()  gives  a   compiler  error  because  NormalClass  is  not  abstract.   class  NormalClass{          abstract  void  abstractMethod();//COMPILER  ERROR   }   What  is  Coupling?   Coupling is a measure of how much a class is dependent on other classes. There should minimal dependencies between classes. So, we should always aim for low coupling between classes. Coupling  Example  Problem   Consider  the  example  below:   class  ShoppingCartEntry  {          public  float  price;          public  int  quantity;   }     class  ShoppingCart  {          public  ShoppingCartEntry[]  items;   }     class  Order  {          private  ShoppingCart  cart;          private  float  salesTax;            public  Order(ShoppingCart  cart,  float  salesTax)  {                  this.cart  =  cart;                  this.salesTax  =  salesTax;          }            //  This  method  know  the  internal  details  of  ShoppingCartEntry  and          //  ShoppingCart  classes.  If  there  is  any  change  in  any  of  those          //  classes,  this  method  also  needs  to  change.          public  float  orderTotalPrice()  {                  float  cartTotalPrice  =  0;                  for  (int  i  =  0;  i  <  cart.items.length;  i++)  {                          cartTotalPrice  +=  cart.items[i].price                                          *  cart.items[i].quantity;                  }                  cartTotalPrice  +=  cartTotalPrice  *  salesTax;                  return  cartTotalPrice;          }   }   Method   orderTotalPrice   in   Order   class   is   coupled   heavily   with   ShoppingCartEntry   and   ShoppingCart  classes.    It  uses  different  properties  (items,  price,  quantity)  from  these  classes.  If  any  of   these  properties  change,  orderTotalPrice  will  also  change.  This  is  not  good  for  Maintenance.    
Java  Interview  Questions  –  www.JavaInterview.in   7     Solution   Consider a better implementation with lesser coupling between classes below: In this implementation, changes in ShoppingCartEntry or CartContents might not affect Order class at all. class  ShoppingCartEntry   {          float  price;          int  quantity;            public  float  getTotalPrice()          {                  return  price  *  quantity;          }   }     class  CartContents   {          ShoppingCartEntry[]  items;            public  float  getTotalPrice()          {                  float  totalPrice  =  0;                  for  (ShoppingCartEntry  item:items)                  {                          totalPrice  +=  item.getTotalPrice();                  }                  return  totalPrice;          }   }     class  Order   {          private  CartContents  cart;          private  float  salesTax;            public  Order(CartContents  cart,  float  salesTax)          {                  this.cart  =  cart;                  this.salesTax  =  salesTax;          }            public  float  totalPrice()          {                  return  cart.getTotalPrice()  *  (1.0f  +  salesTax);          }   }   What  is  Cohesion?   Cohesion   (Video   Link   -­‐   https://www.youtube.com/watch?v=BkcQWoF5124 )   is   a   measure   of   how  related  the  responsibilities  of  a  class  are.    A  class  must  be  highly  cohesive  i.e.  its  responsibilities   (methods)  should  be  highly  related  to  one  another.  
8   Java  Interview  Questions  –  www.JavaInterview.in       Example  Problem   Example   class   below   is   downloading   from   internet,   parsing   data   and   storing   data   to   database.   The   responsibilities  of  this  class  are  not  really  related.  This  is  not  cohesive  class.   class  DownloadAndStore{          void  downloadFromInternet(){          }                    void  parseData(){          }                    void  storeIntoDatabase(){          }                    void  doEverything(){                  downloadFromInternet();                  parseData();                  storeIntoDatabase();          }   }   Solution This is a better way of approaching the problem. Different classes have their own responsibilities. class  InternetDownloader  {          public  void  downloadFromInternet()  {          }   }     class  DataParser  {          public  void  parseData()  {          }   }     class  DatabaseStorer  {          public  void  storeIntoDatabase()  {          }   }     class  DownloadAndStore  {          void  doEverything()  {                  new  InternetDownloader().downloadFromInternet();                  new  DataParser().parseData();                  new  DatabaseStorer().storeIntoDatabase();          }   }   What  is  Encapsulation?   Encapsulation is “hiding the implementation of a Class behind a well defined interface”. Encapsulation helps us to change implementation of a class without breaking other code.
Java  Interview  Questions  –  www.JavaInterview.in   9     Approach  1   In  this  approach  we  create  a  public  variable  score.  The  main  method  directly  accesses  the  score  variable,   updates  it.   public  class  CricketScorer  {          public  int  score;   }     Let’s  use  the  CricketScorer  class.   public  static  void  main(String[]  args)  {   CricketScorer  scorer  =  new  CricketScorer();   scorer.score  =  scorer.score  +  4;   }   Approach  2   In  this  approach,  we  make  score  as  private  and  access  value  through  get  and  set  methods.  However,  the   logic  of  adding  4  to  the  score  is  performed  in  the  main  method.   public  class  CricketScorer  {          private  int  score;            public  int  getScore()  {                  return  score;          }            public  void  setScore(int  score)  {                  this.score  =  score;          }   }     Let’s  use  the  CricketScorer  class.     public  static  void  main(String[]  args)  {   CricketScorer  scorer  =  new  CricketScorer();     int  score  =  scorer.getScore();   scorer.setScore(score  +  4);   }   Approach  3   In  this  approach  -­‐  For  better  encapsulation,  the  logic  of  doing  the  four  operation  also  is  moved  to  the   CricketScorer  class.   public  class  CricketScorer  {          private  int  score;                    public  void  four()  {                  score  +=  4;          }     }     Let’s  use  the  CricketScorer  class.  
10   Java  Interview  Questions  –  www.JavaInterview.in       public  static  void  main(String[]  args)  {   CricketScorer  scorer  =  new  CricketScorer();   scorer.four();   }   Description   In  terms  of  encapsulation  Approach  3  >  Approach  2  >  Approach  1.  In  Approach  3,  the  user  of  scorer  class   does  not  even  know  that  there  is  a  variable  called  score.  Implementation  of  Scorer  can  change  without   changing  other  classes  using  Scorer.   What  is  Method  Overloading?   A method having the same name as another method (in same class or a sub class) but having different parameters is called an Overloaded Method. Example  1   doIt  method  is  overloaded  in  the  below  example:   class  Foo{          public  void  doIt(int  number){                            }          public  void  doIt(String  string){                            }   }   Example  2   Overloading  can  also  be  done  from  a  sub  class.   class  Bar  extends  Foo{          public  void  doIt(float  number){                            }   }   What  is  Method  Overriding?   Creating a Sub Class Method with same signature as that of a method in SuperClass is called Method Overriding. Method  Overriding  Example  1:   Let’s  define  an  Animal  class  with  a  method  shout.   public  class  Animal  {          public  String  bark()  {                  return  "Don't  Know!";          }   }     Let’s  create  a  sub  class  of  Animal  –  Cat    -­‐  overriding  the  existing  shout  method  in  Animal.   class  Cat  extends  Animal  {          public  String  bark()  {  
Java  Interview  Questions  –  www.JavaInterview.in   1 1                    return  "Meow  Meow";          }   }   bark method in Cat class is overriding the bark method in Animal class. What  is  an  Inner  Class?   Inner  Classes  are  classes  which  are  declared  inside  other  classes.  Consider  the  following  example:   class  OuterClass  {            public  class  InnerClass  {          }            public  static  class  StaticNestedClass  {          }     }   What  is  a  Static  Inner  Class?   A  class  declared  directly  inside  another  class  and  declared  as  static.  In  the  example  above,  class  name   StaticNestedClass  is  a  static  inner  class.     Can  you  create  an  inner  class  inside  a  method?   Yes.   An   inner   class   can   be   declared   directly   inside   a   method.   In   the   example   below,   class   name   MethodLocalInnerClass  is  a  method  inner  class.     class  OuterClass  {            public  void  exampleMethod()  {                  class  MethodLocalInnerClass  {                  };          }     }   Constructors   Constructor (Youtube Video link - https://www.youtube.com/watch?v=XrdxGT2s9tc ) is invoked whenever we create an instance(object) of a Class. We cannot create an object without a constructor. If we do not provide a constructor, compiler provides a default no-argument constructor. What  is  a  Default  Constructor?   Default Constructor is the constructor that is provided by the compiler. It has no arguments. In the example below, there are no Constructors defined in the Animal class. Compiler provides us with a default constructor, which helps us create an instance of animal class.   public  class  Animal  {          String  name;            public  static  void  main(String[]  args)  {  
12   Java  Interview  Questions  –  www.JavaInterview.in                      //  Compiler  provides  this  class  with  a  default  no-­‐argument  constructor.                  //  This  allows  us  to  create  an  instance  of  Animal  class.                  Animal  animal  =  new  Animal();          }   }   How  do  you  call  a  Super  Class  Constructor  from  a  Constructor? A constructor can call the constructor of a super class using the super() method call. Only constraint is that it should be the first statement i Both example constructors below can replaces the no argument "public Animal() " constructor in Example 3. public  Animal()  {          super();          this.name  =  "Default  Name";   }   Can  a  constructor  be  called  directly  from  a  method?     A constructor cannot be explicitly called from any method except another constructor. class  Animal  {          String  name;            public  Animal()  {          }            public  method()  {                  Animal();//  Compiler  error          }   }   Is  a  super  class  constructor  called  even  when  there  is  no  explicit  call  from  a   sub  class  constructor? If a super class constructor is not explicitly called from a sub class constructor, super class (no argument) constructor is automatically invoked (as first line) from a sub class constructor. Consider the example below: class  Animal  {          public  Animal()  {                  System.out.println("Animal  Constructor");          }   }     class  Dog  extends  Animal  {          public  Dog()  {                  System.out.println("Dog  Constructor");          }   }     class  Labrador  extends  Dog  {          public  Labrador()  {                  System.out.println("Labrador  Constructor");          }  
Java  Interview  Questions  –  www.JavaInterview.in   1 3     }     public  class  ConstructorExamples  {          public  static  void  main(String[]  args)  {                  Labrador  labrador  =  new  Labrador();          }   }   Program  Output   Animal Constructor Dog Constructor Labrador Constructor Interface   What  is  an  Interface?   An interface (YouTube video link - https://www.youtube.com/watch?v=VangB-sVNgg ) defines a contract for responsibilities (methods) of a class. How  do  you  define  an  Interface?   An   interface   is   declared   by   using   the   keyword   interface.   Look   at   the   example   below:   Flyable   is   an   interface.   //public  abstract  are  not  necessary   public  abstract  interface  Flyable  {          //public  abstract  are  not  necessary          public  abstract  void  fly();   }   How  do  you  implement  an  interface? We can define a class implementing the interface by using the implements keyword. Let us look at a couple of examples: Example  1   Class  Aeroplane  implements  Flyable  and  implements  the  abstract  method  fly().   public  class  Aeroplane  implements  Flyable{          @Override          public  void  fly()  {                  System.out.println("Aeroplane  is  flying");          }   }   Example  2   public  class  Bird  implements  Flyable{          @Override          public  void  fly()  {                  System.out.println("Bird  is  flying");          }   }  
14   Java  Interview  Questions  –  www.JavaInterview.in       Can  you  tell  a  little  bit  more  about  interfaces?   Variables   in   an   interface   are   always   public,   static,   final.   Variables   in   an   interface   cannot   be   declared   private.   interface  ExampleInterface1  {          //By  default  -­‐  public  static  final.  No  other  modifier  allowed          //value1,value2,value3,value4  all  are  -­‐  public  static  final          int  value1  =  10;          public  int  value2  =  15;          public  static  int  value3  =  20;          public  static  final  int  value4  =  25;          //private  int  value5  =  10;//COMPILER  ERROR   }   Interface  methods  are  by  default  public  and  abstract.  A  concrete  method  (fully  defined  method)  cannot   be  created  in  an  interface.  Consider  the  example  below:   interface  ExampleInterface1  {          //By  default  -­‐  public  abstract.  No  other  modifier  allowed          void  method1();//method1  is  public  and  abstract          //private  void  method6();//COMPILER  ERROR!                    /*//Interface  cannot  have  body  (definition)  of  a  method              //This  method,  uncommented,  gives  COMPILER  ERROR!          void  method5()  {                  System.out.println("Method5");          }            */   }   Can  you  extend  an  interface?   An interface can extend another interface. Consider the example below:   interface  SubInterface1  extends  ExampleInterface1{          void  method3();   }   Class   implementing   SubInterface1   should   implement   both   methods   -­‐   method3   and   method1(from   ExampleInterface1)   An interface cannot extend a class. /*  //COMPILE  ERROR  IF  UnCommented        //Interface  cannot  extend  a  Class   interface  SubInterface2  extends  Integer{          void  method3();   }   */   Can  a  class  extend  multiple  interfaces?  
Java  Interview  Questions  –  www.JavaInterview.in   1 5     A class can implement multiple interfaces. It should implement all the method declared in all Interfaces being implemented.   interface  ExampleInterface2  {          void  method2();   }     class  SampleImpl  implements  ExampleInterface1,ExampleInterface2{          /*  A  class  should  implement  all  the  methods  in  an  interface.                If  either  of  method1  or  method2  is  commented,  it  would                  result  in  compilation  error.              */          public  void  method2()  {                  System.out.println("Sample  Implementation  for  Method2");          }            public  void  method1()  {                  System.out.println("Sample  Implementation  for  Method1");          }             }     POPULAR  VIDEOS   Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:  https://www.youtube.com/watch?v=njZ48YVkei0   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:  https://www.youtube.com/watch?v=xyXuo0y-xoU   Java  Interview  :  A  Guide  for  Experienced:  https://www.youtube.com/watch?v=0xcgzUdTO5M Collections  Interview  Questions  1:  https://www.youtube.com/watch?v=GnR4hCvEIJQ Collections  Interview  Questions  2:  https://www.youtube.com/watch?v=6dKGpOKAQqs Collections  Interview  Questions  3:  https://www.youtube.com/watch?v=_JTIYhnLemA Collections  Interview  Questions  4:  https://www.youtube.com/watch?v=ZNhT_Z8_q9s Collections  Interview  Questions  5:  https://www.youtube.com/watch?v=W5c8uXi4qTw  
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Java object oriented programming - OOPS

  • 1. Java  Interview  Questions  –  www.JavaInterview.in   1     OOPS   Following  picture  show  the  topics  we  would  cover  in  this  article.     What  is  the  super  class  of  every  class  in  Java?   Every class in java is a sub class of the class Object. When we create a class we inherit all the methods and properties of Object class. Let’s look at a simple example:   String  str  =  "Testing";   System.out.println(str.toString());   System.out.println(str.hashCode());   System.out.println(str.clone());     if(str  instanceof  Object){          System.out.println("I  extend  Object");//Will  be  printed   }   In the above example, toString, hashCode and clone methods for String class are inherited from Object class and overridden. Can  super  class  reference  variable  can  hold  an  object  of  sub  class?   Yes. Look at the example below:   Actor reference variables actor1, actor2 hold the reference of objects of sub classes of Animal, Comedian and Hero. Since object is super class of all classes, an Object reference variable can also hold an instance of any class.
  • 2. 2   Java  Interview  Questions  –  www.JavaInterview.in       //Object  is  super  class  of  all  java  classes   Object  object  =  new  Hero();       public  class  Actor  {          public  void  act(){                  System.out.println("Act");          };   }     //IS-­‐A  relationship.  Hero  is-­‐a  Actor   public  class  Hero  extends  Actor  {          public  void  fight(){                  System.out.println("fight");          };   }     //IS-­‐A  relationship.  Comedian  is-­‐a  Actor   public  class  Comedian  extends  Actor  {          public  void  performComedy(){                  System.out.println("Comedy");          };   }     Actor  actor1  =  new  Comedian();   Actor  actor2  =  new  Hero();   Is  Multiple  Inheritance  allowed  in  Java?   Multiple Inheritance results in a number of complexities. Java does not support Multiple Inheritance.   class  Dog  extends  Animal,  Pet  {  //COMPILER  ERROR   }   However, we can create an Inheritance Chain class  Pet  extends  Animal  {   }     class  Dog  extends  Pet  {   }   What  is  Polymorphism?   Refer  to  this  video(https://www.youtube.com/watch?v=t8PTatUXtpI)  for  a  clear  explanation  of   polymorphism.   Polymorphism  is  defined  as  “Same  Code”  giving  “Different  Behavior”.  Let’s  look  at  an  example.     Let’s  define  an  Animal  class  with  a  method  shout.   public  class  Animal  {          public  String  shout()  {                  return  "Don't  Know!";          }  
  • 3. Java  Interview  Questions  –  www.JavaInterview.in   3     }     Let’s  create  two  new  sub  classes  of  Animal  overriding  the  existing  shout  method  in  Animal.   class  Cat  extends  Animal  {          public  String  shout()  {                  return  "Meow  Meow";          }   }     class  Dog  extends  Animal  {          public  String  shout()  {                  return  "BOW  BOW";          }            public  void  run(){                            }   }     Look  at  the  code  below.  An  instance  of  Animal  class  is  created.  shout  method  is  called.     Animal  animal1  =  new  Animal();                   System.out.println(                  animal1.shout());  //Don't  Know!     Look  at  the  code  below.  An  instance  of  Dog  class  is  created  and  store  in  a  reference  variable  of  type   Animal.   Animal  animal2  =  new  Dog();     //Reference  variable  type  =>  Animal   //Object  referred  to  =>  Dog   //Dog's  bark  method  is  called.   System.out.println(                  animal2.shout());  //BOW  BOW     When  shout  method  is  called  on  animal2,  it  invokes  the  shout  method  in  Dog  class  (type  of  the  object   pointed  to  by  reference  variable  animal2).   Even  though  dog  has  a  method  run,  it  cannot  be  invoked  using  super  class  reference  variable.   //animal2.run();//COMPILE  ERROR   What  is  the  use  of  instanceof  Operator  in  Java?   instanceof operator checks if an object is of a particular type. Let us consider the following class and interface declarations: class  SuperClass  {   };     class  SubClass  extends  SuperClass  {   };  
  • 4. 4   Java  Interview  Questions  –  www.JavaInterview.in       Java  Interview  Questions  –  www.JavaInterview.in   At  http://www.JavaInterview.in,  we  want  you  to  clear  java  interview  with  ease.  So,  in  addition  to   focussing  on  Core  and  Advanced  Java  we  also  focus  on  topics  like  Code  Reviews,  Performance,     Design  Patterns,  Spring  and  Struts.   We  have  created  more  than  20  videos  to  help  you  understand  these  topics  and  become  an  expert  at   them.  Visit  our  website  http://www.JavaInterview.in  for  complete  list  of  videos.    Other  than  the   videos,  we  answer  the  top  200  frequently  asked  interview  questions  on  our  website.   With  more  900K  video  views  (Apr  2015),  we  are  the  most  popular  channel  on  Java  Interview   Questions  on  YouTube.   Register  here  for  more  updates  :     https://feedburner.google.com/fb/a/mailverify?uri=RithusTutorials   POPULAR  VIDEOS   Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:  https://www.youtube.com/watch?v=njZ48YVkei0   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:  https://www.youtube.com/watch?v=xyXuo0y-xoU   Java  Interview  :  A  Guide  for  Experienced:  https://www.youtube.com/watch?v=0xcgzUdTO5M Collections  Interview  Questions  1:  https://www.youtube.com/watch?v=GnR4hCvEIJQ Collections  Interview  Questions  2:  https://www.youtube.com/watch?v=6dKGpOKAQqs Collections  Interview  Questions  3:  https://www.youtube.com/watch?v=_JTIYhnLemA Collections  Interview  Questions  4:  https://www.youtube.com/watch?v=ZNhT_Z8_q9s Collections  Interview  Questions  5:  https://www.youtube.com/watch?v=W5c8uXi4qTw   interface  Interface  {   };     class  SuperClassImplementingInteface  implements  Interface  {   };     class  SubClass2  extends  SuperClassImplementingInteface  {   };     class  SomeOtherClass  {   };     Let’s  consider  the  code  below.    We  create  a  few  instances  of  the  classes  declared  above.  
  • 5. Java  Interview  Questions  –  www.JavaInterview.in   5     SubClass  subClass  =  new  SubClass();   Object  subClassObj  =  new  SubClass();     SubClass2  subClass2  =  new  SubClass2();   SomeOtherClass  someOtherClass  =  new  SomeOtherClass();     Let’s  now  run  instanceof  operator  on  the  different  instances  created  earlier.   System.out.println(subClass  instanceof  SubClass);//true   System.out.println(subClass  instanceof  SuperClass);//true   System.out.println(subClassObj  instanceof  SuperClass);//true     System.out.println(subClass2                    instanceof  SuperClassImplementingInteface);//true     instanceof  can  be  used  with  interfaces  as  well.  Since  Super  Class  implements  the  interface,  below  code   prints  true.   System.out.println(subClass2                    instanceof  Interface);//true     If  the  type  compared  is  unrelated  to  the  object,  a  compilation  error  occurs.   //System.out.println(subClass     //                        instanceof  SomeOtherClass);//Compiler  Error     Object  referred  by  subClassObj(SubClass)-­‐  NOT  of  type  SomeOtherClass   System.out.println(subClassObj  instanceof  SomeOtherClass);//false   What  is  an  Abstract  Class?   An   abstract   class   (Video   Link   -­‐   https://www.youtube.com/watch?v=j3GLUcdlz1w )   is   a   class   that  cannot  be  instantiated,  but  must  be  inherited  from.  An  abstract  class  may  be  fully  implemented,   but  is  more  usually  partially  implemented  or  not  implemented  at  all,  thereby  encapsulating  common   functionality  for  inherited  classes. In   code   below   ”AbstractClassExample   ex   =   new   AbstractClassExample();”   gives   a   compilation   error   because  AbstractClassExample  is  declared  with  keyword  abstract.     public  abstract  class  AbstractClassExample  {          public  static  void  main(String[]  args)  {                  //An  abstract  class  cannot  be  instantiated                  //Below  line  gives  compilation  error  if  uncommented                  //AbstractClassExample  ex  =  new  AbstractClassExample();          }   }   How  do  you  define  an  abstract  method?   An Abstract method does not contain body. An abstract method does not have any implementation. The implementation of an abstract method should be provided in an over-riding method in a sub class.        //Abstract  Class  can  contain  0  or  more  abstract  methods          //Abstract  method  does  not  have  a  body  
  • 6. 6   Java  Interview  Questions  –  www.JavaInterview.in              abstract  void  abstractMethod1();          abstract  void  abstractMethod2();     Abstract  method  can  be  declared  only  in  Abstract  Class.  In  the  example  below,  abstractMethod()  gives  a   compiler  error  because  NormalClass  is  not  abstract.   class  NormalClass{          abstract  void  abstractMethod();//COMPILER  ERROR   }   What  is  Coupling?   Coupling is a measure of how much a class is dependent on other classes. There should minimal dependencies between classes. So, we should always aim for low coupling between classes. Coupling  Example  Problem   Consider  the  example  below:   class  ShoppingCartEntry  {          public  float  price;          public  int  quantity;   }     class  ShoppingCart  {          public  ShoppingCartEntry[]  items;   }     class  Order  {          private  ShoppingCart  cart;          private  float  salesTax;            public  Order(ShoppingCart  cart,  float  salesTax)  {                  this.cart  =  cart;                  this.salesTax  =  salesTax;          }            //  This  method  know  the  internal  details  of  ShoppingCartEntry  and          //  ShoppingCart  classes.  If  there  is  any  change  in  any  of  those          //  classes,  this  method  also  needs  to  change.          public  float  orderTotalPrice()  {                  float  cartTotalPrice  =  0;                  for  (int  i  =  0;  i  <  cart.items.length;  i++)  {                          cartTotalPrice  +=  cart.items[i].price                                          *  cart.items[i].quantity;                  }                  cartTotalPrice  +=  cartTotalPrice  *  salesTax;                  return  cartTotalPrice;          }   }   Method   orderTotalPrice   in   Order   class   is   coupled   heavily   with   ShoppingCartEntry   and   ShoppingCart  classes.    It  uses  different  properties  (items,  price,  quantity)  from  these  classes.  If  any  of   these  properties  change,  orderTotalPrice  will  also  change.  This  is  not  good  for  Maintenance.    
  • 7. Java  Interview  Questions  –  www.JavaInterview.in   7     Solution   Consider a better implementation with lesser coupling between classes below: In this implementation, changes in ShoppingCartEntry or CartContents might not affect Order class at all. class  ShoppingCartEntry   {          float  price;          int  quantity;            public  float  getTotalPrice()          {                  return  price  *  quantity;          }   }     class  CartContents   {          ShoppingCartEntry[]  items;            public  float  getTotalPrice()          {                  float  totalPrice  =  0;                  for  (ShoppingCartEntry  item:items)                  {                          totalPrice  +=  item.getTotalPrice();                  }                  return  totalPrice;          }   }     class  Order   {          private  CartContents  cart;          private  float  salesTax;            public  Order(CartContents  cart,  float  salesTax)          {                  this.cart  =  cart;                  this.salesTax  =  salesTax;          }            public  float  totalPrice()          {                  return  cart.getTotalPrice()  *  (1.0f  +  salesTax);          }   }   What  is  Cohesion?   Cohesion   (Video   Link   -­‐   https://www.youtube.com/watch?v=BkcQWoF5124 )   is   a   measure   of   how  related  the  responsibilities  of  a  class  are.    A  class  must  be  highly  cohesive  i.e.  its  responsibilities   (methods)  should  be  highly  related  to  one  another.  
  • 8. 8   Java  Interview  Questions  –  www.JavaInterview.in       Example  Problem   Example   class   below   is   downloading   from   internet,   parsing   data   and   storing   data   to   database.   The   responsibilities  of  this  class  are  not  really  related.  This  is  not  cohesive  class.   class  DownloadAndStore{          void  downloadFromInternet(){          }                    void  parseData(){          }                    void  storeIntoDatabase(){          }                    void  doEverything(){                  downloadFromInternet();                  parseData();                  storeIntoDatabase();          }   }   Solution This is a better way of approaching the problem. Different classes have their own responsibilities. class  InternetDownloader  {          public  void  downloadFromInternet()  {          }   }     class  DataParser  {          public  void  parseData()  {          }   }     class  DatabaseStorer  {          public  void  storeIntoDatabase()  {          }   }     class  DownloadAndStore  {          void  doEverything()  {                  new  InternetDownloader().downloadFromInternet();                  new  DataParser().parseData();                  new  DatabaseStorer().storeIntoDatabase();          }   }   What  is  Encapsulation?   Encapsulation is “hiding the implementation of a Class behind a well defined interface”. Encapsulation helps us to change implementation of a class without breaking other code.
  • 9. Java  Interview  Questions  –  www.JavaInterview.in   9     Approach  1   In  this  approach  we  create  a  public  variable  score.  The  main  method  directly  accesses  the  score  variable,   updates  it.   public  class  CricketScorer  {          public  int  score;   }     Let’s  use  the  CricketScorer  class.   public  static  void  main(String[]  args)  {   CricketScorer  scorer  =  new  CricketScorer();   scorer.score  =  scorer.score  +  4;   }   Approach  2   In  this  approach,  we  make  score  as  private  and  access  value  through  get  and  set  methods.  However,  the   logic  of  adding  4  to  the  score  is  performed  in  the  main  method.   public  class  CricketScorer  {          private  int  score;            public  int  getScore()  {                  return  score;          }            public  void  setScore(int  score)  {                  this.score  =  score;          }   }     Let’s  use  the  CricketScorer  class.     public  static  void  main(String[]  args)  {   CricketScorer  scorer  =  new  CricketScorer();     int  score  =  scorer.getScore();   scorer.setScore(score  +  4);   }   Approach  3   In  this  approach  -­‐  For  better  encapsulation,  the  logic  of  doing  the  four  operation  also  is  moved  to  the   CricketScorer  class.   public  class  CricketScorer  {          private  int  score;                    public  void  four()  {                  score  +=  4;          }     }     Let’s  use  the  CricketScorer  class.  
  • 10. 10   Java  Interview  Questions  –  www.JavaInterview.in       public  static  void  main(String[]  args)  {   CricketScorer  scorer  =  new  CricketScorer();   scorer.four();   }   Description   In  terms  of  encapsulation  Approach  3  >  Approach  2  >  Approach  1.  In  Approach  3,  the  user  of  scorer  class   does  not  even  know  that  there  is  a  variable  called  score.  Implementation  of  Scorer  can  change  without   changing  other  classes  using  Scorer.   What  is  Method  Overloading?   A method having the same name as another method (in same class or a sub class) but having different parameters is called an Overloaded Method. Example  1   doIt  method  is  overloaded  in  the  below  example:   class  Foo{          public  void  doIt(int  number){                            }          public  void  doIt(String  string){                            }   }   Example  2   Overloading  can  also  be  done  from  a  sub  class.   class  Bar  extends  Foo{          public  void  doIt(float  number){                            }   }   What  is  Method  Overriding?   Creating a Sub Class Method with same signature as that of a method in SuperClass is called Method Overriding. Method  Overriding  Example  1:   Let’s  define  an  Animal  class  with  a  method  shout.   public  class  Animal  {          public  String  bark()  {                  return  "Don't  Know!";          }   }     Let’s  create  a  sub  class  of  Animal  –  Cat    -­‐  overriding  the  existing  shout  method  in  Animal.   class  Cat  extends  Animal  {          public  String  bark()  {  
  • 11. Java  Interview  Questions  –  www.JavaInterview.in   1 1                    return  "Meow  Meow";          }   }   bark method in Cat class is overriding the bark method in Animal class. What  is  an  Inner  Class?   Inner  Classes  are  classes  which  are  declared  inside  other  classes.  Consider  the  following  example:   class  OuterClass  {            public  class  InnerClass  {          }            public  static  class  StaticNestedClass  {          }     }   What  is  a  Static  Inner  Class?   A  class  declared  directly  inside  another  class  and  declared  as  static.  In  the  example  above,  class  name   StaticNestedClass  is  a  static  inner  class.     Can  you  create  an  inner  class  inside  a  method?   Yes.   An   inner   class   can   be   declared   directly   inside   a   method.   In   the   example   below,   class   name   MethodLocalInnerClass  is  a  method  inner  class.     class  OuterClass  {            public  void  exampleMethod()  {                  class  MethodLocalInnerClass  {                  };          }     }   Constructors   Constructor (Youtube Video link - https://www.youtube.com/watch?v=XrdxGT2s9tc ) is invoked whenever we create an instance(object) of a Class. We cannot create an object without a constructor. If we do not provide a constructor, compiler provides a default no-argument constructor. What  is  a  Default  Constructor?   Default Constructor is the constructor that is provided by the compiler. It has no arguments. In the example below, there are no Constructors defined in the Animal class. Compiler provides us with a default constructor, which helps us create an instance of animal class.   public  class  Animal  {          String  name;            public  static  void  main(String[]  args)  {  
  • 12. 12   Java  Interview  Questions  –  www.JavaInterview.in                      //  Compiler  provides  this  class  with  a  default  no-­‐argument  constructor.                  //  This  allows  us  to  create  an  instance  of  Animal  class.                  Animal  animal  =  new  Animal();          }   }   How  do  you  call  a  Super  Class  Constructor  from  a  Constructor? A constructor can call the constructor of a super class using the super() method call. Only constraint is that it should be the first statement i Both example constructors below can replaces the no argument "public Animal() " constructor in Example 3. public  Animal()  {          super();          this.name  =  "Default  Name";   }   Can  a  constructor  be  called  directly  from  a  method?     A constructor cannot be explicitly called from any method except another constructor. class  Animal  {          String  name;            public  Animal()  {          }            public  method()  {                  Animal();//  Compiler  error          }   }   Is  a  super  class  constructor  called  even  when  there  is  no  explicit  call  from  a   sub  class  constructor? If a super class constructor is not explicitly called from a sub class constructor, super class (no argument) constructor is automatically invoked (as first line) from a sub class constructor. Consider the example below: class  Animal  {          public  Animal()  {                  System.out.println("Animal  Constructor");          }   }     class  Dog  extends  Animal  {          public  Dog()  {                  System.out.println("Dog  Constructor");          }   }     class  Labrador  extends  Dog  {          public  Labrador()  {                  System.out.println("Labrador  Constructor");          }  
  • 13. Java  Interview  Questions  –  www.JavaInterview.in   1 3     }     public  class  ConstructorExamples  {          public  static  void  main(String[]  args)  {                  Labrador  labrador  =  new  Labrador();          }   }   Program  Output   Animal Constructor Dog Constructor Labrador Constructor Interface   What  is  an  Interface?   An interface (YouTube video link - https://www.youtube.com/watch?v=VangB-sVNgg ) defines a contract for responsibilities (methods) of a class. How  do  you  define  an  Interface?   An   interface   is   declared   by   using   the   keyword   interface.   Look   at   the   example   below:   Flyable   is   an   interface.   //public  abstract  are  not  necessary   public  abstract  interface  Flyable  {          //public  abstract  are  not  necessary          public  abstract  void  fly();   }   How  do  you  implement  an  interface? We can define a class implementing the interface by using the implements keyword. Let us look at a couple of examples: Example  1   Class  Aeroplane  implements  Flyable  and  implements  the  abstract  method  fly().   public  class  Aeroplane  implements  Flyable{          @Override          public  void  fly()  {                  System.out.println("Aeroplane  is  flying");          }   }   Example  2   public  class  Bird  implements  Flyable{          @Override          public  void  fly()  {                  System.out.println("Bird  is  flying");          }   }  
  • 14. 14   Java  Interview  Questions  –  www.JavaInterview.in       Can  you  tell  a  little  bit  more  about  interfaces?   Variables   in   an   interface   are   always   public,   static,   final.   Variables   in   an   interface   cannot   be   declared   private.   interface  ExampleInterface1  {          //By  default  -­‐  public  static  final.  No  other  modifier  allowed          //value1,value2,value3,value4  all  are  -­‐  public  static  final          int  value1  =  10;          public  int  value2  =  15;          public  static  int  value3  =  20;          public  static  final  int  value4  =  25;          //private  int  value5  =  10;//COMPILER  ERROR   }   Interface  methods  are  by  default  public  and  abstract.  A  concrete  method  (fully  defined  method)  cannot   be  created  in  an  interface.  Consider  the  example  below:   interface  ExampleInterface1  {          //By  default  -­‐  public  abstract.  No  other  modifier  allowed          void  method1();//method1  is  public  and  abstract          //private  void  method6();//COMPILER  ERROR!                    /*//Interface  cannot  have  body  (definition)  of  a  method              //This  method,  uncommented,  gives  COMPILER  ERROR!          void  method5()  {                  System.out.println("Method5");          }            */   }   Can  you  extend  an  interface?   An interface can extend another interface. Consider the example below:   interface  SubInterface1  extends  ExampleInterface1{          void  method3();   }   Class   implementing   SubInterface1   should   implement   both   methods   -­‐   method3   and   method1(from   ExampleInterface1)   An interface cannot extend a class. /*  //COMPILE  ERROR  IF  UnCommented        //Interface  cannot  extend  a  Class   interface  SubInterface2  extends  Integer{          void  method3();   }   */   Can  a  class  extend  multiple  interfaces?  
  • 15. Java  Interview  Questions  –  www.JavaInterview.in   1 5     A class can implement multiple interfaces. It should implement all the method declared in all Interfaces being implemented.   interface  ExampleInterface2  {          void  method2();   }     class  SampleImpl  implements  ExampleInterface1,ExampleInterface2{          /*  A  class  should  implement  all  the  methods  in  an  interface.                If  either  of  method1  or  method2  is  commented,  it  would                  result  in  compilation  error.              */          public  void  method2()  {                  System.out.println("Sample  Implementation  for  Method2");          }            public  void  method1()  {                  System.out.println("Sample  Implementation  for  Method1");          }             }     POPULAR  VIDEOS   Java  Interview  :  A  Freshers  Guide  -­‐  Part  1:  https://www.youtube.com/watch?v=njZ48YVkei0   Java  Interview  :  A  Freshers  Guide  -­‐  Part  2:  https://www.youtube.com/watch?v=xyXuo0y-xoU   Java  Interview  :  A  Guide  for  Experienced:  https://www.youtube.com/watch?v=0xcgzUdTO5M Collections  Interview  Questions  1:  https://www.youtube.com/watch?v=GnR4hCvEIJQ Collections  Interview  Questions  2:  https://www.youtube.com/watch?v=6dKGpOKAQqs Collections  Interview  Questions  3:  https://www.youtube.com/watch?v=_JTIYhnLemA Collections  Interview  Questions  4:  https://www.youtube.com/watch?v=ZNhT_Z8_q9s Collections  Interview  Questions  5:  https://www.youtube.com/watch?v=W5c8uXi4qTw