Exercise - Inheritance 1

1.     Here is a program that uses a class Video to represent videos available at a rental store. Try to trace out what will this program print. 


class Video

{

  private String  title;    // name of the item

  private int     length;   // number of minutes

  private boolean avail;    // is the video in the store?

 

  // constructor

  public Video( String ttl )

  {

    title = ttl; length = 90; avail = true;

  }

 

  // constructor

  public Video( String ttl, int lngth )

  {

    title = ttl; length = lngth; avail = true;

  }

 

  public void show()

  {

   System.out.println(title + ", " + length + " min. available:" + avail );

  }

}

 

public class VideoStore

{

  public static void main ( String args[] )

  {

    Video item1 = new Video("Jaws", 120 );

    Video item2 = new Video("Star Wars" );

 

    item1.show();

    item2.show();

  }

}

2. Output: 

Jaws, 120 min. available:true

Star Wars, 90 min. available:true


3.     Let’s make a  Movie class that is similar to Video, but also includes the name of the director and a rating. The class is given as follows:


class Movie extends Video

{

  private String  director;     // name of the director

  private String  rating;       // U, SG, SX

  // constructor

  public Movie( String ttl, int lngth, String dir, String rtng )

  {

    super( ttl, lngth );            //use the super class's constructor

    director = dir;  rating = rtng;

  }

} 

4.     The class Movie is a subclass of Video. List all of the members of a Movie object and for each member state whether it is inherited from Video or defined in Movie. 

    title - defined in Video

    length - defined in Video

    avail - defined in Video

    director - defined in Movie

    rating - defined in Movie


5.     Here is an example program that makes use of the two classes:

public class VideoStore

{

  public static void main ( String args[] )

  {

    Video item1 = new Video("Microcosmos", 90 );

    Movie item2 = new Movie("Jaws", 120, "Spielberg", "PG" );

    item1.show();

    item2.show();

  }

}

What does this print?  Can you explain the output?

Output: 

Microcosmos, 90 min. available:true

Jaws, 120 min. available:true

item 1 is defined and printed as a video with title, length and avail while item 2 is also defined and printed as movie with title, length and avail.


6.     To print out all data members of a Movie object, we need to override the show() method that is inherited from Video.  Change the definition of the Movie class to override the parent’s show() method:

@Override

   public void show()

  {

   super.show();   

   System.out.println(director + ", " + rating );

  }


7.     Now compile and run again the VideoStore program given in step 5.  This time what does the program print?

Microcosmos, 90 min. available:true

Jaws, 120 min. available:true

Spielberg, PG


8.      Overload the Movie constructor by adding the following constructor definition in the Movie class and recompile the class (Make sure there is no error).


public Movie( String ttl, String dir, String rtng )

{

  super( ttl );    // invoke the matching parent class constructor 

  director = dir;  rating = rtng;     // initialize members unique to Movie

}


Does a child constructor always invoke a parent constructor? 

Try removing the statement:

 

      super( ttl );     

 

in the above constructor. Now compile the class and you’ll get a compilation error. Explain why? Then make the correction for the error.


Answer: not necessarily. it only invokes when the super method is used. The error happens because the parent class constructor was not invoked. 

Comments

Post a Comment

Popular posts from this blog

Exercise 3 - PredefinedClass

1.            Write TWO (2) different ways to create an object from the String class.                - define literally (String str = “Hello!”; )    - using new operator     2.            What is the output for the following Java statements? String phr = new String("Big,blue sky"); System.out.println(phr.substring(phr.indjexOf(","),7)); Ans: ,blu 3.         Give your description on the output that will be produced by the execution of the following statements. Random rd = new Random(); System.out.println(rd.nextInt(41) + 10); Ans: The output will be a random number from 0 to 40 which is added with 10. 4.      Write Java code segment that use class Random to generate two random numbers that is great...
Read more

Exercise - User definedClass-2

Image
Task 1 // The Triangle class stores and manipulates data for a //triangle.   public class Triangle {      private double height;      private double base;      // The setHeight method accepts an argument which is         //stored in the height field.      public void setHeight(double len)      {           height = len;      }      // The setBase method accepts an argument which is      //stored in the base field.          public void setBase(double b)      {           base = b;      }      //The set method accepts two arguments which are      //stored in the h...
Read more

Exercise - User definedClass-3

Image
TASK 1   Based on the following  UML class diagram :   a)               Write a complete Student class. b)               Next, write  TestStudent  class that invokes the methods of the  Student  class  and applies the concept of an array of objects. c)                Sample run as given below: User’s inputs    Matric No :  s1111   Test 1 :  67   Test 2 :  45   Matric No :  s2222   Test 1 :  67   Test 2 :  98   Matric No :  s3333   Test 1 :  67   Test 2 :  90   Program outputs   *****************************        STUDENT INFORMATION ***************************** Matric No   AverageMark   s1111   ...
Read more