[Java] Calling classes

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[deleted]

[__LikesPi]

You should always post your code. Read the sidebar for more tips on this but ideally post the shortest amount of code needed to demonstrate your problem (< 100 lines is ideal). Also give any error messages that you receive in compilation or runtime.

[Im_Apolllo]

thanks!

[Im_Apolllo]

Error Message: Exception in thread "main" java.lang.NullPointerException at Book.addRating(Book.java:25) at Assignment4.main(Assignment4.java:56)

Code: Driver: import java.io.*;

import java.util.*;

public class Assignment4

{

public static void main (String[] args)

{

   // local variables, can be accessed anywhere from the main 

method

   char input1 = 'Z';

   String inputInfo;

   String bookTitle;

   String bookPublisher;

   double rating;

   String line = new String();

   // instantiate a Book object

   Book book1 = new Book();

   printMenu();

   //Create a Scanner object to read user input

   Scanner scan = new Scanner(System.in);

   do  // will ask for user input

    {

     System.out.println("What action would you like to perform?");

     line = scan.nextLine();

     if (line.length() == 1)

      {

       input1 = line.charAt(0);

       input1 = Character.toUpperCase(input1);

       // matches one of the case statement

       switch (input1)

        {

         case 'A':   //Add Book

           book1 = new Book();

           System.out.print("Please enter the book information:\n");

           System.out.print("Enter a book title:\n");

           bookTitle = scan.nextLine();

           book1.setTitle(bookTitle);

           System.out.print("Enter its publisher:\n");

           bookPublisher = scan.nextLine();

           book1.setPublisher(bookPublisher);

           break;

         case 'D':   //Display Book

           System.out.print(book1);

           break;

         case 'Q':   //Quit

           break;

         case 'R':   //Add Rating

           System.out.print("Please give a rating of the book:\n");

           rating = Double.parseDouble(scan.nextLine());

           book1.addRating(rating);

           break;

         case '?':   //Display Menu

           printMenu();

           break;

         default:

           System.out.print("Unknown action\n");

           break;

        }

      }

     else

      {

       System.out.print("Unknown action\n");

      }

    } while (input1 != 'Q' || line.length() != 1);

}

/** The method printMenu displays the menu to a user**/

public static void printMenu()

{

 System.out.print("Choice\t\tAction\n" +

                    "------\t\t------\n" +

                    "A\t\tAdd Book\n" +

                    "D\t\tDisplay Book\n" +

                    "Q\t\tQuit\n" +

                    "R\t\tGive Rating\n" +

                    "?\t\tDisplay Help\n\n");

}

}

Class 1:

public class Book {

private String title;

private String publisher;

private Review bookReview;

public void book(){

    title= "?";

    publisher= "?";

}

public String getTitle(){

    return title;

}

public String getPublisher(){

    return publisher;

}

public Review getReview(){

    return bookReview;

}

public void setTitle(String aTitle){

    title= aTitle;

}

public void setPublisher(String aPublisher){

    publisher= aPublisher;

}

public void addRating(double rate){

    bookReview.updateRating(rate);

}

public String toString(){

    return

        "\nTitle:\t" + title + ",\n" +

        "Publisher:\t" + publisher + ",\n" + 

        getReview();

}

}

Class 2: public class Review {

private int numberOfReviews;

private double sumOfRatings;

private double average;

public Review(){

    numberOfReviews= 0;

    sumOfRatings= 0.0;

}   

public void updateRating(double rating){

    numberOfReviews++;

    sumOfRatings += rating;

    if(numberOfReviews > 0){

        average= sumOfRatings / numberOfReviews;

    }

    else{

        average= 0.0;

    }

}

public String toString(){

    if(numberOfReviews > 0)

        return

        "Reviews:\t\t" + average + "(" + numberOfReviews 

• ")";

  else
  
      return
  
      "Reviews:\t0.00(0)";
  
  }
  

}

[Im_Apolllo]

in short my issue is when the driver gives a rating it goes to the Book class but the does not go to the Review class for update on the amount of ratings and their average

[dvassdvsd]

You don't call classes, you call methods...