Enthuware Toughest questions on Collections

[Mohit G Gupta]

Toughest Questions for Oracle Java Programmer Certification
Enthunware

Q. No. 11

 
import java.util.*;

class Book {

    private String title, isbn;

    public boolean equals(Object o){

	return (o instanceof Book && ((Book)o).isbn.equals(this.isbn));

    }



    // ... setters and getters for title and isbn

}



class BookStore {

    Map<Book, Integer> map = new HashMap<Book, Integer>();    

    int getNumberOfCopies(Book b){

	return map.get(b);

    }

    public void addBook(Book b, int numberofcopies){

	map.put(b, numberofcopies);

    }

    // ... other useful methods.

}



public class TestClass {	

  static BookStore bs = new BookStore(); 

  

  public static void main(String[] args)   {

    Book b = new Book(); b.setIsbn("111");

    bs.addBook(b, 10);

    System.out.println(bs.getNumberOfCopies(b));

    

    b = new Book(); b.setIsbn("111");

    System.out.println(bs.getNumberOfCopies(b));

  }

}
 

What will it print when it is compiled and run?



(a) It will compile with a warning and run without throwing an exception.
(b) It will compile without a warning and will throw an exception when run.
(c) It will compile and run without warning and exception.
(d) It will print 10 and -1.
(e) It will print 10 and 0.
(f) It will print 10.


answer are b,f

shouldn't be answer be 10 ,null
as get returns null if there is no value corresponding to given key

please help

[Abimaran Kugathasan]

Don't you execute this code? What is the answer?

[Mohit G Gupta]

i have removed
b.setIsbn("111");

from the code and then ran the program

.The output is same as given in the answer 10 followed by a Exception

help

[Sumit Khurana]

HashMap add the key,value pair by using hashcode and equals method but book class does not implement the hash code and equals. so,every new object is different from the previous one and you can't search through the hashmap unless you have the reference of object when you are adding in the hashMap.


Exception occurs because when you execute the method
bs.getNumberOfCopies(b);

this method starts finding the b in HashMap and it does not find any b object because each object is different(because we have not implemented the hashcode). and hashmap returns null.

int getNumberOfCopies(Book b){

return map.get(b); //now here exception occurs because jvm can't convert null to zero..

}

[Sumit Khurana]

Sumit Khurana wrote:
int getNumberOfCopies(Book b){

return map.get(b); //now here exception occurs because jvm can't unbox null to zero..

}

[Arjun Srivastava]

hi sumit,see this question

[Sumit Khurana]

in this question...what is the problem??