Reputation: 29
I'm making a library program that keeps a record of any books that are in the collection and how many copies of each book there are. Here is the code
import java.util.Arrays;
import java.util.Scanner;
public class Library{
static String title;
static String author;
static int id;
static int copies;
static String date;
static Book[] database = new Book[100];
static int count=0;
public static void main(String[] args){
int i;
Scanner s = new Scanner(System.in);
do{
addBook();
System.out.println("would you like to add another book?");
i=s.nextInt();
}while(i == 0);
database[0].viewDetails();
database[1].viewDetails();
checkingOut();
}
public static void addBook(){
Scanner s = new Scanner(System.in);
System.out.println("Enter the title of the book you want to add to the collection");
title=s.nextLine();
System.out.println("Enter the author of the book you want to add to the collection");
author=s.nextLine();
System.out.println("Enter the publishing date of the book you want to add to the collection");
date=s.nextLine();
System.out.println("Enter the ID number of the book you want to add to the collection");
id=s.nextInt();
System.out.println("Enter the the number of copies that will be added into the collection");
copies=s.nextInt();
Book Book1 = new Book(date, author, copies, id, title);
database[count] = Book1;
count++;
}
public static void checkingOut(){
boolean found=false;
int idSearch;
int i=0;
Scanner s = new Scanner(System.in);
System.out.println("Enter the ID number of the book you want to check out");
idSearch=s.nextInt();
while(i<database.length && found!=true){
if(database[i].getIdentificationNumber() == idSearch){
found = true;
}
i++;
}
if(found==true){
database[i].checkOut();
System.out.println("There are "+database[i].getNumberCopies()+" copies left");
}
else{System.out.println("There is no book with that ID number!");}
}
}
I'm getting a null pointer exception on line 55 of my checking out method, I can't figure out why. Please let me know if you can spot what it is any help would be greatly appreciated.
Upvotes: 0
Views: 142
Reputation: 14278
should be if(found==true)
instead of =
use ==
as =
will always evaluate as true
.
Upvotes: 0
Reputation: 95968
if(found=true)
will be always executed because the expression of the assignment returns the assigned value, and this will cause database[i].checkOut();
to be executed where it shouldn't.
You should write:
if(found)
That's why we avoid writing ==
when we compare boolean
s. It's enough to write if(someBoolean)
.
Upvotes: 1