java.lang.nullpointer exception in beginner code

**Bobba** · May 3rd, 2012, 03:08 AM

Hi there writing a small program for borrowing and lending books, the code compiles but alot of the time when i run this code i get "java.lang.NullPointerException" and the program stops working.

in this particular instance its relating to these lines of codes:

public void displayBooks(){
for (int i=0;i<books.length;i++){
System.out.println("code: " + books[i].getCode());
System.out.println("Title: " + books[i].getTitle());
if (books[i].getStatus() == null){
System.out.println("Status: Available");
}else{
System.out.println("Return date: " + books[i].getReturnDate());
}
System.out.println("************************");
}
}

i get the following :

java.lang.NullPointerException
at Library.displayBooks(Library.java:76)
at AssignmentTwo.start(AssignmentTwo.java:28)
at Application.main(Application.java:11)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Nativ e Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Native MethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(De legatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.ru nCommand(JavacCompiler.java:271)

In this same code I cannot also figure out how to make i so it displays the main menu again after executing this code.

Any help would be much appreciated.

Thanks.

**andreas90** · May 3rd, 2012, 03:24 AM

Hello Bobba!
Which line is Library.displayBooks(Library.java:76)? You should print the variables in that line to find out which is null.
Until you provide us with further information, I am only guessing that you have not initialized the books[] array.

**aesguitar** · May 5th, 2012, 02:28 PM

try using a try/catch statement here:

 
if (books[i].getStatus() == null){
System.out.println("Status: Available");
}else{
System.out.println("Return date: " + books[i].getReturnDate());
}

catch a NullPointerException for it.

**deveshinjiit@gmail.com** · May 6th, 2012, 11:59 AM

hii,
please insert try/catch block and and insert
catch(Exception e)
{System.out.println( e);}
then the code will work fine..
Reagrds,
Devesh

**copeg** · May 6th, 2012, 12:30 PM

Originally Posted by aesguitar

catch a NullPointerException

Originally Posted by deveshinjiit@gmail.com

catch(Exception e)
{System.out.println( e);}
then the code will work fine..

Sorry, but this is bad advice. Catching it is like treating the symptoms of a disease when you can treat the disease itself - and will almost always result in a bug in the code. Figure out why it is being thrown and treat the disease, as andreas90 alluded to in post #2.

**Bobba** · May 6th, 2012, 04:30 PM

hello sorry i havent been able to get back to this, been working.

public void displayBooks(){
    for (int i=0;i<books.length;i++){
      System.out.println("Code: " + books[i].getCode());    <---------------------------- 99 is this line here
      System.out.println("Title: " + books[i].getTitle());
      if (books[i].getStatus() == null){
        System.out.println("Status: Available");
      }else{  
        System.out.println("Return date: " + books[i].getReturnDate());  
      }
        System.out.println("************************");
    }
  }
 
java.lang.NullPointerException
	at Library.displayBooks(Library.java:99)
	at AssignmentTwo.start(AssignmentTwo.java:28)
	at Application.main(Application.java:11)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:271)

its a different line because I have been doing more work on it.

It also happens here:

 public void searchByTitle(){
    System.out.println("Please enter the book title that you would like to search: ");
    String userSelection = Keyboard.readInput();
    BookItem book = findTitle(userSelection);
    for(int i=0;i<books.length;i++){
      if (books[i].getTitle() == book.getTitle()){   <--------------------------------------------------- line 78
        System.out.println("Record found in the collection: ");
        System.out.println("Code: " + books[i].getCode());  
        System.out.println("Title: " + books[i].getTitle());
        if (books[i].getStatus() == null){
          System.out.println("Status: Available");
        }else{
          System.out.println("Status: " + books[i].getStatus());
        }if (books[i].getReturnDate() == null){                        
          System.out.println("Return Date: N/A");
        }else{
          System.out.println("Return Date: " + books[i].getReturnDate());  
        }
      }else if (book == null){
        System.out.println("Sorry, this book is not in the collection.");
        i = 1001;
      }
    }
  }
 
java.lang.NullPointerException
	at Library.searchByTitle(Library.java:78)
	at AssignmentTwo.lending(AssignmentTwo.java:52)
	at AssignmentTwo.start(AssignmentTwo.java:25)
	at Application.main(Application.java:11)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:271)

thanks again help is always appreciated

>

**andreas90** · May 6th, 2012, 04:36 PM

Originally Posted by Bobba

hello sorry i havent been able to get back to this, been working.
i have edited the array slightly but still the same error, it works correctly but still shows the following issue,

Are you getting an "java.lang.NullPointerException"? Did you try what I suggested in post#2?
Where do you define and initialize your books array?

**andreas90** · May 6th, 2012, 05:37 PM

As I told in my other two posts, you have to initialize an array before you use it. Assuming that you haven't initialized your array (if you do initialize it somewhere else please post here the code), the exception is thrown because you are trying to access an index of the array (books[i]) that has an invalid value (null).
And please wrap your code with code tags.

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