Re: question about assigning null to a reference object
- From: trippy <silverbells@xxxxxxxxxxxxxx>
- Date: Thu, 1 Feb 2007 00:40:37 -0600
In article <yEbwh.658$R71.10423@xxxxxxxxxxxxxxxxxxxxxxx>, Farcus
Pottysquirt took the hamburger meat, threw it on the grill, and I said
"Oh Wow"...
I have a class called "Book"
package chapter03;
public class Book {
String title = "";
String author = "";
String subject = "";
int numberOfPages = 1;
String getTitle()
{
return this.title;
}
String getAuthor()
{
return this.author;
}
String getSubject()
{
return this.subject;
}
int getPages()
{
return this.numberOfPages;
}
void setTitle(String t)
{
this.title = t;
}
void setAuthor(String a)
{
this.author = a;
}
void setSubject(String s)
{
this.subject = s;
}
void setPages(int p)
{
this.numberOfPages = p;
}
void showAll()
{
String t = getTitle();
String a = getAuthor();
String s = getSubject();
int p = getPages();
// added the if clause to see if I can prevent a null pointer exception
if ( t != null && a != null && s != null && p != 0) {
System.out.print("Book Title: " + t);
System.out.print(" Author: " + a);
System.out.print(" Subject: " + s);
System.out.println(" Pages: " + p);
}
}
}
My main program
package chapter03;
public class BookTestDrive {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Book b = new Book();
Book c = new Book();
b.setAuthor("Buce Eckle");
b.setPages(1123);
b.setSubject("Java");
b.setTitle("Thinking in Java 4th Edition");
b.showAll();
c.setAuthor("Kathy Sierra & Bert Bates");
c.setPages(688);
c.setSubject("Java");
c.setTitle("Head First Java");
c.showAll();
Book d = c;
d.showAll();
c = b;
c.showAll();
b.setAuthor("Bruce Eckle");
b.setPages(11212);
b.setSubject("Javax");
b.setTitle("Thinking in Java 54th Edition");
b.showAll();
c.setAuthor("Kathy Bates & Bert Baccarach");
c.setPages(688);
c.setSubject("Javax");
c.setTitle("Ass Ended Java");
c.showAll();
b = c;
b.showAll();
c = null;
c.showAll();
}
}
When I run the BookTestDrive, my results are what I expect, up until I
call the showAll method on object reference c after I assign it to null.
Book Title: Thinking in Java 4th Edition Author: Buce Eckle Subject:
Java Pages: 1123
Book Title: Head First Java Author: Kathy Sierra & Bert Bates Subject:
Java Pages: 688
Book Title: Head First Java Author: Kathy Sierra & Bert Bates Subject:
Java Pages: 688
Book Title: Thinking in Java 4th Edition Author: Buce Eckle Subject:
Java Pages: 1123
Book Title: Thinking in Java 54th Edition Author: Bruce Eckle Subject:
Javax Pages: 11212
Book Title: Ass Ended Java Author: Kathy Bates & Bert Baccarach Subject:
Javax Pages: 688
Book Title: Ass Ended Java Author: Kathy Bates & Bert Baccarach Subject:
Javax Pages: 688
Exception in thread "main" java.lang.NullPointerException
at chapter03.BookTestDrive.main(BookTestDrive.java:44)
In order to prevent this exception, I attempted to add the if condition
to the showAll method. However this has not worked. If I assign null
to the c reference object, what are the values of the instance variables
after the reference object is assigned to null? And how can I trap
the NullPointerException. I tried using a try..catch on it
package chapter03;
public class Book {
String title = "";
String author = "";
String subject = "";
int numberOfPages = 1;
//snip stuff already shown above
void showAll()
{
String t = getTitle();
String a = getAuthor();
String s = getSubject();
int p = getPages();
try
{
System.out.print("Book Title: " + t);
System.out.print(" Author: " + a);
System.out.print(" Subject: " + s);
System.out.println(" Pages: " + p);
}
catch (NullPointerException e) {}
}
}
This would indicate to me that the NullPointerException cannot be
caught?
No, it's doing exactly what you're telling it to do. When you're setting
c to null, you're saying there is no object. Then you're calling the
methods in your non-existant object. NullPointerException means "I need
something that isn't there". Set c to something, say, a new object, or
set it back to b, or whatever. As long as it's a book object, you're
good to go.
--
trippy
mhm31x9 Smeeter#29 WSD#30
sTaRShInE_mOOnBeAm aT HoTmAil dOt CoM
NP: "The American Way" -- Sacred Reich
"Now, technology's getting better all the time and that's fine,
but most of the time all you need is a stick of gum, a pocketknife,
and a smile."
-- Robert Redford "Spy Game"
.
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- From: Farcus Pottysquirt
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