Two collections and a for loop. (Urgent help needed) Checking an object variable against an inputted String value. Nullpointerexception

Question

Hi there,

I'm relatively new to java, I'm certain the error is trivial. But can't for the life of me spot it. I have an end of term exam on monday and currently trying to get to grips with past papers!

Anyway heregoes, in another method (ALGO_1) I search over elements of and check the value H_NAME equals a value entered in the main. When I attempt to run the code I get a null pointer exception, also upon trying to print (with System.out.println etc) the H_NAME value after each for loop in the snippet I also get a null statement returned to me.

I am fairly certain that the collection is simply not storing the data gathered up by the Scanner. But then again when I check the collection size with size() it is about the right size. Either way I'm pretty lost and would appreciate the help.

Main questions I guess to ask are:

• from the readBackground method is the data.add in the wrong place?
• is the snippet simply structured wrongly?
• oh and another point when I use System.out.println to check the Background object values name, starttime, increment etc they print out fine.

Thanks in advance.(PS im guessing the formatting is terrible, apologies.)

snippet of code:

for(Hydro hd: hydros){    

System.out.println(hd.H_NAME);  
for(Background back : backgs){  
System.out.println(back.H_NAME);  
if(back.H_NAME.equals(hydroName)){ //get error here  




public static Collection<Background> readBackground(String url) throws IOException {

URL u = new URL(url);
InputStream is = u.openStream();  
InputStreamReader isr = new InputStreamReader(is);  
BufferedReader b = new BufferedReader(isr);  
String line ="";  
Vector<Background> data = new Vector<Background>();  
while((line = b.readLine())!= null){  
Scanner s = new Scanner(line);  
String name = s.next();  
double starttime = Double.parseDouble(s.next());  
double increment = Double.parseDouble(s.next());  
double sum = 0;  
double p = 0;  
double nterms = 0;  
while((s.hasNextDouble())){  
p = Double.parseDouble(s.next());  
nterms++;  
sum += p;  
}  
double pbmean = sum/nterms;  
Background SAMP = new Background(name, starttime, increment, pbmean);  
data.add(SAMP);  
}  
return data;  
}  

Edit/Delete Message

Answer 1

If you get a NullPointerException (NPE) when you call back.H_NAME.equals(xxx), it probably means either back is null or back.H_NAME is null.

You say that when you print back.H_NAME, you get null, so that suggests that back.H_NAME is actually null. You haven't shown us any code that describes the class Background, so it's hard to give you any more help.