Java: BufferedWriter NullPointerException

Question

I am attempting to write a method that has to take in a Writer object and use that to provide output to a file. The current code I have is throwing a NullPointerException, presumably because either there is an error in how I am creating my BufferedWriter or else in some instances w (the Writer object) is being passed as null. I have no control over what is passed as w, and cannot change the exceptions that this method is able to throw.

My code is as follows:

public void write(Writer w, Stat s) throws IOException { try{ BufferedWriter writeFile = new BufferedWriter(w); writeFile.write(s.getData()); writeFile.flush(); } catch (IOException e){ ... } }

Is there something I'm doing wrong?

(This assignment arises out of homework, but this question isn't the homework itself)

Have you initialized the w and stat variables? Does stat.getData() actually return data? (I'm assuming s was supposed to be stat and you just typed it wrong in the question. Is that correct?) — Keith
– Keith, Commented Oct 5, 2015 at 18:58
Add null checks for both the writer and stat objects e. g. If(w ! =null & & s ! = null) after entering the try block — user3928919
– user3928919, Commented Oct 5, 2015 at 18:58
you should debug or add some print statement to see what the values of w, and s are. if these input parameters are null you cant do much about this. other than throw an exception — Marquis Blount
– Marquis Blount, Commented Oct 5, 2015 at 18:59
@Keith sorry, that was a type in creating the question. stat.getData() is actually s.getData() and yes, it does return data. w and s are provided in the method call. — DTR
– DTR, Commented Oct 5, 2015 at 19:00

wero · Accepted Answer · 2015-10-05 19:00:00Z

3

You need both Writer wand Stat s to be not null. Therefore you should reject them if they are null.

public void write(Writer w, Stat s) throws IOException { if (w == null) throw new IllegalArgumentException("writer is null"); if (s == null) throw new IllegalArgumentException("stats is null"); ...

answered Oct 5, 2015 at 19:00

wero

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10 Comments

Keith Over a year ago

You should add that although the Stat object may be constructed, its getData() may be returning null, too.

ControlAltDel Over a year ago

this is double exception checking, ie the NullPointerException is ALSO doing this check. Better IMO if you are going to go this route would be to put try {} catch {} blocks around the lines where you could get the nullpointerexceptions, catch the NPEs, and throw an IllegalArgumentException initialized with the NPE exception

wero Over a year ago

@Keith I don't think that this belongs into the sanitizing of arguments. writeFile.write(s.getData());does not throw a NPE if s.getData() returns null.

DTR Over a year ago

Thanks for this, turns out that I was only thinking about the cases where w could be null, but it turns out the problem was being caused by s.

wero Over a year ago

@DTR glad that you found it

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