nullpointer exception at a search method in Java

Question

So I tracked down the bugger, but I am no closer to understanding what is wrong. Here is what the compiler says:

Exception in thread "main" java.lang.NullPointerException at BasicFile.Search(BasicFile.java:215) at TestFile.main(TestFile.java:42)

Line 215 is the one that starts with while, first one.

String Search(String key) throws IOException {

    int lines = 0;
    String line = "";
    String foundAt = "";
    BufferedReader BF = new BufferedReader(new FileReader(f));

    try {
        while ((line = BF.readLine().toLowerCase()) != null) {
            lines++;
            //create tokenizer words with what is in line
            StringTokenizer words = new StringTokenizer(line);
            while(words.hasMoreTokens()) { //while words has tokens left
                //go to next token and compare to key
                if (words.nextToken().equals(key.toLowerCase())) 
                    foundAt = foundAt + "\n" + lines + ":" + line;
                //do nothing continue loop                     
            }
        }
        BF.close();
    } catch(FileNotFoundException e) {
    }
    return foundAt;
}

You put != to check if the value returned is not null. But you're using the value returned by invoking a method on it... — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Mar 2, 2014 at 23:04
Why do you have != null? What could be null? The answer is the value returned by BF.readLine(). If that can be null, why are you invoking a method on it? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Mar 2, 2014 at 23:06
I was trying to be all tricky and doing it all on one line... I should have been more careful. My shame to live with. — user3236502
– user3236502, Commented Mar 2, 2014 at 23:15

jsalonen · Accepted Answer · 2014-03-02 23:05:00Z

2

When your buffer reader runs out of lines it returns null. You are trying to call toLowerCase method on null which ends up throwing the null pointer exception.

Refactor your code in a way that it doesn't require you to execute toLowerCase before ensuring the line is non-null.

For example:

String next;

while ((next = BF.readLine()) != null) {
   String line = next.toLowerCase();
   // ...
}

answered Mar 2, 2014 at 23:05

jsalonen

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1 Comment

user3236502 Over a year ago

ah! I see, now the tricky part will be finding a way to do that.

tommyo · Accepted Answer · 2014-03-02 23:04:51Z

0

while ((line = BF.readLine().toLowerCase()) != null)

What happens if BF.readline() returns null?

answered Mar 2, 2014 at 23:04

tommyo

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3 Comments

user3236502 Over a year ago

null pointer exception... I just learned that 10 seconds ago.

tommyo Over a year ago

@user3236502: Indeed, you are trying to call a method on null. Test for null before doing that.

tommyo Over a year ago

@user3236502: That is why i phrased the answer as a question. Good luck.

Olivier Citeau · Accepted Answer · 2014-03-02 23:07:23Z

0

remove .toLowerCase() from the test

answered Mar 2, 2014 at 23:07

Olivier Citeau

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2 Comments

user3236502 Over a year ago

But I must make it lowercase otherwise it may not return a value like Cat will be not found if I am looking for cat.

Olivier Citeau Over a year ago

jsalonen replied to you. I cannot explain it better than him.

AJMansfield · Accepted Answer · 2014-03-02 23:34:46Z

0

Please, stop it, your code is giving me cancer! There are a number of stylistic errors in the code that you need to fix.

First off in java, method names always begin with a lowercase letter. You are programming in Java, not C#, so you need to use the Java naming conventions. That means your method should be called search, not Search.
The same goes for variable names. What is BF supposed to mean, anyway? Replace it with in, please.
Next up, unless this method is in an object that itself represents that particular file, the global variable f should be passed as a parameter instead.
BufferedReader is AutoCloseable, so you should use a try-with-resources to deal with closing it.
You need to add a javadoc comment to it, documenting its parameters with @param, its return with @return, and exactly why it might need to throw an IOException with @exception.

Here is a mostly-fixed version of your code:

/**
 * Needs Javadoc
 */
String search(String key, File f) throws IOException {

    int lines = 0
    String line = "";
    String foundAt = "";

    try(BufferedReader in = new BufferedReader(new FileReader(f)) {

        while ((line = in.readLine().toLowerCase()) != null) { //the line in question

            lines++;
            StringTokenizer words = new StringTokenizer(line);

            while(words.hasMoreTokens()) 
                if (words.nextToken().equals(key.toLowerCase()))
                    foundAt = foundAt + "\n" + lines + ":" + line;                     
        }

    } catch(FileNotFoundException e){}

    return foundAt;
}

Now, the problem here is that in.readline() returns a null sometimes. Calling a method on a null is always a NullPointerException. Therefore you get a NullPointerException when you attempt to call that null's missing toLowerCase() method.

You need to convert it toLowerCase after you ensure it is non-null.

edited Mar 2, 2014 at 23:34

answered Mar 2, 2014 at 23:23

AJMansfield

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3 Comments

user3236502 Over a year ago

I just do things the way the professor asks, he is the one that set up the global variable. BF and Search.

AJMansfield Over a year ago

@user3236502 don't let yourself be cheated out of proper education! You should demand that to be taught real java.

AJMansfield Over a year ago

@user3236502 If I were teaching a programming class, I would take off major points from any code that didn't use proper style, even if the bad style was from bad template code given to students.

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