NullPointerException error in Hashmap

Question

I have spent countless hours on this one part of my code which is giving me a Null Pointer Exception. I do not understand why it is happening or how to stop it. I have tried every method previously posted on stackoverflow. Here is the function that reads a data file and puts each line in a hashmap.

public ArrayList<HashMap<String, String>> readDataFromFile(){
    this.openFileForReading();
    ArrayList<HashMap<String, String>> list = new ArrayList<HashMap<String,String>>();
    try{
        String line;

        while ((line = this.reader.readLine()) != null){ 

            while (!(line.equals(""))){

                if (line.equals("[type = book]")){
                    HashMap<String, String> data= new HashMap<>();
                    line = this.reader.readLine();

                    while (!(line.equals(""))){<----this is where the null exception error is

                        String tokens[] = line.split("=");
                        data.put(tokens[0], tokens[1]);
                        System.out.println(tokens[0] + " " + tokens[1]);
                        line = this.reader.readLine();

                    }
                   list.add(data);
                }
                else{
                    break;
                }
            }

        }
    }
    catch (IOException exception) {
        list = null;
        System.err.println("(FileIO): " + exception);            
    }
    // Close the file when we have finished reading or if an error occurs
    finally {
        try {
            this.reader.close();                
        } catch (IOException exception) {
            System.err.println("(FileIO): " + exception);
        }
    }

    return list;
}

I then run it in main with this:

public static void main (String args[]) {                                
    FileIO fileIO = new FileIO(DATA_FILE_PATH);
    ArrayList<HashMap<String, String>> list = new ArrayList<HashMap<String,String>>();
    list = fileIO.readDataFromFile();

}

run:
callnumber �QA76.73.J38S265� authors �Walter Savitch, Kenrich Mock� title �Absolute Java� publisher �Addison-Wesley� year �2009� callnumber �P98.C6116� title �Computational Linguistics� Exception in thread "main" java.lang.NullPointerException organization �Association for Computational Linguistics� at librarysearch.FileIO.readDataFromFile(FileIO.java:160) year �2008� at librarysearch.FileIO.main(FileIO.java:405) Java Result: 1 BUILD SUCCESSFUL (total time: 1 second)

I wrote in the code where the error occurs. Why is this happening? I don't understand, if anyone saavy could please help that would be greatly appreciated.

First thing to do: read the exception stack trace (or post it if you can't understand it). Error messages are intended to be read. They tell you what and where the problem is. By not reading them, you're shooting yourself in the foot, and what could be solved in 10 seconds taks days to be solved. — JB Nizet
– JB Nizet, Commented Nov 13, 2013 at 21:59

rob · Accepted Answer · 2013-11-13 22:20:46Z

3

You're consuming a new line without checking for null:

 HashMap<String, String> data= new HashMap<>();
 line = this.reader.readLine(); <--- i mean this, and line can be null

 while (!(line.equals(""))){<----this is where the null exception error is

I give you a general advise:

Never compare a String variable against a String literal in this way:

if (myStringWhichCanBeNull.equals("my literal which is never null")) // ...

but always like this:

if ("my literal which is never null".equals(myStringWhichCanBeNull)) // ...

In many cases you will avoid NPE and have more robust code!

(I don't say that in this case this had saved your live...)

edited Nov 13, 2013 at 22:20

rob

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answered Nov 13, 2013 at 22:01

hgoebl

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1 Comment

user2989421 Over a year ago

This worked! sort of. I added a while (line != null) before line.equals(") and it now doesnt give an error but stops at the first blank line

JB Nizet · Accepted Answer · 2013-11-13 22:02:45Z

1

line = this.reader.readLine();

while (!(line.equals(""))) {

You're reading a line, and then don't check if it's null before calling equals() on it. It will be null if there is nothing to read anymore in the file, as documented in the javadoc of the readLine() method.

answered Nov 13, 2013 at 22:02

JB Nizet

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Comments

DavidC · Accepted Answer · 2013-11-13 22:06:59Z

0

 line = this.reader.readLine();

probably returns null on line 13 (unlucky for some)

then you call .equals on it

So maybe add some more error checking

answered Nov 13, 2013 at 22:06

DavidC

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Comments

rob · Accepted Answer · 2013-11-13 22:19:42Z

0

You need to check the result of BufferedReader.readLine() for null everywhere you call the method. As the Java API documentation states, BufferedReader.readLine() returns:

A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached

Your outer loop checks for null, but you have additional nested loops which perform additional reads but don't check for null. Consider restructuring your code and modifying your logic so you're only using one loop instead of nesting several loops to perform the readLine() operations. Otherwise, you will need to add more null checks and either add break statements or modify your loop conditions, which may make the code more difficult to read.

edited Nov 13, 2013 at 22:19

answered Nov 13, 2013 at 22:07

rob

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