How can I fix the java.lang.NullPointerException in the given code?

Question

When I run the program below (in NETBEANS 6.1.0 ), I get a 'java.lang.NullPointerException' in line 21 and 49. I'm a java novice please help to fix the error.

line 21. database.loadFile(fileToPath("contextPasquier99.txt"));

line 49. return java.net.URLDecoder.decode(url.getPath(),"UTF-8");

---

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  package ca.pfv.spmf.test;

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.net.URL;

import ca.pfv.spmf.algorithms.frequentpatterns.eclat_and_charm.AlgoCharm;
import ca.pfv.spmf.input.transaction_database_list_integers.TransactionDatabase;
import ca.pfv.spmf.patterns.itemset_set_integers_with_tids.Itemsets;

/**
 * Example of how to use the CHARM algorithm from the source code.
 * @author Philippe Fournier-Viger (Copyright 2009)
 */
   public class MainTestCharm_saveToMemory {

public static void main(String [] arg) throws IOException{
    // Loading the transaction database
    TransactionDatabase database = new TransactionDatabase();
    try {
        database.loadFile(fileToPath("contextPasquier99.txt"));
    } catch (IOException e) {
        e.printStackTrace();
    }
    database.printDatabase();

    // Applying the Charm algorithm
    AlgoCharm algo = new AlgoCharm();
    Itemsets closedItemsets = algo.runAlgorithm(null, database, 100000, 0.4, true);
    // NOTE 0: We use "null" as output file path, because in this
    // example, we want to save the result to memory instead of
    // saving to a file

    // NOTE 1: if you  use "true" in the line above, CHARM will use
    // a triangular matrix  for counting support of itemsets of size 2.
    // For some datasets it should make the algorithm faster.

    // NOTE 2:  1000000 is the hashtable size used by CHARM for
    // storing itemsets.  Most users don't use this parameter.

    // print the statistics
    algo.printStats();
    // print the frequent itemsets found
    closedItemsets.printItemsets(database.size());
}

public static String fileToPath(String filename) throws UnsupportedEncodingException{
    URL url = MainTestCharm_saveToMemory.class.getResource(filename);
     return java.net.URLDecoder.decode(url.getPath(),"UTF-8");
}

}

I get the following:

Exception in thread "main" java.lang.NullPointerException
        at ca.pfv.spmf.test.MainTestCharm_saveToMemory.fileToPath(MainTestCharm_saveToMemory.java:49)
        at ca.pfv.spmf.test.MainTestCharm_saveToMemory.main(MainTestCharm_saveToMemory.java:21)

Answer 1

Your .getResource(filename) is probably returning a null. You should test that url has a value before using it; something like:

public static String fileToPath(String filename) throws UnsupportedEncodingException{
    URL url = MainTestCharm_saveToMemory.class.getResource(filename);

    if (url != null) {
        return java.net.URLDecoder.decode(url.getPath(),"UTF-8");
    }

    System.out.println("file: " + filename + "not found");
    System.exit(-1); // or return empty string or null
}

EDIT: For getResource(filename) to work outside the package, the filename should start with "/"; for example:

database.loadFile(fileToPath("/contextPasquier99.txt"));

If it's called like:

database.loadFile(fileToPath("contextPasquier99.txt"));

it will only look inside the package ca.pfv.spmf.test.

Answer 2

Without line numbers, this is a bit tricky, but I'm going to go out on a limb...

What does MainTestCharm_saveToMemory.class.getResource(filename); return? I'm guessing null, causing a NPE on the next line when you do url.getPath()

Answer 3

The cause of NullPointerExcception is located in line 48.

URL url = MainTestCharm_saveToMemory.class.getResource(filename);

When you tried to obtain the URL of resource you got the null as stated in documentation of method getResource. Thus, you must check the existence of resource before run the code. It will help you to solve this problem. BTW do not forgot that the resource is not a regular file located anywhere on the filesystem. It's a file located in the the search path used to load classes.

Answer 4

Mostly your file "contextPasquier99.txt" exist in some path but you might have not specified it with complete correct path.

if so your URL url = MainTestCharm_saveToMemory.class.getResource(filename);

returns null for invalid file path and this propogates the null pointer exception to below line

 return java.net.URLDecoder.decode(url.getPath(),"UTF-8");

as url is calling its method but seems url is null.

and this exception propogates to caller i e

 database.loadFile(fileToPath("contextPasquier99.txt"));