如何在给定代码中修复java.lang.NullPointerException？

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");

---

---

  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

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 2

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 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 ie

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