factory method pattern with class registration produces a nullpointer exception

Question

well, i searched the internet to this problem but didn't find any proper solution

in http://www.oodesign.com/factory-pattern.html

the author described away to register classes using reflection or object creation

i tried the object creation approach by the following code: the factory class

package com.mf.egyptse;

import java.util.HashMap;

public abstract class ParserFactory {

    private static HashMap parsers;

    static
    {
        parsers= new HashMap();

        System.out.println("This is first static block");
    }


    static void putParser(Object key,Object parser)
    {
        parsers.put(key, parser);

    }
    static Object getParser(Object key)
    {
        return parsers.get(key);

    }

}

each parser register itself in the factory: public class NormalParser extends ParserFactory implements ParsingBehavior{

/**
 * Define the number of nested columns or tags to be parsed
 */
final static int NO_OF_COLOUMNS = 13;
static String input = null;
static String[] elements= {"name","sector", "p.c", "open", "close", "chgpercent", "lastprice", "high", "low","value","volume","trades","marketcap"};



static
{
    ParserFactory.putParser("normal", new NormalParser());

}

and the main is :

public class Main {

    /**
     * @param args
     * @throws IOException 
     * @throws InterruptedException 
     */
    public static void main(String[] args) throws IOException, InterruptedException {

//      NileParser.parseNile();
//      OTCParser.parseOTC();
//      NormalParser.parseNormal();


        ParsingBehavior parser = (ParsingBehavior) ParserFactory.getParser("normal");



        parser.parseToXML(null, null);

    }

}

the interface is:

package com.mf.egyptse;

import java.io.File;
import java.io.IOException;

public interface ParsingBehavior {

    void parseToXML(CharSequence input,File file) throws IOException;

}

this code return always nullpointer exception while executing. the porblem is that the static block don't executed. so what is the solution ?

Please post the stacktrace, otherwise we have to guess where your NPE is coming from. — Preston, Aug 29 '11 at 02:24

score 2 · Answer 1 · answered Sep 05 '12 at 05:07

As answered by "Snicolas", your problem is that the HashMap is not populated by the time its being used. Your static block in main should load all the necessary parser classes such that these classes register themselves first.

public class Main {

    static {
        // Load necessary parser classes
        Class.forName("normal");
    }

    /**
     * @param args
     * @throws IOException 
     * @throws InterruptedException 
     */
    public static void main(String[] args) throws IOException, InterruptedException {
        ParsingBehavior parser = (ParsingBehavior) ParserFactory.getParser("normal");
        parser.parseToXML(null, null);
    }

}

score 1 · Answer 2 · edited Dec 14 '11 at 14:29

1

Getparser should return a ParsingBehavior. Cast inside it.

But your problem comes from the fact that your parser class is not loaded by the jvm, as it is not used by your main. So static code is not executed.

Your are mixing your factory with a bus. Let the main register your parser in the factory.

edited Dec 14 '11 at 14:29

answered Aug 29 '11 at 02:34

Snicolas

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factory method pattern with class registration produces a nullpointer exception

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