getting an error at Instance inst =new Instance(1.0, new double[attrs.size()]); Line

Question

I am getting an error at the following line :

Instance inst =new Instance(1.0, new double[attrs.size()]);

Can any body explain how I can fix this error?

Here is the code:

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import weka.core.Attribute;
import weka.core.FastVector;
import weka.core.Instance;
import weka.core.Instances;
import weka.core.converters.ArffSaver;


public class ContextualFeatureExtractor implements FeatureExtractor{
    private static final int NUMBASEATTR = 1;
    private static final int[] HOURCATEGORY = {3,3,3,3,0,0,0,0,0,0,1,1,1,1,1,1,2,2,2,2,2,2,3,3};
    private static final String[] TIMENAME = {"Morning","Afternoon","Evening","Night"};
    private static final int MINEMOTCOUNT = 100;

    private FastVector attrs;
    private Pattern emotpat;
    private ArrayList<String> emotslist;

    public void setupAttributes(List<Tweet> tweets)
    {
        attrs = new FastVector();
        // Determine attributes

        FastVector hourvals = new FastVector();
        hourvals.addElement(TIMENAME[0]); hourvals.addElement(TIMENAME[1]);
        hourvals.addElement(TIMENAME[2]); hourvals.addElement(TIMENAME[3]);
        Attribute timeofday = new Attribute("TimeOfDay", hourvals);
        attrs.addElement(timeofday);

        // Find frequent emoticons
        emotpat = Pattern.compile("\\p{Graph}*\\p{Punct}\\p{Graph}*");
        HashMap<String,Integer> emots = new HashMap<String,Integer>(100);
        for(Tweet t: tweets)
        {
            Matcher emotmat = emotpat.matcher(t.text);
            while (emotmat.find())
            {
                String curemot = emotmat.group();
                if(curemot.length() > 1 && curemot.length() < 5) {
                    if(emots.containsKey(curemot)) {
                        int curcount = emots.get(curemot);
                        curcount++;
                        emots.put(curemot, curcount);
                    }
                    else
                        emots.put(curemot, 1);
                }
            }
        }
        Set<Map.Entry<String,Integer>> emotset = emots.entrySet();
        Set<String> emotrem = new HashSet<String>(100);
        for(Map.Entry<String,Integer> emotmap : emotset)
        {
            if(emotmap.getValue() < MINEMOTCOUNT)
                emotrem.add(emotmap.getKey());
        }
        Set<String> goodemots = emots.keySet();
        goodemots.removeAll(emotrem);
        emotslist = new ArrayList<String>();
        for(String emot : goodemots)
        {
            Attribute attr = new Attribute("confeature:" + emot);
            attrs.addElement(attr);
            emotslist.add(emot);
        }
    }

    public Instances extractFeatures(List<Tweet> tweets)
    {
        if(attrs == null)
            setupAttributes(tweets);
        Instances feats = new Instances("Contextual Features", attrs, tweets.size());
        feats.setClassIndex(0);
        // Record features

        for(Tweet t: tweets)
        {

            Instance inst =new Instance(1.0, new double[attrs.size()]);
            inst.setDataset(feats);

            int hrcat = HOURCATEGORY[t.hour];
            inst.setValue(0, TIMENAME[hrcat]);

            Matcher emotmat = emotpat.matcher(t.text);
            while (emotmat.find())
            {
                String curemot = emotmat.group();
                if(emotslist.contains(curemot))
                {
                    int attrind = emotslist.indexOf(curemot)+NUMBASEATTR;
                    //double val = inst.value(attrind);
                    //val += 1.0;
                    inst.setValue(attrind, 1.0);
                }

            }           

            feats.add(inst);
        }

         ArffSaver saver = new ArffSaver();
         saver.setInstances(feats);
         try {
            saver.setFile(new File("output/contextual.arff"));
            saver.writeBatch();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return feats;
    }

    public static void main(String[] args) {
        ArrayList<Tweet> tweets = TweetFileParser.parseFile("data/train.40000.2009.05.25");
        ContextualFeatureExtractor cfe = new ContextualFeatureExtractor();
        Instances insts = cfe.extractFeatures(tweets);
        System.out.println(insts.toSummaryString());
    }
}

Answer 1

It appears from the Javadocs that Instance is an interface - which means you can't instantiate it, but must instead construct one of its implementing classes. (This appears to be one of BinarySparseInstance, DenseInstance or SparseInstance.)

I do see however that in some other version, Instance was a concrete class. If you're running through examples from one version with another version of the library, this may lead you to have hit this error. Ensure that you're using the version you expect, and check the documentation for that version to see what you can call.

See also:

• Cannot instantiate the type...
• Cannot instantiate the type List<Product>

Answer 2

weka - LibSVM => libsvm.jar library is not in classpath. Download it from here. Or if you have library then add it to your classpath.