title: weka.core.UnassignedDatasetException when creating an unlabeled instance
body: <p>I trained an IBK classifier with some training data that I created manually as following:</p>
  
  <pre><code>ArrayList&lt;Attribute&gt; atts = new ArrayList&lt;Attribute&gt;();
  ArrayList&lt;String&gt; classVal = new ArrayList&lt;String&gt;();
  classVal.add("C1");
  classVal.add("C2");
  atts.add(new Attribute("a"));
  atts.add(new Attribute("b"));
  atts.add(new Attribute("c"));
  atts.add(new Attribute("d"));
  atts.add(new Attribute("@@class@@", classVal));
  
  Instances dataRaw = new Instances("TestInstances", atts, 0);
  dataRaw.setClassIndex(dataRaw.numAttributes() - 1);
  double[] instanceValue1 = new double[]{3,0,1,0,0};
  dataRaw.add(new DenseInstance(1.0, instanceValue1));
  
  double[] instanceValue2 = new double[]{2,1,1,0,0};
  dataRaw.add(new DenseInstance(1.0, instanceValue2));
  
  double[] instanceValue3 = new double[]{2,0,2,0,0};
  dataRaw.add(new DenseInstance(1.0, instanceValue3));
  
  double[] instanceValue4 = new double[]{1,3,0,0,1};
  dataRaw.add(new DenseInstance(1.0, instanceValue4));
  
  double[] instanceValue5 = new double[]{0,3,1,0,1};
  dataRaw.add(new DenseInstance(1.0, instanceValue5));
  
  double[] instanceValue6 = new double[]{0,2,1,1,1};
  dataRaw.add(new DenseInstance(1.0, instanceValue6));
  </code></pre>
  
  <p>Then I build up the classifier:</p>
  
  <pre><code>IBk ibk = new IBk();
  try {
      ibk.buildClassifier(dataRaw);
  
  } catch (Exception e) {
      e.printStackTrace();
  }
  </code></pre>
  
  <p>I want to create a new instance with unlabeled class and classify this instance, I tried the following with no luck.</p>
  
  <pre><code>IBk ibk = new IBk();
  try {
      ibk.buildClassifier(dataRaw);
      double[] values = new double[]{3,1,0,0,-1};
      DenseInstance newInst = new DenseInstance(1.0,values);
      double classif = ibk.classifyInstance(newInst);
      System.out.println(classif);
  } catch (Exception e) {
      e.printStackTrace();
  }
  </code></pre>
  
  <p>I just get the following errors</p>
  
  <pre><code>weka.core.UnassignedDatasetException: DenseInstance doesn't have access to a dataset!
  at weka.core.AbstractInstance.classAttribute(AbstractInstance.java:98)
  at weka.classifiers.AbstractClassifier.classifyInstance(AbstractClassifier.java:74)
  at TextCategorizationTest.instancesWithDoubleValues(TextCategorizationTest.java:136)
  at TextCategorizationTest.main(TextCategorizationTest.java:33)
  </code></pre>
  
  <p>Looks like I am doing something wrong while creating a new instance. How can I create an unlabeled instance exactly ?</p>
  
  <p>Thanks in Advance</p>