validation - Weka - Update Instances during evaluation -

i using weka api test algorithm. during evaluation, want update of instances (feature vectors) after evaluate each instance. here method now

    for(int = 0; < testdataset.numinstances(); i++) {         // current validation instance         instance curinst = testdataset.instance(i);         // actual class index         int actualclassidx = (int)curinst.classvalue();         // predicted class index         int predclassidx = (int)bestclassifier.classifyinstance(curinst);          if(actualclassidx == predclassidx) {             numcorrect++;         }         // update validation dataset         updatefeatures(testdataset);     }     double accuracy = (double)numcorrect / testdataset.numinstances();     system.out.println("testing accuracy = " + accuracy * 100 + "%"); 

this works, however, want use things evaluation object me because need more statistic data (e.g: confusion matrix). there sufficient way achieve that?