In the following some code snippets that explain how to generate the output Weka generates when one runs a classifier from the commandline. When referring to the Evaluation class, the weka.classifiers.Evaluation class is meant. This article provides only a quick overview, for more details, please see the Javadoc of the Evaluation class.

Model

A classifier's model, if that classifier supports the output of it, can be simply output by using the toString() method after it got trained:
 Instances data = ... // from somewhere
 Classifier cls = new weka.classifiers.trees.J48();
 cls.buildClassifier(data);
 System.out.println(cls);
NB: Weka always outputs the model based on the full training set (provided with the option -t), no matter whether cross-validation is used or a designated test set (via -T). The 10 models generated during a 10-fold cross-validation run are never output. If you want to output these models you have to simulate the crossValidateModel method yourself (see article Generating cross-validation folds (Java approach)), or use the KnowledgeFlow (see article Displaying results of cross-validation folds).

Statistics

The statistics, also called the summary of an evaluation, can be be generated via the toSummaryString methods. Here is an example of the summary from a cross-validated J48:
 Classifier cls = new J48();
 Evaluation eval = new Evaluation(data);
 Random rand = new Random(1);  // using seed = 1
 int folds = 10;
 eval.crossValidateModel(cls, data, folds, rand);
 System.out.println(eval.toSummaryString());

Detailed class statistics

In order to generate the detailed statistics per class (on the commandline via option -i), one can use the toClassDetailsString methods. Once again a code snippet featuring a cross-validated J48:
 Classifier cls = new J48();
 Evaluation eval = new Evaluation(data);
 Random rand = new Random(1);  // using seed = 1
 int folds = 10;
 eval.crossValidateModel(cls, data, folds, rand);
 System.out.println(eval.toClassDetailsString());

Confusion matrix

The confusion matrix is simply output with the toMatrixString() or toMatrixString(String) method of the Evaluation class. In the following an example of cross-validating J48 on a dataset and outputting the confusion matrix to stdout.
 Classifier cls = new J48();
 Evaluation eval = new Evaluation(data);
 Random rand = new Random(1);  // using seed = 1
 int folds = 10;
 eval.crossValidateModel(cls, data, folds, rand);
 System.out.println(eval.toMatrixString());

See also