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datasets.java

 import weka.core.converters.ConverterUtils.DataSource;
 ...
 DataSource source = new DataSource("/some/where/data.arff");
 Instances data = source.getDataSet();
 // setting class attribute if the data format does not provide this information
 // For example, the XRFF format saves the class attribute information as well
 if (data.classIndex() == -1)
   data.setClassIndex(data.numAttributes() - 1);