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I used NumericToNominal to convert the class label form countinus to be nomanil BY USING WEKA API
used this code :
NumericToNominal convert= new NumericToNominal();
String class_index=instances.numAttributes() - 1;
String options= new String;
options=class_index;//where class index is 183 and i dont want to put a specific number in my code