Weka is a collection of machine learning algorithms for data mining tasks. The algorithms can either be applied directly to a dataset or called from your own Java code. Weka contains tools for data pre-processing, classification, regression, clustering, association rules, and visualization. It is also well-suited for developing new machine learning schemes.
You can test this by testing the arff file with the Instances class. At the command line type the following:
java -cp weka.jar; weka.core.Instances filename.arff
where you substitute ``filename.arff'' with the actual name of file. This should return an error if there is a problem reading the file, or show some statistics if the file is ok. The error message might give an indication of what is wrong. A common problem people have with arff files is that labels can only have spaces if they are enclosed in single quotes, i.e. a label such as: some value should be written either 'some value' or some_value in the file.
Source: Weka FAQ