Java Weka: How to specify split percentage?

In the UI class ClassifierPanel's method startClassifier(), I found the following code:

// Percent split

int trainSize = (int) Math.round(inst.numInstances() * percent
    / 100);
int testSize = inst.numInstances() - trainSize;
Instances train = new Instances(inst, 0, trainSize);
Instances test = new Instances(inst, trainSize, testSize);

so after randomizing your dataset...

trainingSet.randomize(new java.util.Random(0));

... I suggest you split your trainingSetin the same way:

int trainSize = (int) Math.round(trainingSet.numInstances() * 0.8);
int testSize = trainingSet.numInstances() - trainSize;
Instances train = new Instances(trainingSet, 0, trainSize);
Instances test = new Instances(trainingSet, trainSize, testSize);

then use Classifier#buildClassifier(Instances data) to train the classifier with 80% of your set instances:

model.buildClassifier(train);

UPDATE: thanks to @ChengkunWu's answer, I added the randomizing step above.

You might also want to randomize the split as well.

data.randomize(new java.util.Random(0));