Tutorial 2: Introduction to the API of MOA

by Albert Bifet, Richard Kirkby on October 15, 2017 using MOA 2017.06.

Using the API

It’s easy to use the methods of MOA inside Java Code. For example, this is the Java code for a prequential evaluation:


Exercise 1 Write a program in Java that learns a model using one stream and tests the model using another stream.

Writing a new classifier

To demonstrate the implementation and operation of learning algorithms in the system, the Java code of a simple decision stump classifier is studied. The classifier monitors the result of splitting on each attribute and chooses the attribute the seems to best separate the classes, based on information gain. The decision is revisited many times, so the stump has potential to change over time as more examples are seen. In practice it is unlikely to change after sufficient training.

To describe the implementation, relevant code fragments are discussed in turn, with the entire code listed at the end. The line numbers from the fragments match up with the final listing.

A simple approach to writing a classifier is to extend moa.classifiers.AbstractClassifier and implement MultiClassClassifier(line 16), which will take care of certain details to ease the task.

	public IntOption gracePeriodOption = new IntOption("gracePeriod", 'g',
			"The number of instances to observe between model changes.",
			1000, 0, Integer.MAX_VALUE);

	public FlagOption binarySplitsOption = new FlagOption("binarySplits", 'b',
			"Only allow binary splits.");

	public ClassOption splitCriterionOption = new ClassOption("splitCriterion",
			'c', "Split criterion to use.", SplitCriterion.class,

To set up the public interface to the classifier, the options available to the user must be specified. For the system to automatically take care of option handling, the options need to be public members of the class, that extend the moa.options.Option type.

The decision stump classifier example has three options, each of a different type.
The meaning of the first three parameters used to construct options are consistent between different option types. The first parameter is a short name used to identify the option. The second is a character intended to be used on the command line. It should be unique—a command line character cannot be repeated for different options otherwise an exception will be thrown. The third standard parameter is a string describing the purpose of the option. Additional parameters to option constructors allow things such as default values and valid ranges to be specified.

The first option specified for the decision stump classifier is the “grace period”. The option is expressed with an integer, so the option has the type IntOption. The parameter will control how frequently the best stump is reconsidered when learning from a stream of examples. This increases the efficiency of the classifier-evaluating after every single example is expensive, and it is unlikely that a single example will change the decision of the current best stump. The default value of 1000 means that the choice of stump will be re-evaluated only after 1000 examples have been observed since the last evaluation. The last two parameters specify the range of values that are allowed for the option-it makes no sense to have a negative grace period, so the range is restricted to integers 0 or greater.

The second option is a flag, or a binary switch, represented by a FlagOption. By default all flags are turned off, and will be turned on only when a user requests so. This flag controls whether the decision stumps should only be allowed to split two ways. By default the stumps are allowed have more than two branches.

The third option determines the split criterion that is used to decide which stumps are the best. This is a ClassOption that requires a particular Java class of the type SplitCriterion. If the required class happens to be an OptionHandler then those options will be used to configure the object that is passed in.

	protected AttributeSplitSuggestion bestSplit;

	protected DoubleVector observedClassDistribution;

	protected AutoExpandVector<AttributeClassObserver> attributeObservers;

	protected double weightSeenAtLastSplit;

	public boolean isRandomizable() {
		return false;

Four global variables are used to maintain the state of the classifier.

The bestSplit field maintains the current stump that has been chosen by the classifier. It is of type AttributeSplitSuggestion, a class used to split instances into different subsets.

The observedClassDistribution field remembers the overall distribution of class labels that have been observed by the classifier. It is of type DoubleVector, which is a handy class for maintaining a vector of floating point values without having to manage its size.

The attributeObservers field stores a collection of AttributeClassObservers, one for each attribute. This is the information needed to decide which attribute is best to base the stump on.

The weightSeenAtLastSplit field records the last time an evaluation was performed, so that it can be determined when another evaluation is due, depending on the grace period parameter.

The isRandomizable() function needs to be implemented to specify whether the classifier has an element of randomness. If it does, it will automatically be set up to accept a random seed. This classifier is does not, so false is returned.

	public void resetLearningImpl() {
		this.bestSplit = null;
		this.observedClassDistribution = new DoubleVector();
		this.attributeObservers = new AutoExpandVector<AttributeClassObserver>();
		this.weightSeenAtLastSplit = 0.0;

This function is called before any learning begins, so it should set the default state when no information has been supplied, and no training examples have been seen. In this case, the four global fields are set to sensible defaults.

	public void trainOnInstanceImpl(Instance inst) {
		this.observedClassDistribution.addToValue((int) inst.classValue(), inst
		for (int i = 0; i < inst.numAttributes() - 1; i++) {
			int instAttIndex = modelAttIndexToInstanceAttIndex(i, inst);
			AttributeClassObserver obs = this.attributeObservers.get(i);
			if (obs == null) {
				obs = inst.attribute(instAttIndex).isNominal() ?
					newNominalClassObserver() : newNumericClassObserver();
				this.attributeObservers.set(i, obs);
			obs.observeAttributeClass(inst.value(instAttIndex), (int) inst
					.classValue(), inst.weight());
		if (this.trainingWeightSeenByModel - this.weightSeenAtLastSplit >=
				this.gracePeriodOption.getValue()) {
			this.bestSplit = findBestSplit((SplitCriterion)
			this.weightSeenAtLastSplit = this.trainingWeightSeenByModel;

This is the main function of the learning algorithm, called for every training example in a stream. The first step, lines 47-48, updates the overall recorded distribution of classes. The loop from line 49 to line 59 repeats for every attribute in the data. If no observations for a particular attribute have been seen previously, then lines 53-55 create a new observing object. Lines 57-58 update the observations with the values from the new example. Lines 60-61 check to see if the grace period has expired. If so, the best split is re-evaluated.

	protected AttributeClassObserver newNominalClassObserver() {
		return new NominalAttributeClassObserver();

	protected AttributeClassObserver newNumericClassObserver() {
		return new GaussianNumericAttributeClassObserver();

	protected AttributeSplitSuggestion findBestSplit(SplitCriterion criterion) {
		AttributeSplitSuggestion bestFound = null;
		double bestMerit = Double.NEGATIVE_INFINITY;
		double[] preSplitDist = this.observedClassDistribution.getArrayCopy();
		for (int i = 0; i < this.attributeObservers.size(); i++) {
			AttributeClassObserver obs = this.attributeObservers.get(i);
			if (obs != null) {
				AttributeSplitSuggestion suggestion =
				if (suggestion.merit > bestMerit) {
					bestMerit = suggestion.merit;
					bestFound = suggestion;
		return bestFound;

These functions assist the training algorithm. newNominalClassObserver and newNumericClassObserver are responsible for creating new observer objects for nominal and numeric attributes, respectively. The findBestSplit() function will iterate through the possible stumps and return the one with the highest `merit’ score.

	public double[] getVotesForInstance(Instance inst) {
		if (this.bestSplit != null) {
			int branch = this.bestSplit.splitTest.branchForInstance(inst);
			if (branch >= 0) {
				return this.bestSplit
		return this.observedClassDistribution.getArrayCopy();

This is the other important function of the classifier besides training – using the model that has been induced to predict the class of examples. For the decision stump, this involves calling the functions branchForInstance() and resultingClassDistributionFromSplit() that are implemented by the AttributeSplitSuggestion class.

Putting all of the elements together, the full listing of the tutorial class is given below.


Exercise 2 Write a classifier in Java that implements a K nearest-neighbor algorithm.