DataSetIterator.java

package eu.europeana.cloud.mcs.driver;

import eu.europeana.cloud.common.model.DataSet;
import eu.europeana.cloud.common.response.ResultSlice;
import eu.europeana.cloud.common.utils.FirstFlag;
import eu.europeana.cloud.mcs.driver.exception.DriverException;
import eu.europeana.cloud.service.mcs.exception.MCSException;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * Class for iterating through DataSets of given provider.
 *
 * The best way to initialise iterator is to obtain it by calling
 * {@link DataSetServiceClient#getDataSetIteratorForProvider(String)} method.
 *
 * Iterator obtains {@link DataSet} objects in chunks, obtaining new chunk only
 * if needed, inside overridden {@link Iterator} class methods.
 *
 */
public class DataSetIterator implements Iterator<DataSet> {

    //iterator parameters
    private final DataSetServiceClient client;
    private final String providerId;
    //variables for holding state
    private FirstFlag firstTime = new FirstFlag();
    private String nextSlice = null;
    private Iterator<DataSet> dataSetListIterator;

    /**
     * Creates instance of DataSetIterator.
     *
     * @param client properly initialised client for internal communication with
     * MCS server (required)
     * @param providerId id of the provider (required)
     */
    public DataSetIterator(DataSetServiceClient client, String providerId) {
        if (client == null) {
            throw new DriverException("DataSetServiceClient for DataSetIterator cannot be null");
        }
        this.client = client;

        if (providerId == null || providerId.equals("")) {
            throw new DriverException("ProviderId for DataSetIterator cannot be null/empty");
        }
        this.providerId = providerId;

    }

    /**
     * Returns <code>true</code> if the iteration has more elements.
     *
     * The first call to this method might take longer time than the others, as
     * there might be a need to obtain first chunk of data.
     *
     * @return {@code true} if the iteration has more elements, false if not.
     */
    @Override
    public boolean hasNext() {
        if (firstTime.unpack()) {
            obtainNextChunk();
        }

        return (dataSetListIterator.hasNext() || nextSlice != null);
    }

    /**
     * Returns next element in the iteration.
     *
     * Some calls to this method might take longer time than the others, if the
     * new chunk of data has to be obtained.
     *
     * @return next element in the iteration
     * @throws NoSuchElementException if there are no more elements
     */
    @Override
    public DataSet next() {
        if (firstTime.unpack()) {
            obtainNextChunk();
        }
        if (dataSetListIterator.hasNext()) {
            return dataSetListIterator.next();
        }
        if (nextSlice != null) {
            obtainNextChunk();
            return dataSetListIterator.next();
        }
        throw new NoSuchElementException("Calling next on exhausted DataSet iterator.");

    }

    //TODO there is a technical possibility to support this, should we?
    @Override
    public void remove() {
        throw new UnsupportedOperationException("Not supported.");
    }

    //this method does not check if this is valid to obtain next chunk now
    private void obtainNextChunk() {

        ResultSlice<DataSet> currentChunk;
        try {
            currentChunk = client.getDataSetsForProviderChunk(providerId, nextSlice);
        } catch (MCSException ex) {
            throw new DriverException("Error when trying to obtain DataSet list chunk for iterator", ex);
        }
        dataSetListIterator = currentChunk.getResults().iterator();
        nextSlice = currentChunk.getNextSlice();
    }

}