Page.java

/*
 * Copyright 2015-2016 Red Hat, Inc. and/or its affiliates
 * and other contributors as indicated by the @author tags.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.hawkular.inventory.api.paging;

import java.io.Closeable;
import java.io.IOException;
import java.util.Iterator;
import java.util.List;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.stream.Collectors;
import java.util.stream.StreamSupport;

import org.hawkular.inventory.api.Log;

/**
 * A read-only list representing a single page of some results.
 *
 * <p>Contains a reference to the paging state object that describes the position of the page in some overall results
 * .<p/>
 *
 * <p>This implements the {@link AutoCloseable} so make sure you call the {@link #close()} method or you use the
 * try-with-resource statement, in order to prevent potential memory leaks. </p>
 *
 * @author Lukas Krejci
 * @since 0.0.1
 */
public class Page<T> implements Iterator<T>, AutoCloseable, Iterable<T> {
    private Iterator<T> wrapped;
    private final PageContext pageContext;
    private final long totalSize;

    public Page(Iterator<T> wrapped, PageContext pageContext, long totalSize) {
        this.wrapped = wrapped;
        this.pageContext = pageContext;
        this.totalSize = totalSize;
    }

    protected Page(PageContext pageContext, long totalSize) {
        this(null, pageContext, totalSize);
    }

    /**
     * @return the information about the page of the results that this object represents
     */
    public PageContext getPageContext() {
        return pageContext;
    }

    /**
     * @return the total number of results of which this page is a subset of
     */
    public long getTotalSize() {
        return totalSize;
    }

    /**
     * Try to avoid calling this method in production code, because it can have bad impact on performance
     * <p>
     * Note that this operation {@link #close() closes} this page.
     *
     * @return results in a list form
     */
    public List<T> toList() {
        try {
            return StreamSupport.stream(Spliterators.spliteratorUnknownSize(this, Spliterator.ORDERED), false)
                    .collect(Collectors.toList());
        } finally {
            close();
        }
    }

    @Override public boolean hasNext() {
        return wrapped != null && wrapped.hasNext();
    }

    @Override public T next() {
        if (wrapped == null) {
            throw new IllegalStateException("the iterator has been already closed");
        }
        Log.LOGGER.trace("Page: Obtaining next element from the wrapped iterator.");
        return wrapped.next();
    }

    /**
     * @throws IllegalStateException if the close fails
     */
    @Override public void close() {
        try {
            //the iterator usually fetches data from some data store so it might need closing, too.
            if (wrapped instanceof Closeable) {
                ((Closeable) wrapped).close();
            }
        } catch (IOException e) {
            throw new IllegalStateException("Failed to close the wrapped result iterator.", e);
        } finally {
            this.wrapped = null;
        }
    }

    @Override public Iterator<T> iterator() {
        return this;
    }
}