ResultSetIterator.java

/*******************************************************************************
 * Copyright (c) 1998, 2015 Oracle and/or its affiliates. All rights reserved.
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
 * which accompanies this distribution.
 * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
 * and the Eclipse Distribution License is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *     Oracle - initial API and implementation from Oracle TopLink
******************************************************************************/
package org.eclipse.persistence.tools.workbench.utility.iterators;

import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * A <code>ResultSetIterator</code> wraps a <code>ResultSet</code>
 * and transforms its rows for client consumption. Subclasses can override
 * <code>#buildNext(ResultSet)</code> to build the expected object from
 * the current row of the result set.
 * <p>
 * To use, supply:<ul>
 * <li> a <code>ResultSet</code>
 * <li> an <code>Adapter</code> that converts a row in the <code>ResultSet</code>
 * into the desired object
 * (alternatively, subclass <code>ResultSetIterator</code>
 * and override the <code>buildNext(ResultSet)</code> method)
 * </ul>
 * <p>
 */
public class ResultSetIterator
    implements Iterator
{
    private final ResultSet resultSet;
    private final Adapter adapter;
    private Object next;

    private static final Object END = new Object();


    /**
     * Construct an iterator on the specified result set that returns
     * the objects produced by the specified adapter.
     */
    public ResultSetIterator(ResultSet resultSet, Adapter adapter) {
        super();
        this.resultSet = resultSet;
        this.adapter = adapter;
        this.next = this.buildNext();
    }

    /**
     * Construct an iterator on the specified result set that returns
     * the first object in each row of the result set.
     */
    public ResultSetIterator(ResultSet resultSet) {
        this(resultSet, Adapter.DEFAULT_INSTANCE);
    }

    /**
     * Build the next object for the iterator to return.
     * Close the result set when we reach the end.
     */
    private Object buildNext() {
        try {
            if (this.resultSet.next()) {
                return this.buildNext(this.resultSet);
            }
            this.resultSet.close();
            return END;
        } catch (SQLException ex) {
            throw new RuntimeException(ex);
        }
    }

    /**
     * By default, return the first object in the current row
     * of the result set. Any <code>SQLException</code>s will
     * be caught and wrapped in a <code>RuntimeException</code>.
     */
    protected Object buildNext(ResultSet rs) throws SQLException {
        return this.adapter.buildNext(rs);
    }

    /**
     * @see java.util.Iterator#hasNext()
     */
    public boolean hasNext() {
        return this.next != END;
    }

    /**
     * @see java.util.Iterator#next()
     */
    public Object next() {
        if (this.next == END) {
            throw new NoSuchElementException();
        }
        Object temp = this.next;
        this.next = this.buildNext();
        return temp;
    }

    /**
     * @see java.util.Iterator#remove()
     */
    public void remove() {
        throw new UnsupportedOperationException();
    }


    // ********** interface **********

    /**
     * Used by <code>ResultSetIterator</code> to convert a
     * <code>ResultSet</code>'s current row into the next object
     * to be returned by the <code>Iterator</code>.
     */
    public interface Adapter {

        /**
         * Return an object corresponding to the result set's
         * "current" row. Any <code>SQLException</code>s will
         * be caught and wrapped in a <code>RuntimeException</code>.
         * @see java.sql.ResultSet
         */
        Object buildNext(ResultSet rs) throws SQLException;


        Adapter DEFAULT_INSTANCE =
            new Adapter() {
                // return the first object in the current row of the result set
                public Object buildNext(ResultSet rs) throws SQLException {
                    // result set columns are indexed starting with 1
                    return rs.getObject(1);
                }
            };
    }

}