    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.collections4.sequence;
    018
    019/**
    020 * Abstract base class for all commands used to transform an objects sequence
    021 * into another one.
    022 * <p>
    023 * When two objects sequences are compared through the
    024 * {@link SequencesComparator#getScript SequencesComparator.getScript} method,
    025 * the result is provided has a {@link EditScript script} containing the commands
    026 * that progressively transform the first sequence into the second one.
    027 * <p>
    028 * There are only three types of commands, all of which are subclasses of this
    029 * abstract class. Each command is associated with one object belonging to at
    030 * least one of the sequences. These commands are {@link InsertCommand
    031 * InsertCommand} which correspond to an object of the second sequence being
    032 * inserted into the first sequence, {@link DeleteCommand DeleteCommand} which
    033 * correspond to an object of the first sequence being removed and
    034 * {@link KeepCommand KeepCommand} which correspond to an object of the first
    035 * sequence which <code>equals</code> an object in the second sequence. It is
    036 * guaranteed that comparison is always performed this way (i.e. the
    037 * <code>equals</code> method of the object from the first sequence is used and
    038 * the object passed as an argument comes from the second sequence) ; this can
    039 * be important if subclassing is used for some elements in the first sequence
    040 * and the <code>equals</code> method is specialized.
    041 *
    042 * @see SequencesComparator
    043 * @see EditScript
    044 *
    045 * @since 4.0
    046 */
    047public abstract class EditCommand<T> {
    048
    049    /** Object on which the command should be applied. */
    050    private final T object;
    051
    052    /**
    053     * Simple constructor. Creates a new instance of EditCommand
    054     *
    055     * @param object  reference to the object associated with this command, this
    056     *   refers to an element of one of the sequences being compared
    057     */
    058    protected EditCommand(final T object) {
    059        this.object = object;
    060    }
    061
    062    /**
    063     * Returns the object associated with this command.
    064     *
    065     * @return the object on which the command is applied
    066     */
    067    protected T getObject() {
    068        return object;
    069    }
    070
    071    /**
    072     * Accept a visitor.
    073     * <p>
    074     * This method is invoked for each commands belonging to
    075     * an {@link EditScript EditScript}, in order to implement the visitor design pattern
    076     *
    077     * @param visitor  the visitor to be accepted
    078     */
    079    public abstract void accept(CommandVisitor<T> visitor);
    080
    081}