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 * <p>
029 * There are only three types of commands, all of which are subclasses of this
030 * abstract class. Each command is associated with one object belonging to at
031 * least one of the sequences. These commands are {@link InsertCommand
032 * InsertCommand} which correspond to an object of the second sequence being
033 * inserted into the first sequence, {@link DeleteCommand DeleteCommand} which
034 * correspond to an object of the first sequence being removed and
035 * {@link KeepCommand KeepCommand} which correspond to an object of the first
036 * sequence which {@code equals} an object in the second sequence. It is
037 * guaranteed that comparison is always performed this way (i.e. the
038 * {@code equals} method of the object from the first sequence is used and
039 * the object passed as an argument comes from the second sequence) ; this can
040 * be important if subclassing is used for some elements in the first sequence
041 * and the {@code equals} method is specialized.
042 * </p>
043 *
044 * @param <T> the type of object to apply this command.
045 * @see SequencesComparator
046 * @see EditScript
047 * @since 4.0
048 */
049public abstract class EditCommand<T> {
050
051    /** Object on which the command should be applied. */
052    private final T object;
053
054    /**
055     * Simple constructor. Creates a new instance of EditCommand
056     *
057     * @param object  reference to the object associated with this command, this
058     *   refers to an element of one of the sequences being compared
059     */
060    protected EditCommand(final T object) {
061        this.object = object;
062    }
063
064    /**
065     * Accept a visitor.
066     * <p>
067     * This method is invoked for each command belonging to
068     * an {@link EditScript EditScript}, in order to implement the visitor design pattern
069     *
070     * @param visitor  the visitor to be accepted
071     */
072    public abstract void accept(CommandVisitor<T> visitor);
073
074    /**
075     * Returns the object associated with this command.
076     *
077     * @return the object on which the command is applied
078     */
079    protected T getObject() {
080        return object;
081    }
082
083}