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.text.diff;
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 StringsComparator#getScript StringsComparator.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</code> an object in the second sequence. It is
037 * guaranteed that comparison is always performed this way (i.e. the
038 * <code>equals</code> 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</code> method is specialized.
042 * </p>
043 *
044 * <p>
045 * This code has been adapted from Apache Commons Collections 4.0.
046 * </p>
047 *
048 * @see StringsComparator
049 * @see EditScript
050 *
051 * @param <T> object type
052 * @since 1.0
053 */
054public abstract class EditCommand<T> {
055
056    /** Object on which the command should be applied. */
057    private final T object;
058
059    /**
060     * Simple constructor. Creates a new instance of EditCommand
061     *
062     * @param object  reference to the object associated with this command, this
063     *   refers to an element of one of the sequences being compared
064     */
065    protected EditCommand(final T object) {
066        this.object = object;
067    }
068
069    /**
070     * Returns the object associated with this command.
071     *
072     * @return the object on which the command is applied
073     */
074    protected T getObject() {
075        return object;
076    }
077
078    /**
079     * Accept a visitor.
080     * <p>
081     * This method is invoked for each commands belonging to
082     * an {@link EditScript EditScript}, in order to implement the visitor design pattern
083     *
084     * @param visitor  the visitor to be accepted
085     */
086    public abstract void accept(CommandVisitor<T> visitor);
087
088}