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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.commons.collections4.sequence;

/**
 * Abstract base class for all commands used to transform an objects sequence
 * into another one.
 * 
 * When two objects sequences are compared through the
 * {@link SequencesComparator#getScript SequencesComparator.getScript} method,
 * the result is provided has a {@link EditScript script} containing the commands
 * that progressively transform the first sequence into the second one.
 * 

 * There are only three types of commands, all of which are subclasses of this
 * abstract class. Each command is associated with one object belonging to at
 * least one of the sequences. These commands are {@link InsertCommand
 * InsertCommand} which correspond to an object of the second sequence being
 * inserted into the first sequence, {@link DeleteCommand DeleteCommand} which
 * correspond to an object of the first sequence being removed and
 * {@link KeepCommand KeepCommand} which correspond to an object of the first
 * sequence which equals an object in the second sequence. It is
 * guaranteed that comparison is always performed this way (i.e. the
 * equals method of the object from the first sequence is used and
 * the object passed as an argument comes from the second sequence) ; this can
 * be important if subclassing is used for some elements in the first sequence
 * and the equals method is specialized.
 *
 * @see SequencesComparator
 * @see EditScript
 *
 * @since 4.0
 */
public abstract class EditCommand {

    /** Object on which the command should be applied. */
    private final T object;

    /**
     * Simple constructor. Creates a new instance of EditCommand
     *
     * @param object  reference to the object associated with this command, this
     *   refers to an element of one of the sequences being compared
     */
    protected EditCommand(final T object) {
        this.object = object;
    }

    /**
     * Returns the object associated with this command.
     *
     * @return the object on which the command is applied
     */
    protected T getObject() {
        return object;
    }

    /**
     * Accept a visitor.
     * 
     * This method is invoked for each commands belonging to
     * an {@link EditScript EditScript}, in order to implement the visitor design pattern
     *
     * @param visitor  the visitor to be accepted
     */
    public abstract void accept(CommandVisitor visitor);

}