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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;

/**
 * This interface should be implemented by user object to walk
 * through {@link EditScript EditScript} objects.
 * 
 * Users should implement this interface in order to walk through
 * the {@link EditScript EditScript} object created by the comparison
 * of two sequences. This is a direct application of the visitor
 * design pattern. The {@link EditScript#visit EditScript.visit}
 * method takes an object implementing this interface as an argument,
 * it will perform the loop over all commands in the script and the
 * proper methods of the user class will be called as the commands are
 * encountered.
 * 

 * The implementation of the user visitor class will depend on the
 * need. Here are two examples.
 * 

 * The first example is a visitor that build the longest common
 * subsequence:
 * 
 * import org.apache.commons.collections4.comparators.sequence.CommandVisitor;
 *
 * import java.util.ArrayList;
 *
 * public class LongestCommonSubSequence implements CommandVisitor {
 *
 *   public LongestCommonSubSequence() {
 *     a = new ArrayList();
 *   }
 *
 *   public void visitInsertCommand(Object object) {
 *   }
 *
 *   public void visitKeepCommand(Object object) {
 *     a.add(object);
 *   }
 *
 *   public void visitDeleteCommand(Object object) {
 *   }
 *
 *   public Object[] getSubSequence() {
 *     return a.toArray();
 *   }
 *
 *   private ArrayList a;
 *
 * }
 * 
 * 
 * The second example is a visitor that shows the commands and the way
 * they transform the first sequence into the second one:
 * 
 * import org.apache.commons.collections4.comparators.sequence.CommandVisitor;
 *
 * import java.util.Arrays;
 * import java.util.ArrayList;
 * import java.util.Iterator;
 *
 * public class ShowVisitor implements CommandVisitor {
 *
 *   public ShowVisitor(Object[] sequence1) {
 *     v = new ArrayList();
 *     v.addAll(Arrays.asList(sequence1));
 *     index = 0;
 *   }
 *
 *   public void visitInsertCommand(Object object) {
 *     v.insertElementAt(object, index++);
 *     display("insert", object);
 *   }
 *
 *   public void visitKeepCommand(Object object) {
 *     ++index;
 *     display("keep  ", object);
 *   }
 *
 *   public void visitDeleteCommand(Object object) {
 *     v.remove(index);
 *     display("delete", object);
 *   }
 *
 *   private void display(String commandName, Object object) {
 *     System.out.println(commandName + " " + object + " ->" + this);
 *   }
 *
 *   public String toString() {
 *     StringBuffer buffer = new StringBuffer();
 *     for (Iterator iter = v.iterator(); iter.hasNext();) {
 *       buffer.append(' ').append(iter.next());
 *     }
 *     return buffer.toString();
 *   }
 *
 *   private ArrayList v;
 *   private int index;
 *
 * }
 * 
 *
 * @since 4.0
 */
public interface CommandVisitor {

    /**
     * Method called when an insert command is encountered.
     *
     * @param object object to insert (this object comes from the second sequence)
     */
    void visitInsertCommand(T object);

    /**
     * Method called when a keep command is encountered.
     *
     * @param object object to keep (this object comes from the first sequence)
     */
    void visitKeepCommand(T object);

    /**
     * Method called when a delete command is encountered.
     *
     * @param object object to delete (this object comes from the first sequence)
     */
    void visitDeleteCommand(T object);

}