小弟我在读一段原码的时候，看到一个有关问题代码

/*Copyleft (C) 2005 H?lio Perroni Filhoxperroni@yahoo.comICQ: 2490863This file is part of ChatterBean.ChatterBean is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.ChatterBean is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.You should have received a copy of the GNU General Public License along with ChatterBean (look at the Documents/ directory); if not, either write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA, or visit (http://www.gnu.org/licenses/gpl.txt). */package bitoflife.chatterbean.text;import java.util.Arrays;public class Request {    /*     * Attributes     */    private Sentence[] sentences;    private String original;    /*     * Constructor     */    public Request() {    }    public Request(String original) {        this.original = original;    }    public Request(String original, Sentence... sentences)// 可变参数个数    {        this.original = original;        this.sentences = sentences;    }    /*     * Methods     */    public boolean empty() {        return (sentences == null || sentences.length == 0);    }    /**     * 比较相等必须sentences 和 original 2个变量都要相等。     */    public boolean equals(Object obj) {        if (obj == null || !(obj instanceof Request))            return false;        Request compared = (Request) obj;        return original.equals(compared.original)                && Arrays.equals(sentences, compared.sentences);    }        public Sentence lastSentence(int index) {   //////问题在这里？？？        return sentences[sentences.length - (1 + index)];    }     public String toString() {        return original;    }    public String trimOriginal() {        return original.trim();    }    /*     * Properties     */    public String getOriginal() {        return original;    }    public void setOriginal(String original) {        this.original = original;    }    public Sentence[] getSentences() {        return sentences;    }    public Sentence getSentences(int index) {        return sentences[index];    }    public void setSentences(Sentence[] sentences) {        this.sentences = sentences;    }}

public Sentence lastSentence() {           return sentences[sentences.length - 1];    }