Question

我开发的应用程序可以从互联网上获取一些公交车时刻表。

三个班级： MainClass.java， TKLReader.java， Logger.java（仅用于记录）

一个外部图书馆：Jsoup

程序按以下顺序运行：主----＆GT;

-> initApp (inits app)
-> TKLReader.connect (connects to the website)
-> TKLReader.read (reads the information needed and gathers it to list)
-> printTimetable (prints out the timetable)

这是我的问题：我运行程序，它应该连接。我得到的信息，我可以用我的Logger类登录。当我尝试从列表中打印东西时，我（尝试）将信息存储在List<List<String>>但是我收到以下错误：

23:20Exception in thread "Thread-1" java.lang.IndexOutOfBoundsException:    Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(Unknown Source)
at java.util.ArrayList.get(Unknown Source)
at TKLReader.read(TKLReader.java:101)
at MainClass$1.run(MainClass.java:42)
at java.lang.Thread.run(Unknown Source)

我知道列表中有这些项目，因为我可以打印它的大小而不是0.由于某种原因，该程序认为它有0项。

以下是TKLReader.java的代码：

public class TKLReader {

    public static Document doc;

    public static int num_lines;
    public static int num_stops;

    public static final boolean debug = false; //For debugging purpose

    public void connect(final String url, final int num_lines, final int num_stops) {

        //This function connects to the website

        TKLReader.num_lines = num_lines;
        TKLReader.num_stops = num_stops;

        Thread t = new Thread(new Runnable() {

            public void run() {

                try {
                    //When Jsoup connects, it sets the doc variable
                    doc = Jsoup.connect(url).get();
                } catch (IOException e) {
                    Logger.log("TKLHUB", e.getMessage());
                }

            }

        });

        t.start();

    }

    public List<List<String>> read() {

        //Initializing variables...
        List<List<String>> returnList = new ArrayList<List<String>>();
        List<String> tempList = new ArrayList<String>();

        Elements tdList;
        int counter = 0, lineCounter = 0;


        //These int-arrays define the td-elements' indexes on the page. They vary depending if
        //the user selects one or more bus lines.
        int[] tdIndexesSingle = {
                4,6,7,8,9
        };

        int[] tdIndexesMulti = {
                5,7,8,9,10,
                12,14,15,16,17,
                19,21,22,23,24,
                26,28,29,30,31,
                33,35,36,37,38
        };

        //this selects the array to use
        int[] tdIndexes = num_lines == 1 ? tdIndexesSingle : tdIndexesMulti;

        if(doc == null) return null;

        tdList = doc.getElementsByTag("td");

        if(tdList.size() == 0) return null;


        for(int i = 0; i < tdList.size(); i++) {

            String item = tdList.get(i).text();

            if(!debug) {

                if(contains(tdIndexes, i) && lineCounter < num_lines) {

                    tempList.add(item); //here I clearly add an item to the tempList, which is later added to the returnList
                    Logger.log("Added item: " + item); //Logger is a class that I created for logging
                    counter++;  //The tds are in groups of five (number, destination, time1, time2, time3)
                                //When counter hits 5, the loop starts a new list

                    if(counter == 5) {

                        Logger.log("Reset: " + item);
                        Logger.log("tempList Size: " + tempList.size());

                        returnList.add(tempList); //Here I add the tempList
                        tempList.clear(); //and then clear it

                        counter = 0; // and set the counter back to 0
                        lineCounter++; //mark that this line has been handled

                        Logger.log("Size: " + returnList.size()); //I log the size of the returnList

                        if(returnList.size() != 0) {
                            //Logger.log(returnList.get(0).get(0));     //Here I get an error, it says Index 0, Size 0
                                                                    //although I've just added items
                        }

                    }

                }

            } else {

                Logger.log("(" + i + ")" + item); //For debugging

            }

        }

        return returnList; //This returns the list

    }

    public static boolean contains(int[] a, int key) { //Checks whether a specific key is inside an array

        for(int i = 0; i < a.length; i++) {

            if(a[i] == key) return true;

        }

        return false;

    }

}

Logger.java：

public class Logger {

    public static void log(final String tag, final String txt) {
        System.out.println(tag + ": " + txt);
    }

    public static void log(final String txt) {
        System.out.println(txt);
    }

}

请帮帮我！如果需要请求澄清！

Answer 1

问题在于：

returnList.add(tempList); //Here I add the tempList
tempList.clear(); //and then clear it

您不能将tempList的副本放入returnList;你将tempList置于其中。所以当你清除它时，returnList中的项目（同一个对象）也会被清除。

然后，当你拨打这个电话时：

returnList.get(0).get(0)

您有效地致电

tempList.get(0)

哪个无效，因为tempList现在为空。

在tempList循环中移动for的声明，然后移除clear。这将在每次迭代时为您提供一个新的空列表，然后可以安全地添加。

Java List有项目但不允许我访问它们

1 个答案: