Question

使用以下代码从文件中读取最后四行但是将第一行返回为null为什么？如何解决这个问题呢？请帮我？

public void read(){

    StringBuilder text = new StringBuilder();
    File sdCard = Environment.getExternalStorageDirectory();
    File dir = new File (sdCard.getAbsolutePath()+File.separator+"GPS");
    dir.mkdirs();
    String fname =  "gps.txt";
    File file = new File (dir, fname);
    String[] last4 = new String[4];
    int count=0;
    try {
        BufferedReader br = new BufferedReader(new FileReader(file));
        while(br.ready()){
            last4[count++%4]=br.readLine();

        }
        for (int i=0; i<4;i++){
            text.append(last4[(i+count)%4]);
            text.append('\n');

        }
        br.close();
    }
    catch (IOException e) {
        e.printStackTrace();

    }

}

Answer 1

您必须检查文件是否存在。根据您的代码复制粘贴，

public void read() throws FileNotFoundException{

    StringBuilder text = new StringBuilder();
    String fname =  "asdf.txt";
    String path = Environment.getExternalStorageDirectory()+File.separator+"GPS"+File.separator+fname;

    //You have to check if file exists
    File file = new File(path);
    if(!file.exists()){
        //TODO do smth if your file doesnt exist
        return;
    }

    BufferedReader br = null;
    String[] last4 = new String[4];
    int count=0;
    try {

        String sCurrentLine;

        br = new BufferedReader(new FileReader(path));

        while ((sCurrentLine = br.readLine()) != null ) {
            String str = sCurrentLine;
            last4[count%4] = str;
            count++;

        }

         for (int i=0; i<4;i++){
                text.append(last4[i]);
                text.append('\n');

            }
        System.out.println(text.toString());

    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            if (br != null)br.close();
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

}

Answer 2

试试这个。

private static BufferedReader innerReader;
private static BufferedReader innerReader1;
private static final int UNTIL_LINE = 4;

public static int countLine(Reader reader) throws IllegalArgumentException
{
    int countLine = 0;
    if(reader == null)
    {
        throw new IllegalArgumentException("Null reader");
    }
    String line;
    innerReader1 = new BufferedReader(reader);
    try
    {
        while((line = innerReader1.readLine()) != null)
        {
            countLine++;
        }
    }catch(IOException e){}
    return countLine;
}

public static List<String> loadFile(Reader reader, int countLine)
        throws IllegalArgumentException{
    List<String> fileList = new ArrayList<String>();
    if(reader == null)
    {
        throw new IllegalArgumentException("Null Reader");
    }
    String line;
    int thisLine = 0;
    innerReader = new BufferedReader(reader);
    try
    {
    while((line = innerReader.readLine()) != null)
    {
        if (line == null || line.trim().isEmpty())
            throw new IllegalArgumentException(
                    "Line Empty");

        thisLine++;
        if(thisLine > countLine-UNTIL_LINE)
        {
            fileList.add(line);
        }
    }
    } catch (IOException e) {
    }
    return fileList;
}

测试代码。

    int lines = 0;
    List<String> test = new ArrayList<String>();
    try {
        lines = countLine(new FileReader("YourFile.txt"));
    } catch (IOException e1) {
        e1.printStackTrace();
    }
    try {
        test = loadFile(new FileReader("YourFile.txt"), lines);
    } catch (IOException e) {
        e.printStackTrace();
    }
    for(String s : test)
    {
        System.out.println(s);
    }

完整文件示例：

1行

2行

3行

4行

5行

6行

结果：

3行

4行

5行

6行

Answer 3

您确定该文件存在且您具有读取权限吗？在阅读之前创建父目录很奇怪。检查logcat以查找堆栈跟踪。另外，请确保您在"android.permission.WRITE_EXTERNAL_STORAGE"中声明了AndroidManifest权限。

我快速写了下面的方法。应该适合你。

public static String[] tail(File file, int tail) throws IOException {
    BufferedReader reader = new BufferedReader(new FileReader(file));
    List<String> lines = new ArrayList<>();
    for (String line = reader.readLine(); line != null; line = reader.readLine()) {
        lines.add(line);
    }
    reader.close();
    return lines.subList(Math.max(0, lines.size() - tail), lines.size()).toArray(new String[tail]);
}

使用示例：

try {
    File gps = new File(Environment.getExternalStorageDirectory(), "GPS/gps.txt");
    String[] lastFourLines = tail(gps, 4);
} catch (IOException e) {
    // Are you sure the file exists?
    // Did you declare "android.permission.WRITE_EXTERNAL_STORAGE" permission?
}

如何从文件中读取最后四行？

3 个答案: