java - 从文件构建数组中的 NullPointerException

Question

*以下代码根据文本文件中的字符串构建“2D”数组。目前它正在返回一个 NullPointException 错误:

temp = thisLine.split(delimiter); My question is, am I correct in understanding that temp is returning null? If so, why, and how do I add a check for null? I'm rather new to Java, and this is my first attempt at creating a string array of arrays from a file.*

--------编辑--------

以上问题已解决。

对于那些感兴趣的人，下面是返回 IndexOutOfBoundsException 的代码。 具体来说是这一行:

fileContents.set(i, fileContents.get(i).replace(hexLibrary[i][0], hexLibrary[i][1]));

System.out.println("SnR after this");

    String[][] hexLibrary;    // calls the replaces array from the LibToArray method
    hexLibrary = LibToArray();

    for(int i=0;i<502;i++){
        {
        fileContents.set(i, fileContents.get(i).replace(hexLibrary[i][0], hexLibrary[i][1]));
        }       
        }
    for (String row : fileContents) {
        System.out.println(row);        // print array to cmd
        }

<小时/>

_____________ _________________

<小时/>

    public static String[][] LibToArray()
    {

        String thisLine;  
         String[] temp;  
         String delimiter=",";  
         String [][] hexLibrary = new String[502][2];  
    try
        {
            BufferedReader br= new BufferedReader(new FileReader("hexlibrary.txt"));  
            for (int j=0; j<502; j++) {  
                    thisLine=br.readLine(); 
                    temp = thisLine.split(delimiter);  
             for (int i = 0; i < 2; i++) {  
                hexLibrary[j][i]=temp[i];  
             }  
            }   
        }
        catch (IOException ex) {    // E.H. for try
        JOptionPane.showMessageDialog(null, "File not found.  Check name and directory."); // error message
        }
    return hexLibrary;
    }

Answer 1

thisLine 更有可能为 null。如果您在读取 ​​502 行之前用完输入，就会发生这种情况。如果 thisLine 不是 null，则 thisLine.split(delimiter) 将不会返回 null。您应该始终检查 null 行:

for (int j=0; j<502; j++) {  
    thisLine=br.readLine(); 
    if (thisLine != null) {
        temp = thisLine.split(delimiter);  
        for (int i = 0; i < 2; i++) {  
            hexLibrary[j][i]=temp[i];  
        }  
    } else {
        // report error: premature end of input file
        break; // no point in continuing to loop
    }
}

就我个人而言，我会编写您的方法来不假设任何特定的文件长度:

public static String[][] LibToArray() {
    List<String[]> lines = new ArrayList<>();
    String delimiter=",";  
    try (BufferedReader br= new BufferedReader(new FileReader("hexlibrary.txt"))) {
        String line = br.readLine();
        while (line != null) {
            String[] tmp = line.split(delimiter);
            // the next line is dangerous--what if there was only one token?
            // should add a check that there were at least 2 elements.
            lines.add(new String[] {tmp[0], tmp[1]});
            line = br.readLine();
        }
    } catch (IOException ex) {
        JOptionPane.showMessageDialog(null, "File not found.  Check name and directory.");
    }
    String[][] hexLibrary = new String[lines.length][];
    lines.toArray(hexLibrary);
    return hexLibrary;
}

(上面使用了新的 Java 7 try-with-resources syntax 。如果您使用的是早期的 Java，则应该在方法之前添加一个 finally 子句来关闭 br返回。