ArrayList 内容超出范围，并在 Java 中的 While 循环后删除

Question

我正在尝试使用 while 循环将列表列表保存到 ArrayList，该循环在扫描仪中的行上循环。扫描仪正在读取一个 12 行的二进制文本文件。列表列表（ArrayList）创建成功，但是一旦while循环终止变量ArrayList为空，返回一个空的列表列表。我还通过在声明列表列表的同时声明一个计数器来测试代码，并且计数器在 while 循环中递增并在循环后保留数据。

我对编码还是很陌生！提前谢谢你。

public static void main(String[] args) throws Exception{
        
        
        try { 
            readFile();
            data = dataPrep();
        }
        catch (Exception e) {
            e.printStackTrace ();
        }
        
    }
    
    public static void readFile() throws FileNotFoundException {
 
        try {
            File inputtxt = new File("test.txt");
            scanner = new Scanner(inputtxt);
        }
    
        catch (FileNotFoundException error) {
            System.out.println(error);
        }
    }



public static ArrayList<ArrayList> dataPrep(){
        ArrayList<ArrayList> allBinaryNumbers = new ArrayList<ArrayList>();
        ArrayList<Integer> singleBinaryNumber = new ArrayList<Integer>();  
        
        int counter = 0;
      
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine();
            char[] charLine = line.toCharArray();
            
            for (char numb : charLine){
                singleBinaryNumber.add(Integer.parseInt(String.valueOf(numb)));
            }
            allBinaryNumbers.add(singleBinaryNumber);
            System.out.println(allBinaryNumbers); 
            singleBinaryNumber.clear();     
            counter++;
        }
        
        System.out.println(allBinaryNumbers);
        System.out.println(counter);
        return allBinaryNumbers;
        
    }

我的test.txt是这个

Answer 1

您正在重复使用完成填充后清除的同一个 singleBinaryNumber。请记住，这是一个 reference（指针），这意味着您在每次迭代中添加相同的列表而不是新列表。

你的代码应该是这样的：

public static ArrayList<ArrayList> dataPrep(){
        ArrayList<ArrayList> allBinaryNumbers = new ArrayList<ArrayList>();
        
        int counter = 0;
      
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine();
            char[] charLine = line.toCharArray();
            
            ArrayList<Integer> singleBinaryNumber = new ArrayList<Integer>(); // create a new list for each iteration 
            for (char numb : charLine){
                singleBinaryNumber.add(Integer.parseInt(String.valueOf(numb)));
            }
            allBinaryNumbers.add(singleBinaryNumber);
            System.out.println(allBinaryNumbers); 
            // singleBinaryNumber.clear(); <-- remove this line
            counter++;
        }
        
        System.out.println(allBinaryNumbers);
        System.out.println(counter);
        return allBinaryNumbers;
        
    }

Answer 2

我认为更好的方法是让 readFile() 方法做到这一点，读取文件而不是仅仅打开它，并让这个方法 return 一个 ArrayList。它还应该接受一个字符串参数，该参数将是文件路径（带有文件名），而不是直接在方法本身中对文件路径进行硬编码，例如：

// Class instance member variable with Getter & Setter methods.
private String sourceFile; 

// In your main() method:  
// List of Lists. Each internal list is from a different file.
List<List<String>> filesBinaries = new ArrayList<>(); 

// List for current file to be read.
List<String> binaries = readFile(sourceFile);

// Add the current file List object to the 
// List of Lists (filesBinaries).
if (!binaries.isEmpty()) { 
    filesBinaries.add(binaries);
}

这样，您的 readFile() 方法可能看起来像这样：

public static List<String> readFile(String filePath) throws FileNotFoundException {
    File file = new File(filePath);
    if (!file.exists()) {
        throw new FileNotFoundException("readFile() Method Error! The "
                + "supplied file in the path shown below does not exist!" + System.lineSeparator()
                + file.getAbsolutePath() + System.lineSeparator());
    }

    List<String> binaryLines = new ArrayList<>();
    // 'Try With Resources' is used here to auto-close the reader.
    try (Scanner reader = new Scanner(file)) {
        String line = "";
        while (reader.hasNextLine()) {
            line = reader.nextLine().trim();
            // Data Line Validation:
            /* Skip past blank lines (if any) and any lines
            that do not contain a valid binary string. Valid
            lines would be: 100100  or  00100 11100 01110      */
            if (line.isEmpty() || !line.matches("[01 ]+")) {
                continue;
            }
            binaryLines.add(line);
        }
    }
    return binaryLines;
}

要触发此方法，您的 main() 方法可能如下所示：

public static void main(String[] args) {
    List<List<String>> filesData = new ArrayList<>();
    String sourceFile = "BinaryData.txt";  // The CURRENT file to read

    try {
        // Read the desired file...
        List<String> binaryData = readFile(sourceFile);
        // If the binaryData list is not empty then add it to the fileData List.
        if (!binaryData.isEmpty()) {
            filesData.add(binaryData);
        }
    }
    catch (FileNotFoundException ex) {
        System.err.println("The file: \"" + sourceFile + "\" could not be "
                + "found!" + System.lineSeparator() + ex.getMessage());
    }
    
    // Display the contens of the filesData List...
    for (int i = 0; i < filesData.size(); i++) {
         System.out.println("File #" + (i + 1) + " Binary Data:");
         System.out.println("====================");
         for (String binaryString : filesData.get(i)) {
             System.out.println(binaryString);
         }
    }
}

ArrayList 内容超出范围，并在 Java 中的 While 循环后删除

ArrayList contents Out of Scope, and deleted, after a While Loop in Java

java

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