Question

我正在编写一个程序，需要打开一个文件，添加它，然后关闭它。出于某种原因，当我尝试这样做时，我得到一个nullPointerException。以下是我在主要课程中的内容：

public static filer f = new filer();

然后：

f.addStuff("hi");
f.closeFile();

现在这里是我在文件类中的内容，我认为现在的问题是：公共类文件管理器{

private static Formatter f;//to add to file
private static Scanner s; //to read file
public static File file;
private static boolean ftf = false;

public static void openFile() {
    try{ //exception handling
         file = new File("jibberish.txt");
          //shows if file created
         System.out.println("File created: "+ ftf);
         // deletes file from the system
         file.delete();
         // delete() is invoked
         System.out.println("delete() method is invoked");
         // tries to create new file in the system
         ftf = file.createNewFile();
         // print
         System.out.println("File created: "+ ftf);

        Formatter f = new Formatter(file);
        Scanner s = new Scanner(file);
        FileReader r = new FileReader(file);
        /*
        f = new Formatter("jibberish.txt");  //will automatically create jibberish.txt if no exist
        s = new Scanner("jibberish.txt");
        */  //don't us these because scanner will take type string, easier to clear all other way
    }
    catch(IOException ioe){ 
        System.out.println("Trouble reading from the file: " + ioe.getMessage());
    }
}

public static void addStuff(String toAdd){
    f.format("%s ", toAdd);
    System.out.printf("%s added", toAdd);
}

public static void closeFile(){ //need to do this to avoid errors
    f.close();
    s.close();
}

其他课程都有效，我有所有适当的进口和东西哦，当然这是控制台出来的东西：

File created: false
delete() method is invoked
File created: true
Exception in thread "main" java.lang.NullPointerException
at filer.addStuff(filer.java:48)
at transMain.main(transMain.java:40)

Answer 1

您收到NullPointerException因为当您调用addStuff方法时，f尚未初始化，因此null。在对null对象的引用上调用方法将导致NullPointerException。

问题在于openFile中的以下行。您正在创建一个名为f的新局部变量，该变量隐藏了在类级别声明的名为f的字段：

Formatter f = new Formatter(file);

更改以上行，以便类级别字段f是在openFile中初始化的字段：

f = new Formatter(file);

Java文件格式化正确的顺序

1 个答案: