java - 为什么调用构造函数后 PrintWriter 仍然为 null？

Question

我正在尝试写入一个文件，这是给图书馆系统中拥有一本书的人的简单注释。

try {
    PrintWriter outFile = new PrintWriter(
        "/Users/Test/Desktop/Java/Notifacation.txt");
} catch (FileNotFoundException fnfe) {
    System.out.println(fnfe);
}
System.out.println("Book currently loaned out and unavailable to other "
    + "members. Returning to the main menu.");
outFile.println("==========================================================="
    + "=======================================================");
outFile.println("Notifacation for " + b.whoIsCurrentLoaner());
outFile.println("We have received a request for the book currently in your posession; " + b.getTitle());
outFile.println("It would be greatly appreciated if you could return that book as soon as possible when you have finished with it");
outFile.println("==========================================================="
    + "=======================================================");
welcome();
outFile.close();

但是它抛出了 NullPointerException，当我使用调试器单步执行代码时，它说 PrintWriter 为 null。

PrintWriter怎么可能是null？我在创建 PrintWriter 时没有遇到 Exception。

Answer 1

你已经shadowed the variable ;来自链接的维基百科文章当在某个范围(决策 block 、方法或内部类)中声明的变量与在外部范围中声明的变量具有相同名称时，就会发生变量遮蔽。注释掉类型在 try block 中。

/* PrintWriter */ 
outFile = new PrintWriter("/Users/Test/Desktop/Java/Notifacation.txt");

编辑

创建文件路径的另一种方法是使用 System.getProperty(String)获取“user.home”，例如

 outFile = new PrintWriter(System.getProperty("user.home") 
     + "/Desktop/Java/Notifacation.txt");

这样您就可以在 Windows、Mac 和/或 Linux 上获取当前登录用户的主文件夹。