NullPointerException与扫描仪 [英] NullPointerException with Scanner

问题描述

我在第5行收到NullPointerException.我不太确定为什么或如何解决它...

I am getting a NullPointerException on the 5th line. I am not really sure why or how to fix it...

    public static Scanner getInputScanner(Scanner console){
    Scanner inputFile = new Scanner(System.in);
    Scanner file = null;
    String userInputFile = null;
    while (file.equals(null)) {
        try {
            System.out.print("Enter input file: ");
            userInputFile = inputFile.nextLine();
            file = new Scanner(new File(userInputFile));
        } catch (FileNotFoundException e) {
            System.out.print(userInputFile + " (No such file or directory)");
            file = null;
            return file;
        }
    }
    return file;
}

有指针吗?

推荐答案

尽管通常使用equals方法比较Java对象，但null比较是一个明显的例外:没有Java Object将equal与null-Java类需要满足此要求:

Although you generally compare Java objects using the equals method, null comparison is a notable exception: no Java Object compares equal to null - Java classes need to satisfy this requirement:

对于任何非空参考值x，x.equals(null)应该返回false.

此外，file是以null开头的，因此调用其中的任何方法(包括equals)都将导致NPE.

Moreover, file is null to start with, so calling any methods on it, including equals, will result in NPE.

因此，您需要改用引用相等:

Therefore, you need to use reference equality instead:

while (file == null) {
    ...
}

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