在Java中使用try / catch打开文件有替代方法吗？

Question

I remember in my c++ class we would use the following code to handle bugs when opening a file:

ifstream in;
in.open("foo.txt");
if(in.fail()){
   cout << "FAILURE. EXITING..." << endl;
   exit(0);
}

Now that I'm learning java , I'm having trouble using try/catch statements, because when I create a scanner to read my input file, it isn't recognized outside of that try code block. Is there any equivalent to fail() and exit(0) in java , or is there a better method?

Answer 1

I'm having trouble using try/catch statements, because when I create a scanner to read my input file, it isn't recognized outside of that 'try' code block.

Good! You shouldn't be using it outside your try block. All of the relevant processing of the file should be inside the try block, eg:

try (
    InputStream istream = new FileInputStream(/*...*/); // Or Reader or something
    Scanner scanner = new Scanner(istream)
    ) {
    // Use `scanner` here
}
// Don't use `scanner` here

(That's using the newish try-with-resources.)

In the above, I'm assuming when you said Scanner, you were specifically talking about the Scanner class.

Answering your actual question: No, that's just not the standard practice in Java code. Java embraces exceptions.

Answer 2

To have the Scanner be visible outside of the try...catch block, just declare the variable outside of the block:

Scanner scanner = null;

try {
    scanner = ... //Initialize scanner
} catch (IOException e) {
    //Catch
}

Now you can use your scanner object outside of the try...catch block. To check if the initialization was successful you can check against null if you really have to, but usually error handling should be done inside the catch block, eg

try {
    scanner = ... //Initialize scanner
} catch (IOException e) {
    System.out.println("Failure. Exiting");
    exit(1);
}

Answer 3

You can add throws Exception to the method in which you are using scanner.

void method_name() throws Exception{definition of method}

By this, method knows about some part of code will throw exception and needed to be handled.