Java扫描仪良好做法

Question

I am reading from a file, and I want to handle the exceptions using try/catch blocks. I wrote my code. However, eclipse gave me an error with just declaring my scanner object, and I had to initialize it to null. Below I wrote two versions of my code. Which is considered a better practice? Also, is it better to use try/catch block than just throw the exception at the end of the constructor/function header?

Code version #1:

java.util.Scanner in = null;

try {
    in = new java.util.Scanner (f);
    /* use scanner */
} catch (FileNotFoundException e) {
    System.err.println("File was not found. Make sure the file exist.");
    System.err.println("Message: " + e.getMessage());
} catch (IOException e) {
    System.err.println("File could not be opened.");
    System.err.println("Message: " + e.getMessage());
} finally {
    in.close();
}

Code version #2:

try {
    java.util.Scanner in = new java.util.Scanner (f);
    /* use scanner */
    in.close();
} catch (FileNotFoundException e) {
    System.err.println("File was not found. Make sure the file exist.");
    System.err.println("Message: " + e.getMessage());
} catch (IOException e) {
    System.err.println("File could not be opened.");
    System.err.println("Message: " + e.getMessage());
}

Answer 1

None of the snippets is right. The first one will cause a NullPointerException if the Scanner constructor throws an exception, because it doesn't check for null before invoking close().

The second one fails to close the scanner in case of any exception in the try block.

The first one is closer to the correct code than the second one, obviously.

Regarding what is best regarding the exception, it's a matter of correctness. If the method can handle the exception correctly (and that means the program will continue to work as expected after the exception is handled), it should catch it. If it can't, it should propagate it to the caller and let it handle it.

Answer 2

Java 7 introduced 'Try with Resources' to help prevent memory leaks caused by resources left open in instances like code version #2, bonus: it's simple to use.

    try( Scanner in = new Scanner( System.in ) ) {

    }

And done. The above will automatically close the Scanner after the braces, no matter what happens inside the braces.

Catch and Finally blocks may appear in conjunction with Try with Resources, as usual, but don't have to.

Answer 3

In code version #2 if exception will be thrown before in.close(); file wont be closed. Version #1 doesn't have that problem.

You can combine these two version using try with resource

try (java.util.Scanner in = new java.util.Scanner (new File("your file"))){
    /* use scanner */
} catch (FileNotFoundException e) {
    System.err.println("File was not found. Make sure the file exist.");
    System.err.println("Message: " + e.getMessage());
} catch (IOException e) {
    System.err.println("File could not be opened.");
    System.err.println("Message: " + e.getMessage());
}
//"in" will be closed automatically 

Answer 4

Well, I think the second version seems helpless. If it does throw some exception in the try block, this code won't run.

in.close();

Why not put it in a finally block?