CODEWITHSUNDEEP
java
VDG
VDG

Reputation: 83

How to properly close a Scanner class inside a method?

I have this code that calls to a method in order to check if a number is a certain length, and if not, a new number should be reintroduced by the user.

Problem here is, I can't find the proper way to close a Scanner class in order to prevent a resource leak.

Here's the code so far.

    public static void setIdentification(Person p, int dni) {
    Scanner input = new Scanner (System.in);
    String lengthChecker = Integer.toString(dni);
    if (lengthChecker.length() < 1 || lengthChecker.length() > 8) {
        int dni1; 
        do{
            System.out.println("The ID number isn't valid. Please, introduce a valid number: ");
            dni1 = input.nextInt();
            lengthChecker = Integer.toString(dni1);
            } while (lengthChecker.length() > 8 || lengthChecker.length() < 1 );
        p.dni = dni1;
    } else {
        p.dni = dni;
    }
}
public static void main(String[] args) {
    Scanner input1 = new Scanner (System.in);
    int dni = input1.nextInt();
    Person person1 = new Person();
    setIdentification(person1, dni);
}

I have tried to set the input.close(); in a number of different locations but I always end up getting a run-time error.

There's probably a million ways to optimize this code but right now I'm just really curious as to how to get those Scanners closed.

Upvotes: 2

Views: 2805

Answers (2)

Stephen C
Stephen C

Reputation: 719249

@Jules is correct. In this case, it is neither necessary or advisable to close the Scanner.

But assuming that you did, then the recommended way to do it is this:

try (Scanner input1 = new Scanner (System.in)) {
    int dni = input1.nextInt();
    Person person1 = new Person();
    setIdentification(input1, person1, dni);
}

... noting that you must change your setIdentification method so that it doesn't attempt to open its own scanner. Creating multiple scanners for the same input stream is a mistake. Indeed, it is the mistake in your code.

This works for Java 7 and later. (Under most circumstances, you should not be writing new code for older versions of Java. Android is an exception, because support for Java 7 extensions has only recently become available in Android toolchains.

So why can't you open and close multiple scanners on the same stream? Two reasons:

  1. When you close a Scanner, you automatically close the underlying stream. That means if you then attempt to open / use another scanner on the stream, it will fail when you attempt to read from the closed stream. Streams cannot be reopened.

  2. Even if you don't close the scanner / stream, creating a second scanner on a stream is likely to lead to trouble. A scanner has to read ahead in order to figure out where the token boundaries are. It then keeps any read-ahead characters in an internal buffer. Each scanner has its own buffer. So if you have two or more scanners for the same stream, and interleave their use, one scanner is liable to "grab" characters that the other scanner needs.

Upvotes: 4

Jules
Jules

Reputation: 15199

Closing your scanner will close the input stream it was created using. In your case, this is System.in. This stream is a special case: it is opened by the environment before your program starts running, and therefore should not usually be closed by your program.

In this case, it's fine to not close your Scanner. Just let the garbage collector deal with it.

Upvotes: 4

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