JAVA - Method works for 1st call, but not for 2nd

Question

I have a method called isValidFile which takes in a user input string and determines if that is a valid file. The method output is of the File type. I am able to take in 2 file names in the command line and process them properly. I am also able to take in 1 valid, and 1 invalid. The isValidFile will check validity and promt the user for a valid file name if the one entered is not correct. However, it will not work with 2 invalid inputs. I call them as follows....

    String[] inLines = commandLine(args);
    File reservedWords = isValidFile(inLines[0]);
    File toParse = isValidFile(inLines[1]);

Here is my isValidFile method...

public static File isValidFile(String userFile){
    System.out.println("START OF isValidFile");
    File inFile = new File(userFile);
    Scanner in = null;
    while(!inFile.exists() || inFile.isDirectory()){
        System.out.println("Please enter a valid file name or Q to quit.");
        in = new Scanner(System.in);
        String validFile = in.nextLine();
        System.out.println("IS VALID FILE? " + validFile);            // TEST LINE
        if(validFile.equalsIgnoreCase("q")){  
            System.out.println("\nProgram terminated by user.");
            in.close();
            return null;
        } else inFile = new File(validFile);
        if (inFile.exists()) in.close();
    }
    System.out.println("END OF isValidFile\n");
    return inFile;
}

You can see where I put in test lines to show the start, end, and the file name printed out if it is accepted. Here is my output. You can see that once I entered a valid file for the first argument, it asks for the file name of the 2nd argument but does not allow me to enter any information. It just throws a NoSuchElementException. Why would it work for the first call to the method and not for the second call?

START OF isValidFile

Please enter a valid file name or Q to quit.

in.txt

IS VALID FILE? in.txt

END OF isValidFile

START OF isValidFile

Please enter a valid file name or Q to quit.

Exception in thread "main" java.util.NoSuchElementException: No line found

at java.util.Scanner.nextLine(Unknown Source)

at Project6.isValidFile(Project6.java:53)

at Project6.main(Project6.java:9)

--- Line 53 is the line: String validFile = in.nextLine();

--- Line 9 is the line: File toParse = isValidFile(inLinse[1]);

You should initialize the `Scanner` before the loop, not inside. And don't close it ! — Arnaud, Apr 14 '16 at 15:44
Dont' close scanner.. That will make it close `System.in`... And you can't take anymore input... — Codebender, Apr 14 '16 at 15:46
commandLine(args) is my method that takes in the arguments from the command line when the program is run. — Misspelled99, Apr 14 '16 at 18:13

score 0 · Answer 1 · answered Apr 14 '16 at 16:38

You do not want to close the Scanner, b/c it will close System.in. System.in (i.e. the standard input) is shared by the entire process, so each method call to isValidFile is reading from the same standard input. If it is closed at the end of the first call to isValidFile, it will fail the next time it is called. Thus, it works for the first input, but not the second.

public File isValidFile(String userFile){
    System.out.println("START OF isValidFile");
    File inFile = new File(userFile);
    Scanner sc = new Scanner(System.in);
    while(!inFile.exists() || inFile.isDirectory()){
        System.out.println("Please enter a valid file name or Q to quit.");
        String validFile = sc.nextLine();
        System.out.println("IS VALID FILE? " + validFile);            // TEST LINE
        if(validFile.equalsIgnoreCase("q")){  
            System.out.println("\nProgram terminated by user.");
            return null;
        } else inFile = new File(validFile);
    }
    System.out.println("END OF isValidFile\n");
    return inFile;
}

JAVA - Method works for 1st call, but not for 2nd

1 Answers1