File input java, scanner using if.. else statment. error provided

Question

Ok guys so i am trying to read in from 1 out of 3 files normally with out the if ... else clause

just Scanner myScanner = new Scanner(new File("maze1.txt")); would run the problem, but i want the user to select what maze he/she would like to run

public class Main {
    private static Scanner myScanner;

    public static void main(String[] args) throws FileNotFoundException {

        System.out.println("Please Enter 1,2 or 3 to pick the different maze you would like being solved.");
        Scanner myScannerinput = new Scanner(System.in);



        int Mazenumber = myScannerinput.nextInt();

        if(Mazenumber == 1){
            Scanner myScanner = new Scanner(new File("maze1.txt"));
        }
        else if(Mazenumber == 2){
            Scanner myScanner = new Scanner(new File("maze2.txt"));
        }
        else if(Mazenumber == 3){
            Scanner myScanner = new Scanner(new File("maze3.txt"));
        }
        else{
            System.out.println("You did not choose one of the 3 mazes");
        }



        int numRows = myScanner.nextInt();
        int numCols = myScanner.nextInt();
        myScanner.nextLine();

        int startX = 0;
        int startY = 0;'

Please Enter 1,2 or 3 to pick the different maze you would like being solved.

Exception in thread "main" java.lang.NullPointerException
at Main.main(Main.java:33)

The line myScanner.nextLine(); should be generating a compiler error, because myScanner's scope each time is only within the if and else if blocks. — rgettman
– rgettman, Commented Oct 4, 2013 at 23:59
Notice that you've declared myScanner as an automatic local 3 times, with none of those declarations accessible outside the if ladder. — Hot Licks
– Hot Licks, Commented Oct 5, 2013 at 0:15
@rgettman - It doesn't create a compile error because he's also declared myScanner as a static. — Hot Licks
– Hot Licks, Commented Oct 5, 2013 at 0:16
(You shouldn't make variables that are essentially local to a method "static". They should be declared inside the method.) — Hot Licks
– Hot Licks, Commented Oct 5, 2013 at 0:17
@HotLicks I didn't see the static variable myScanner before. Thanks for pointing that out. — rgettman
– rgettman, Commented Oct 5, 2013 at 0:20

Prabhakaran Ramaswamy · Accepted Answer · 2013-10-05 00:01:30Z

1

don't create individual new Scanner inside the if condition. Create one Scanner before the if condition and assign the file based on your preference.

        int Mazenumber = myScannerinput.nextInt();
        Scanner myScanner =null;
        if(Mazenumber == 1){
            myScanner = new Scanner(new File("maze1.txt"));
        }
        else if(Mazenumber == 2){
            myScanner = new Scanner(new File("maze2.txt"));
        }
        else if(Mazenumber == 3){
            myScanner = new Scanner(new File("maze3.txt"));
        }
        else{
            System.out.println("You did not choose one of the 3 mazes");
        }
        if(myScanner!=null){
           int numRows = myScanner.nextInt();
           int numCols = myScanner.nextInt();
           myScanner.nextLine();
       }

answered Oct 5, 2013 at 0:01

Prabhakaran Ramaswamy

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Comments

Germann Arlington · Accepted Answer · 2013-10-04 23:59:06Z

0

You define and initialise myScanner inside the if block, so it will be out of scope as soon as you leave it

answered Oct 4, 2013 at 23:59

Germann Arlington

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2 Comments

arshajii Over a year ago

His code compiles. The variable declared within the if-statements shadows another variable in the outer scope.

Germann Arlington Over a year ago

@arshajii The variable that the value is assigned to gets out of scope, whatever other variable (not shown in given code anyway) never gets a value.

TheKojuEffect · Accepted Answer · 2013-10-05 00:06:20Z

0

Instead of

 int Mazenumber = myScannerinput.nextInt();

        if(Mazenumber == 1){
            Scanner myScanner = new Scanner(new File("maze1.txt"));
        }
        else if(Mazenumber == 2){
            Scanner myScanner = new Scanner(new File("maze2.txt"));
        }
        else if(Mazenumber == 3){
            Scanner myScanner = new Scanner(new File("maze3.txt"));
        }
        else{
            System.out.println("You did not choose one of the 3 mazes");
        }

Use

Scanner myScanner;
int Mazenumber = 0;
do {
    System.out.println("Choose 1, 2 or 3");
    Mazenumber = myScannerinput.nextInt();
    if(Mazenumber >=1 && Mazenumber <=3) {
         String fileName = "maze"+Mazenumber+".txt";
        myScanner = new Scanner(new File(fileName));
    } else {
           System.out.println("You did not choose one of the 3 mazes");      
     }

} while(Mazenumber>3 && Mazenumber <1);

Exception in thread "main" java.lang.NullPointerException at Main.main(Main.java:33)

The exception is because, you've declared Scanner myScanner inside if-else block.

edited Oct 5, 2013 at 0:06

answered Oct 4, 2013 at 23:58

TheKojuEffect

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4 Comments

arshajii Over a year ago

That won't compile. You can only do the URL trick if some statement or block comes after it. Besides, let's not confuse the OP with that...

TheKojuEffect Over a year ago

@arshajii OP wants with out the if ... else clause. I hope it'll compile. What's the problem with this?

ClearMist Over a year ago

int Mazenumber = myScannerinput.nextInt(); String fileName = "maze"+Mazenumber+".txt"; Scanner myScanner = new Scanner(new File(fileName));

user2800123 Over a year ago

@ClearMist It is a good practice in stackoverflow to close the question if you've got your answer.

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