Java Scanner not working, difference between next() and nextLine()

When I was a newbie programmer, I used to be frustrated a lot every time I used the Scanner class. At the time, it seemed like it behaved erratically and no matter how much debugging I did, it was always wrong.

The mistake I was making was that I failed to realize that whenever I used a .next call, like nextInt or nextDouble, the Scanner would stay on the same line. If then, I wanted to scan a new input that was on a different line, the Scanner will appear to skip it. What happens is that the Scanner is actually scanning the invisible newline character!

So, whenever you want to Scan numbers with a .next() call, make sure you add an empty .nextLine() at the end so that the Scanner moves to the correct position.

Ex:


import java.util.Scanner; 

class myClass {
    public static void main(String[] args) {
        Scanner scn = new Scanner(System.in);
        double grade=0;
        String name=null;
        System.out.print("Please enter the grade: ");
        grade = scn.nextDouble(); //Could also be scn.next(), scn.nextInt(), and so on
        //Add an empy .nextLine() to skip the invisible newline character
        scn.nextLine(); //Try commenting this line out with '//' to see what happens
        System.out.print("Please enter name: ");
        name = scn.nextLine();
        System.out.println("Name: "+name+", grade: "+grade);
    }
}

Sample run without the empty scn.nextLine:

Please enter the grade: 76
Please enter name: Name: , grade: 76.0

Sample run with the empty scn.nextLine:

Please enter the grade: 100
Please enter name: Rommel
Name: Rommel, grade: 100.0

Beautiful!

« Previous Post

aga
8 Nov, 2011

Thank you very much, man! I spend about one hour, trying to figure out what’s going on and why I’m getting continuous loop, while working with Scanner.next(). You saved me a lot of time, so thank you again.
Rommel
9 Nov, 2011

Glad I was able to help, aga.
Dominic
7 Dec, 2011

Thanks, finally got this working. Teacher says: ‘Think logically’. This makes no sense! Why isn’t it working! No logic is present.
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5 May, 2012

mmmm testing this, i found a error in your code that is “name = scn.nextLine();”….that line is why you get the error…it has to be only name = scn.next(); and you won’t need the scn.nextLine(); below the “scn.nextDouble”