Java Scanner doesn't wait for user input

java io java.util.scanner
user2976389 picture user2976389 · May 4, 2014 · Viewed 57k times · Source

I am using Java's Scanner to read user input. If I use nextLine only once, it works OK. With two nextLine first one doesnt wait for user to enter the string(second does).

Output:

X: Y: (wait for input)

My code

System.out.print("X: ");
x = scanner.nextLine();
System.out.print("Y: ");
y = scanner.nextLine();

Any ideas why could this happen? Thanks

Answer

Alexis C. picture Alexis C. · May 4, 2014

It's possible that you are calling a method like nextInt() before. Thus a program like this:

Scanner scanner = new Scanner(System.in);
int pos = scanner.nextInt();
System.out.print("X: ");
String x = scanner.nextLine();
System.out.print("Y: ");
String y = scanner.nextLine();

demonstatres the behavior you're seeing.

The problem is that nextInt() does not consume the '\n', so the next call to nextLine() consumes it and then it's waiting to read the input for y.

You need to consume the '\n' before calling nextLine(). 

System.out.print("X: ");
scanner.nextLine(); //throw away the \n not consumed by nextInt()
x = scanner.nextLine();
System.out.print("Y: ");
y = scanner.nextLine();

(actually a better way would be to call directly nextLine() after nextInt()).

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