How Scanner.next() interact with user input

Question

I found a serious problem that can be replicated using the following code:

Scanner scanner = new Scanner(System.in);
System.out.println("First entry:");
System.out.println("->" + scanner.next());
System.out.println("Second entry:");
System.out.println("->" + scanner.nextLine());

If the user entry is "a", the call to nextLine() will catch an empty String. I found out why this happen in other posts [1], [2]. The next() call leaves just the '\n' caracter at the end of the line.

What I don't undestand is why the following code works:

System.out.println("First entry:");
System.out.println("->" + scanner.next());
System.out.println("Second entry:");
System.out.println("->" + new Scanner(System.in).nextLine());

Example:

For the input "a Hello World" the first block get the "a" in the next() call and the "Hello World" in the nextLine() call.

But in the second block of code the get the "a" in the next() call but "discard" the "Hello World" in the nextLine() call and waits for the user to promt again

*I know why in the first block of code the nextLine() call gets an empty String

``new Scanner(System.in)`` create a new object that parse from beginning. — mmuzahid, Jan 22 '16 at 05:08
The first Scanner instance "locks" the last line or something like that? It parses from the beginning of what exactly? — Rafael Teles, Jan 22 '16 at 05:10
@NayukiMinase this is not a duplicated from that question, I just didn't have a better title to use — Rafael Teles, Jan 22 '16 at 05:20

score 0 · Answer 1 · answered Jan 22 '16 at 05:12

When you use Scanner.next(), you need to be careful about pressing "enter"/"carriage return" keys. As soon as you do that, scanner will read the input as carriage returned empty line.

If you run your first code sample with inputs

"a" helloWorld

, it should still work.

With your second sample, as you are creating a new Scanner, so it will read the input even if you have pressed enter key

Ajmal Muhammad · Answer 2 · 2016-01-22T05:19:05.037

0

new Scanner(System.in) will create a new object and that parse from beginning.

check the below code .

    Scanner scanner = new Scanner(System.in);
    System.out.println("First entry:");
    System.out.println("->" + scanner.nextLine());
    System.out.println("Second entry:");
    System.out.println("->" + scanner.nextLine());

that works in both cases.

edited Jan 22 '16 at 05:19

answered Jan 22 '16 at 05:13

Ajmal Muhammad

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How Scanner.next() interact with user input

2 Answers2