java - Why does input.nextint method have a as a leftover?

Question

In the answer to this question I don't understand why the input.nextInt has a newline character as a leftover.

Scanner is skipping nextLine() after using next(), nextInt() or other nextFoo()?

Answer 1

The answer is quite simple:

Scanner#nextInt() reads the next integer but not the newline (ENTER) the user presses to submit the integer.

To understand this you must know that everything you type will be written to a buffer, from which the Scanner methods try to read from.

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Regard the following example:

System.out.println("Enter a number: ");
int i = scanner.nextInt();
System.out.println("Enter a line: ");
String l = scanner.nextLine();

What will happen:

• The user sees Enter a number: and will press 17 and ENTER.
• The scanner will read the 17 but leave the newline of the ENTER.
• The program will write Enter a line: but the scanner will read the newline of the ENTER instead of waiting for input.