java - Trouble with order of reading next line with Scanner

I am doing some basic java program using 'Scanner'. I read Integer, Double and String.

I have some issues with using scanner for String with other scanners like int and double.

Declaration part:

    Scanner scan = new Scanner(System.in);

    int i2;
    double d2;

    String s2;

Order#1:

    i2 = scan.nextInt();
    d2 = scan.nextDouble();
    s2 = scan.nextLine();     

Result:
Compiler waits to get input for i2 and d2 but not waiting for input for s2. It execute line after s2 = scan.nextLine(); instantly. When i debug, s2 is empty.

Order#2:

    i2 = scan.nextInt();
    s2 = scan.nextLine();
    d2 = scan.nextDouble();     

Result:
Compiler waits to get input for i2 and s2 this time. When i give input hello it throws an error.

1
hello
Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Scanner.java:864)
    at java.util.Scanner.next(Scanner.java:1485)
    at java.util.Scanner.nextDouble(Scanner.java:2413)
    at HelloWorld.main(HelloWorld.java:18)

Order#3:

    s2 = scan.nextLine();
    i2 = scan.nextInt();
    d2 = scan.nextDouble();     

Result:

works fine !!

So why order is playing role here?

Answer

The difference in execution, with change in orders, is due to the fact that new-line is not consumed by nextInt(), nextDouble(), next() or nextFoo() methods.

Therefore, whenever you place a call to nextLine() after any of those methods, it consumes that newline, and virtually skips over that statement.

The fix is simple, don't use nextFoo() methods before nextLine(). Try :-

i2 = Integer.parseInt(scan.nextLine());
d2 = Double.parseDouble(scan.nextLine());
s2 = scan.nextLine();

Or else, you could consume the new-line by

i2 = scan.nextInt();
d2 = scan.nextDouble();
scan.nextLine(); //---> Add this before the nextLine() call

s2 = scan.nextLine();

Order#3 works fine, as nextLine() is the first statement, and therefore, there are no left-over characters to consume.

Related: Scanner is skipping nextLine() after using next(), nextInt() or other nextFoo() methods