整数等于isEmpty（）吗？

Question

Below is the script I have at the moment

import java.util.Arrays;
import java.util.Scanner;

public class SeeWhatTo 
{   
     public static void main(String args[]) {
       Scanner scan = new Scanner(System.in); //define scan  
       int a = scan.nextInt();
       int sum =0;
           while (a>0 )
     {                   
            sum = sum +a;
            a = scan.nextInt();    
     }
        System.out.println(sum);   //print out the sum
    }    
}

Currently, it stores an input value in a and then adds it to sum and once a negative or zero is given as an input, it suspends itself and outputs the sum.

I was wondering if there's an integer equivalent of isEmpty so that i can do while (! a.isEmpty() ) so when there's no input but an enter, then it would stop and prints out the sum.

A natural followup from that would be, is there a way to assign an input integer to a and check if it is empty or not at the same time in the while condition as in while ( ! (a=scan.nextInt()).isEmpty() )

Answer 1

There isn't an equivalent in the sense that you describe, since String is a variable-length collection of characters, and having zero characters is still a valid String. One integer cannot contain zero integers, since by definition, it is already an integer.

However, your problem revolves around how Scanner works, rather than how int works.

Take a look at scan.hasNextInt() , which returns true if there is an int to read, and false otherwise. This may give you what you want, using something like:

Scanner scan = new Scanner(System.in);
int sum = 0;
while(scan.hasNextInt())
{
    int a = scan.nextInt();
    sum = sum + a;
}
System.out.println(sum);

Answer 2

Scanner can do 2 things:

• Read line-by-line ( nextLine ).
• Read token-by-token ( next or eg nextInt ).

These are really two different functionalities of Scanner , and if you're reading tokens then your Scanner basically doesn't know about empty lines.

If you call nextInt , Scanner does two things:

• Finds the next token (default: delimited by any whitespace).
• Tries to turn it in to an int .

The tokenizing behavior is an important feature of Scanner . If you enter 1 2\\n and call nextInt twice, you get 1 and 2 . However, if you enter an empty line, the tokenizing Scanner just skips it as whitespace and keeps looking for another token.

So the straightforward answer is "no": you can never get an "empty" int from a call to nextInt in a simply way and still retain the token-by-token behavior. (That's beyond the fact that a primitive variable in Java can't be "empty".)

One easy way to do what you're asking is to use line-by-line reading instead and call parseInt yourself:

Scanner systemIn = new Scanner(System.in);
int sum = 0;

String line;
while (!(line = systemIn.nextLine()).isEmpty()) {
    sum += Integer.parseInt(line);
}

But you lose the tokenizing behavior. Now, if you enter 1 2\\n , an exception is thrown because nextLine finds 1 2 .

You can still read token-by-token with nextInt , but it's more complicated, using a second Scanner :

Scanner systemIn = new Scanner(System.in);
int sum = 0;

String nextLine;
while (!(nextLine = systemIn.nextLine()).isEmpty()) {
    Scanner theInts = new Scanner(nextLine);

    while (theInts.hasNextInt()) {
        sum += theInts.nextInt();
    }
}

Here, we can enter 1 2\\n , get 1 2 as our next line, then ask the second Scanner to tokenize it.

So yes, you can program the functionality you're looking for, but not in an easy way, because Scanner is more complicated.

edit

Possibly another way is to use a delimiter on the line separator:

// use System.getProperty("line.separator") in 1.6
Scanner systemIn = new Scanner(System.in).useDelimiter(System.lineSeparator());
int sum = 0;

while (systemIn.hasNextInt()) {
    sum += systemIn.nextInt();
}

Now, nextInt tokenizes the same way as nextLine . This will break the loop for any input that's not an int , including empty tokens. (Empty tokens aren't possible with the default delimiter.) I'm never really sure if people actually expect Scanner 's default delimiting to work the way it does or not. It's possible creating a Scanner in this way makes it behave closer to what people seem to expect for reading the console, just line-by-line.