How to reject whitespace as input in java

Question

I am trying to create a code that would reject an answer that only contains whitespace/spaces. So far I already made a code to reject empty inputs but once the user inputs spaces, the program accepts it.

How do I prevent this? Thank you!

    do {
        while(true) {
            System.out.print("Dog's name: ");
            String dogName = null;
            dogName = scan.nextLine().toLowerCase();
            String n = dogName.replaceAll("\\s", "");

            if(dogName.isEmpty()) {
                System.out.println("Error: Name can't be empty.");
                continue;   
            }

`n` is your string with the spaces removed. You can just check `if (n.isEmpty())`. — khelwood, Jan 14 '19 at 11:26

Ricola · Answer 1 · 2019-01-14T11:39:29.000

4

The method trim() would be quite handy in this case. From the documentation :

Returns a copy of the string, with leading and trailing whitespace omitted.

You can use it this way:

while (true) {
    System.out.print("Dog's name: ");
    String dogName = scan.nextLine().trim().toLowerCase();

    if (dogName.isEmpty()) {
        System.out.println("Error: Name can't be empty.");
        continue;
    }
}

edited Jan 14 '19 at 11:39

answered Jan 14 '19 at 11:25

Ricola

2,621
12
22

It will remove the leading and trainling white spaces only – JineshEP Jan 14 '19 at 11:29
4

Well if the String only contains whitespaces, then all of the whitespaces are both leading and trailing so the result is an empty String. – Ricola Jan 14 '19 at 11:32
1

This will alter (trim) valid inputs too. In the case of dog names it's *probably* not going to matter, unless you have a dog called i.e. " Fido" with a leading or trailing space. If you use this code, you should be aware of that. – kscherrer Jan 14 '19 at 12:44

score 1 · Answer 2 · answered Jan 15 '19 at 13:51

1

Thank you for the help everyone! I just added .trim() to the code :)

So now it's: String dogName = scan.nextLine().toLowerCase().trim();

answered Jan 15 '19 at 13:51

M Ann

107
1
9

score 0 · Answer 3 · answered Jan 14 '19 at 11:35

0

You may use String.matches to check for empty strings :

           if (dogName.matches("^\\s*$") || dogName.isEmpty()) {
                System.out.println("Error: Name can't be empty.");
                continue;
            }

answered Jan 14 '19 at 11:35

JineshEP

738
4
7

score 0 · Answer 4 · answered Jan 14 '19 at 11:48

dogName.trim() will remove leading and trailing white space, if the entire String is made of white space then this will replace all and leave an empty string. e.g.

" dog " -> "dog"

" dog name " -> "dog name"

If checking for empty/blanks is going to be common then take a look at the StringUtils.isNotBlank for "inspiration" and write your own Static util methods based of those.

Lai · Answer 5 · 2019-01-14T14:33:19.713

A simple solution, update your code with

dogName.replaceAll("[^a-zA-Z0-9]", "");

so your code becomes, ...also remember to upadte your if statement.

do {
        while(true) {
            System.out.print("Dog's name: ");
            String dogName = scan.nextLine().toLowerCase();
            String n = dogName.replaceAll("[^a-zA-Z0-9]", "");

            if(n.isEmpty()) {
                System.out.println("Error: Name can't be empty.");
                continue;   
            }
   }

How to reject whitespace as input in java

5 Answers5