如何检查 integer 字符串

Question

I'm supposed to write a program that takes in a string representing an integer as input, and outputs yes if every character is a digit 0-9.

I have gone back through my chapters reading and gone through Google, but I'm still having trouble. I know my code is a mess but I am lost. I may have bits and pieces correct or be wrong all together but this is what I have.

import java.util.Scanner; 

public class LabProgram {
   public static void main(String[] args) {
      Scanner scnr = new Scanner(System.in);
      String userString;
  // Add more variables as needed

      userString = scnr.next();
      boolean check1 = Character.isDigit(userString);
  
      while (check1 = True) {
         System.out.println ("Yes");
    }
      System.out.println ("No");

Where did I go wrong?

Answer 1

As pointed out by people in comments, there are some mistakes in your code. Character.isDigit won't work with String. What you need to use is Integer.parseInt(...) . You can put it in a try catch block; Print yes if parsing is successful and print No if exception is thrown:

userString = scnr.next();
try {
  int parsedValue = Integer.parseInt(userString);
  // Do something with parsedValue
  System.out.println("Yes");
} catch (NumberFormatException numberFormatException) {
  System.out.println("No");
}

Answer 2

You could use regular expressions for that:

import java.util.regex.Matcher;
import java.util.regex.Pattern;

// your input, can also be from a scanner
String input = "1234567890";

// regex pattern to match a string that consists only of numbers and is also not empty
String regex = "^[0-9]+$";

// to match negative numbers as well: "^[\\-]?[0-9]+$"

// create a pattern object
Pattern pattern = Pattern.compile(regex);

// create a matcher object
Matcher matcher = pattern.matcher(input);

// check if the input matches
if (matcher.find()) {
  System.out.println("It's a number, yay!");
};

How does the regex work:

• ^ matches the beginning of the input to make sure there is nothing before the number characters
• [0-9]+ tells it to match the range 0-9 one or more times ( + ), which also makes sure that an empty string does not match
• $ matches the end of the string to make sure there is no non-numbers after the match

If you want to match negative numbers as well, there is an additional part of the regex: [\\-]? this matches one minus sign if there is on or none if there is no sign ( ? )

https://www.tutorialspoint.com/java/java_regular_expressions.htm

It's also possible the way you tried it:

// open a scanner
Scanner scanner = new Scanner(System.in);

// read user input from the scanner
String input = scanner.next();

// assume it is a number until we find a non-digit character below
String isNumber = true;

// loop over each character of the user input
for (char character : input) {
  // check if the character is not a digit
  if (!Character.isDigit(character)) {
    System.out.println("NOT a number: ", character);

    // set the is number variable to false
    isNumber = false;

    // break the loop, because it cannot be a number because we already found a non-digit
    break;
  };
};

System.out.println("Is it a number? ", isNumber);