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How do we find out if first character of a string is a number in java?
Using the isDigit() method
The isDigit() method of the java.lang.Character class accepts a character as a parameter and determines whether it is a digit or not. If the given character is a digit this method returns true else, this method returns false.
Therefore, to determine whether the first character of the given String is a digit.
The charAt() method of the String class accepts an integer value representing the index and returns the character at the specified index.
The toCharArray() method of this class converts the String to a character array and returns it you can get its first character as array[0].
Retrieve the 1st character of the desired String using either of the methods.
Then, determine whether it is a digit or not by passing it as a parameter to the isDigit() method.
Example
import java.util.Scanner;
public class FirstCharacterOfString {
public static void main(String args[]) {
//Reading String from user
System.out.println("Enter a String");
Scanner sc = new Scanner(System.in);
String str = sc.next();
//Converting String to a character array
char charArray[] = str.toCharArray();
boolean bool = Character.isDigit(charArray[0]);
if(bool) {
System.out.println("First character is a digit");
} else {
System.out.println("First character is not a digit");
}
}
}Output1
Enter a String krishna First character is not a digit
Output2
Enter a String 2sample First character is a digit
Using the regular expressions.
The matches() method of the String class accepts a regular expression and verifies it matches with the current String, if so, it returns true else, it returns false.
The regular expression to match String which contains a digit as first character is “^[0-9].*$”. Pass this as a parameter to the matches() method of the String class.
Example
import java.util.Scanner;
public class FirstCharacterOfString {
public static void main(String args[]) {
//reading String from user
System.out.println("Enter a String");
Scanner sc = new Scanner(System.in);
String str = sc.next();
boolean bool = str.matches("^[0-9].*$");
if(bool) {
System.out.println("First character is a digit");
} else {
System.out.println("First character is not a digit");
}
}
}Output1
Enter a String krishna First character is not a digit
Output2
Enter a String 2sample First character is a digit
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