How to check if first character of a string is a number or not in java

• To check if first character of a string is a number or not in java we can check in multiple ways.
• By using Character.isDigit() method by passing character of string using for loop or substring method.
• Other way is to check str.substring(0, 1).matches("[0-9]").
• Lets see an example program in java to check first character of a string is digit or character. 

Program #1: Java example program to check the each character of string is number or not   character is a letter or number in Java without using regexes?

1. package StringInterviewprograms;
2. /**
3.  * Check whether the given character is a number /digit or not by using java character isdigit
4.  *method
5.  * @author www.instanceofjava.com
6.  */
7. public class CheckEachCharacterNumberOrNot {
8.     public static void main(String[] args) {
9.         String str = "instanceofjava37";
10.  
11.         //Check whether the given character is a number /digit or not ?
12.  
13.         for(int i=0; i<str.length();i++)
14.         {
15.             Boolean flag = Character.isDigit(str.charAt(i));
16.  
17.             if(flag){
18.  
19.                System.out.println("'"+str.charAt(i)+"' is a number");
20.             }
21.             else{
22.                System.out.println("'"+str.charAt(i)+"' is not a number");
23.             }
24.  
25.         }
26.         
27. }
28.  
29. }

Output:

1. 'i' is not a number
2. 'n' is not a number
3. 's' is not a number
4. 't' is not a number
5. 'a' is not a number
6. 'n' is not a number
7. 'c' is not a number
8. 'e' is not a number
9. 'o' is not a number
10. 'f' is not a number
11. 'j' is not a number
12. 'a' is not a number
13. 'v' is not a number
14. 'a' is not a number
15. '3' is a number
16. '7' is a number

     
Program #2: Java example program to check the First character of string is number or not by using regex.

1. package StringInterviewprograms;
2. /**
3.  * Check whether the given character is a number /digit or not
4.  * @author www.instanceofjava.com
5.  */
6. public class CheckFirstCharacterNumberOrNot {
7.  
8.     public static void main(String[] args) {
9.         String str = "instanceofjava37";
10.  
11.           Boolean flag1 = str.substring(0, 1).matches("[0-9]");
12.  
13. if(flag1){
14.  
15.  System.out.println("First Character is a number..!");
16.  
17.  }
18.  else{
19.    System.out.println("First character is not a number..!");
20. }
21. }
22.  
23. }

Output:

1. First character is not a number..!

Program #3: Java example program to check the First character of string is number or not using eclipse IDE.