How to use isDigit in Java?

To use isDigit in Java, call it like so: Character.isDigit('1').

With a character variable

If you have a single char variable, then you can determine whether the char is a digit like so:

char digit = '1';
char notDigit = 'c';

System.out.println(Character.isDigit(digit)); // true
System.out.println(Character.isDigit(notDigit)); // false

With a string variable

If you want to know whether a single char in a String is a digit, first extract the char at index, then call isDigit:

String string = "Hello World 42!";

System.out.println(Character.isDigit(string.charAt(1))); // e -> false
System.out.println(Character.isDigit(string.charAt(12))); // 4 -> true