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In Java, there are several ways to check if a string is a number. One way is to use the isDigit() method of the Character class. This method returns true if the character is a digit, and false otherwise. For example:

public class CheckIfStringIsNumber {    public static void main(String[] args) {        String str = "123";        boolean isNumber = true;        for (int i = 0; i < str.length(); i++) {            if (!Character.isDigit(str.charAt(i))) {                isNumber = false;                break;            }        }        if (isNumber) {            System.out.println("The string is a number.");        } else {            System.out.println("The string is not a number.");        }    }}

Another way to check if a string is a number is to use the parseInt() method of the Integer class. This method attempts to parse the string as a signed integer. If the string is a valid integer, the method returns the integer value. Otherwise, it throws a NumberFormatException. For example:

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