How can Java determine if the input is a number?

In Java, you can utilize the Scanner class to read user input and use the hasNextDouble() method to determine if the input is a number.

Here is an example code:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("请输入一个数字：");
        if(scanner.hasNextDouble()) {
            double number = scanner.nextDouble();
            System.out.println(number + " 是一个数字。");
        } else {
            String input = scanner.next();
            System.out.println(input + " 不是一个数字。");
        }

        scanner.close();
    }
}

In the example above, we use the hasNextDouble() method of the Scanner class to determine if the input is a number. If the input is a number, we use the nextDouble() method to read the input and store it in a double variable. If the input is not a number, we use the next() method to read the input and store it in a String variable.

Please note that the hasNextDouble() method automatically ignores leading spaces and newline characters when determining if the input is a number. Therefore, any spaces or newlines before or after user input will not affect the outcome of the determination.

Additionally, it is important to note that the hasNextDouble() method can only determine if the input is a double type number. If you need to determine if the input is a different type of number, you can use methods like hasNextInt() (for integers) or hasNextLong() (for long integers).