如何检查字符串是否为数字？

我有一个gpa程序，它适用于 equalsIgnoreCase（）方法，它比较两个字符串，字母a到用户输入，检查它们是否放a与否。但是现在我想添加一个异常，并在输入数字时执行错误消息。我希望程序意识到整数输入与字符串不同并给出错误消息。我可以使用哪些方法将类型 String 变量与 int 类型的输入进行比较，并抛出异常？

I have a gpa program, and it works with the equalsIgnoreCase() method which compares two strings, the letter "a" to the user input, which checks if they put "a" or not. But now I want to add an exception with an error message that executes when a number is the input. I want the program to realize that the integer input is not the same as string and give an error message. Which methods can I use to compare a type String variable to input of type int, and throw exception?