如何验证用户在 java 中输入的数据

Question

I was writing a program in which user is supposed to enter double value but I want to prompt the user again if he/she enters a string value.

This is my code so far:

public static void main(String[] args) {
    Scanner input = new Scanner(System.in); 
    double purchaseAmount = 0; 
    if (input.hasNext("mp")) { 
        do { 
            System.out.println("Enter your loan amount: ");
            purchaseAmount = input.next(); 
        } while(!input.hasNextDouble()); 
    }
}
    

Answer 1

When you prompt a user in Java (assuming you're using System.in or a JTextField) what you're reading from user input is text.

If you want to convert the text to a double there are several approaches, but you could use:

do {
   String promptValue = prompt();
   NumberFormat nf = NumberFormat.getInstance();
   nf.setMaximumFractionDigits(16); // maximum for double
   Number num = nf.parse(promptValue):
} while (null == num);

Notice also that you could ask character by character and parse progressively so you don't need to prompt again but rather discard anything that doesn't match. This would work well in Swing or you could use jline3 if you really want to get sophisticated.

To learn how to parse progressively look at: https://docs.oracle.com/javase/7/docs/api/java/text/NumberFormat.html

Answer 2

If the input given by the user doesn't match the expected format then an exception will be thrown, you can catch this exception to again request for the long value.

    public static Long getLongInputFromUser(){
        Scanner sc= new Scanner(System.in);
        try {
            System.out.println("Print the long value: ");
            Long inputInteger = sc.nextLong();
            return inputInteger;
        }catch(InputMismatchException inputMismatchException){
            System.out.println("The input is not a valid long, please enter again");
            return getLongInputFromUser();
        }
    }

    public static void main(String[] args) {
        Long input = getLongInputFromUser();
        System.out.println("The final value is " + input);
    }

The output of this program is:

Print the long value: 
abc 
The input is not a valid long, please enter again 
Print the long value: 
pqr 
The input is not a valid long, please enter again
Print the long value: 
6 
The final value is 6 

Answer 3

You can use Java exception mechanism

Scanner input=new Scanner(System.in);
double purchaseAmount;
while(true){
    try{
        System.out.println("please input a double value");
        purchaseAmount=input.nextDoble();
    }catch(Exception e){
        System.out.println("You input a string,not a double value");
    }
}