Re-prompt user after invalid input in Java

问题

I'm writing this program in java where I need to re-prompt the user after an invalid input. I came to a solution only to discover that if the user enters another invalid input after the re-prompt then it continues. Can someone please show me a better solution to this? I'll show you what I had anyway:

System.out.println("What is your age?\n");
    age = userInput.nextInt();

    if((age > 120) || (age < 1)) {//error message
        System.out.println("ERROR Please enter a valid age");
        System.out.println("");
        System.out.println("What is your age?\n");
        age = userInput.nextInt();
    }//end if

if the user entered an invalid input after they were prompted again, the program would just continue, how can I overcome this?

回答1:

Replace if with while.

BAM, problem solved.

回答2:

Use a while loop.

while (true) {
    System.out.println("What is your age?\n");
    age = userInput.nextInt();
    if ((age > 120) || (age < 1))
        System.out.println("ERROR Please enter a valid age\n");
    else
        break;
}

回答3:

You could put it in to a while loop so that it keeps looping until the conditions are met --

System.out.println("What is your age?\n");
age = userInput.nextInt();

while((age > 120) || (age < 1)) {//error message
    System.out.println("ERROR Please enter a valid age");
    System.out.println("");
    System.out.println("What is your age?\n");
    age = userInput.nextInt();
}//end if

回答4:

use do-while:

boolean valid;
do {
     System.out.println("What is your age?\n");
     age = userInput.nextInt();
     valid = age > 1 && age < 120;
     if (!valid) {
       System.out.println("ERROR Please enter a valid age");
     }
}while (!valid);

回答5:

What about this

---->One time check - is your input is empty or just pressed the spacebar

Scanner scnr = new Scanner(System.in);      
System.out.println("Enter a string: ");
String input = scnr.nextLine(); 

boolean isEmpty = input == null || input.trim().length() == 0;
if (isEmpty){
    System.out.println("Enter a string again: ");
    input = scnr.nextLine(); 
}

------> Multiple time check- is your input is empty or just pressed the spacebar

 Scanner scnr = new Scanner(System.in);       
do {
    System.out.println("Enter a string: ");
    input = scnr.nextLine();
}
     while (input == null || input.trim().length() == 0);

Important: Don't forget that input should be static string in this case.

static String input="";

回答6:

// Using do-while loop, this problem can be tackled.

    do {
        System.out.println("Enter a pin: ");
        pin = sc.nextInt();
    } while (pin != 12345);
    System.out.println("Welcome to program");

来源：https://stackoverflow.com/questions/18721884/re-prompt-user-after-invalid-input-in-java

标签

java

loops

for-loop

while-loop

do-while