try catch for Scanner

问题

I am using Scanner to get user input. If the user inputs a name, I add that to the ArrayList. If the user does not enter a name then I want to throw an exception, but I want to continue the loop that gets the answer.

for(int i = 0; i < totalLanes; i++){
    runArr.add(this.addRacer());
}

public static String addRacer() throws NullPointerException{
    System.out.print("Enter a name for a racer: ");//Method uses try catch to catch a    NullPointerException.   
    Scanner sc = new Scanner(System.in);
    String rName = null;
    try {            
        if(!sc.nextLine().isEmpty()){
            rName = sc.nextLine();
        }else{
            throw new NullPointerException("name cannot be blank");
        }
    }

    catch (NullPointerException e) {
        System.out.println(e.toString());
        System.out.print("Enter a name for a racer: ");
        addRacer();
    }
    return rName;
}

• Why would this recurse infinitely?
• What is the best way to retrieve the input from the user but make sure they are inputting valid data?

Thanks in advance.

回答1:

The problem is that you read input twice. I mean you have two calls of sc.nextLine() method in your code. Try this instead:

String rName = sc.nextLine();
try {
    if(rName.isEmpty()){
        throw new NullPointerException("Name cannot be blank.");
    }
}

回答2:

You shouldn't throw an exception for that. Just use a while loop:

String rName = sc.nextLine();
while (rName.isEmpty()) {
    System.out.println("Name can't be blank. Try again.");
    rName = sc.nextLine();
}
return rName;

After that loop you are guaranteed to have a non-empty name in your variable, and you can use that name to add a new racer. You do not need recursion.

回答3:

You can use a do{}while() in your case, this can be better way :

Scanner sc = new Scanner(System.in);
String rName;

do {

    System.out.print("Enter a name for a racer: ");
    rName = sc.nextLine();
    try {
        if (rName.isEmpty()) {
            //throw and exception
            throw new NullPointerException("name cannot be blank");
        }
    } catch (NullPointerException e) {
        //print the exception
        System.out.println(e.getMessage());
    }

} while (rName.isEmpty());

return rName;

So you can't break your loop until the value in not empty.

回答4:

If the user does not enter a name then I want to throw an exception, but I want to continue the loop that gets the answer.

For that you need to use a while loop

What is the best way to retrieve the input from the user but make sure they are inputting valid data?

Use a while loop which should execute until user inputs the valid input. You don't need to use recursion for what you are trying to achieve.

  public static String addRacer() throws NullPointerException{
    System.out.print("Enter a name for a racer: ");
    Scanner sc = new Scanner(System.in);
    String rName = null;
    try {  
         String rName = sc.nextLine();  

        if(rName.isEmpty()){
            throw new NullPointerException("name cannot be blank");
        }
            while (rName.isEmpty()) {
               System.out.println("Name can't be blank. Try again.");
               rName = sc.nextLine();
            }
    }

    catch (NullPointerException e) {
        System.out.println(e.toString());
    }
    return rName;
}

回答5:

Don't call addRacer() function in catch. And also remove the line i have marked.Use if else condition for recursion.

catch (NullPointerException e) {
        System.out.println(e.toString());
        System.out.print("Enter a name for a racer: ");//remove this
        addRacer();//remove this
    }

来源：https://stackoverflow.com/questions/41924749/try-catch-for-scanner