在Java中递归地捕获异常

Question

I´m currently working on a little game and I´ve ran into a problem I cannot fix myself. I´ve got a method which reads the input and stores it in variables. For any wrong input will an IllegalArgumentException be thrown where you can try the input once again. But if you´re doing it wrong once again, it will just proceed to the next input type. But I want it to ask for the input till the input is valid. My tutor told me to do it with try and catch which I did but as said it will only do it twice and then proceed.

Here´s the code:

 public void readSettings(){
    Scanner userinput = new Scanner(System.in);
    System.out.println("Wie lange soll der Code sein? (4-10):");
    String input = userinput.nextLine();
    //Eingabe der Länge des zu lösenden Codes. Bei einer Eingabe außerhab des Wertebereichs wird dies gemeldet und neu gefragt.
    try{
        if (input.matches("[4-9]|10")) {
            this.codelength = Integer.parseInt(input);  
        }
        else {
            throw new IllegalArgumentException();
        }
    }

    catch (IllegalArgumentException e) {
        System.out.println("Eingabe außerhalb des Wertebreichs!"); 
        //readSettings();
        System.out.println("Wie lange soll der Code sein? (4-10):");
        input = userinput.nextLine();
        if (input.matches("[4-9]|10")) {
            this.codelength = Integer.parseInt(input);  
        }
        else {
            throw new IllegalArgumentException(e);
        }
    } finally {

        System.out.println("Welche Ziffern sind erlaubt? 0- (1-9):");
        input = userinput.nextLine();
        //Hier wird die valuerange(Also die Maximale Zahl in der Reihe) abgefragt. 
        //Auch hier wird eine falche Eingabe abgefangen und der Input neu gestartet. (Leider nicht sehr elegant und benutzerfreundlich.
        try {
            if (input.matches("[1-9]")) {
                this.valuerange = Integer.parseInt(input);  
            }
            else {
                throw new IllegalArgumentException();
            }
        }
        catch (IllegalArgumentException f) {
            System.out.println("Eingabe außerhalb des Wertebreichs!"); 
            //readSettings();
            System.out.println("Welche Ziffern sind erlaubt? 0- (1-9):");
            input = userinput.nextLine();
            if (input.matches("[1-9]")) {
                this.valuerange = Integer.parseInt(input);  
            }
            else {
                throw new IllegalArgumentException(f);
            }
        } finally {

            //Falls der Modus nicht Cpmouter gegen Computer ist, wird ein Spielername mit abgefragt.
            try {
                if(!cpumode) {
                    System.out.println("Spielername:");
                    this.spielername = userinput.nextLine();
                    //Falls kein input bein Namen vorhanden, wird ein Fehler ausgegeben.
                    if (spielername.length()==0) {
                        throw new IllegalArgumentException("Fehler, kein Spielername eingegeben!" );  

                    }     
                }
            } catch (IllegalArgumentException e) {
                System.out.println("Spielername:");
                this.spielername = userinput.nextLine();
                if (spielername.length()==0) {
                    //throw new IllegalArgumentException("Fehler, kein Spielername eingegeben!" );  
                    throw new IllegalArgumentException(e);
                }     

            }   
        }
    }
}       

I hope you can help me. Thank you!

Answer 1

The first step is to realize that you're repeating your "read code length" all over. This could generally mean two things: 1) you should extract the code into a method and call that method multiple times, or 2) you should use a loop and write the code in the loop. (or both)

the suggestion of a loop should ring a bell, because only that (or recursion...) allows you to repeat the operation a non-fixed number of times, as you want it. In pseudocode:

repeat
    readNumber
until number is valid

as you see, the condition is at the bottom of the loop. In Java, you'd use do {...} while(...); - note that you have to reverse the condition from the pseudocode, because it's while , not until .

On a side note, as you throw the exception yourself, you could just skip the throwing and instead leave your codelength at a value you recognize as invalid, eg -1 . You'll need that anyway, as you have to catch the Exception as part of the readNumber part, which is still inside your loop.