How to re-write an input loop to not contain code repetition?-CodePudding

I have the following code, which continues to ask the user to enter a letter as long as the letter is either "a" or "b":

import java.util.Scanner;

public class Main
{   
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
        
        String letter;
        
        System.out.print("Enter a letter: ");
        letter = scan.nextLine();
        
        while(letter.equals("a") || letter.equals("b"))
        {
            System.out.println("You entered: "   letter);
            
            System.out.print("Enter a letter: ");
            letter = scan.nextLine();
        }
    }
}

But the following code is repeated twice:

System.out.print("Enter a letter: ");
letter = scan.nextLine();

Is there a way to make the above code only appear one time?

CodePudding user response：

    while (true) {
        System.out.print("Enter a letter: ");
        String letter = scan.nextLine();
        if (!letter.equals("a") && !letter.equals("b"))
            break;
        System.out.println("You entered: "   letter);
    }

This is the classic example of a loop that is neither naturally while-do nor do-while — it needs to exit from the middle, if you want the same behavior and also to reduce code duplication. Notice also that the variable declaration letter has been moved to an inner scope since it is no longer needed in the outer scope.

As an alternative to while (true) some languages allow degenerate for-loop as in for(;;).

The below reverses the logic of the conditional loop exit test, at the expense of more control flow logic.

    while (true) {
        System.out.print("Enter a letter: ");
        String letter = scan.nextLine();
        if (letter.equals("a") || letter.equals("b")) {
            System.out.println("You entered: "   letter);
            continue;
        }
        break;
    }

(There is no difference between these in efficiency terms — these are equivalent at the level of machine code.)

CodePudding user response：

Simply

List<Character> expectedChars = new ArrayList<>();
expectedChars.add('a');
expectedChars.add('b');
while(!expectedChars.contains(line = scan.nextLine())) {
System.out.println("Not expected");
}

// Now has a expected char. Proceed.

CodePudding user response：

As Bobulous mentioned, a do-while loop is another simple solution. If duplicating the conditional is still a deal-breaker for you, though, you can also create a function that returns a boolean, and, when true, prints the extra text.

public static void main(String[] args)
{
    Scanner scan = new Scanner(System.in);
    
    String letter;
    
    do
    {
        System.out.print("Enter a letter: ");
        letter = scan.nextLine();
    } while(inputIsAOrB(letter));
}

public static boolean inputIsAOrB(String input) {
    if (input.equals("a") || input.equals("b"))
    {
        System.out.println("You entered: "   input);
        return true;
    }
    else
    {
        return false;
    }
}