How can I ensure user enters a valid choice?

Question

I'm making a simple rock paper scissors program but am unsure how to ensure that the user only enters a valid choice. I need to be able to reprompt them if they don't type some varriant of "rock", "paper", or "scissors" (capitalization doesn't matter) and later "yes" or "no". Suggestions?

import java.util.*;

public class RockPaperScissors {

private int wins = 0;
private int losses = 0;
private int ties = 0;

public static void main(String[] args) {
    // TODO Auto-generated method stub

    RockPaperScissors model = new RockPaperScissors();
    Scanner scan = new Scanner(System.in);

    while (true) {
        System.out.println("Rock, Paper, Scissors... Pick one! (Type Rock, Paper, or Scissors)");
        String playerChoice = scan.next();

        String computerChoice = model.getRandomChoice();

        System.out.println("You chose " + playerChoice + ".");
        System.out.println("The computer chose " + computerChoice + ".");

        RockPaperScissors.GameOutcome outcome = model.getGameOutcome(
                playerChoice, computerChoice);

        if (outcome == RockPaperScissors.GameOutcome.WIN) {
            System.out.println("You won! Congratulations");
        } else if (outcome == RockPaperScissors.GameOutcome.LOSE) {
            System.out.println("You lose! Better luck next time!");
        } else {
            System.out.println("Tie!");
        }

        System.out.print("Do you want to play again? (Yes/No):");

        String answer = scan.next();
        if (answer.equalsIgnoreCase("no")) {
            break;
        }
    }

    System.out.println("Thanks for playing!");
    System.out.println("Wins: " + model.getWins());
    System.out.println("Losses: " + model.getLosses());
    System.out.println("Ties: " + model.getTies());
    scan.close();
}

public static enum GameOutcome {
    WIN, LOSE, TIE;
}

public GameOutcome getGameOutcome(String userChoice, String computerChoice) {

    if (userChoice.equalsIgnoreCase("Rock")) {

        if (computerChoice.equalsIgnoreCase("Paper")) {
            losses++;
            return GameOutcome.LOSE;
        } else if (computerChoice.equalsIgnoreCase("Scissors")) {
            wins++;
            return GameOutcome.WIN;
        }
    } else if (userChoice.equalsIgnoreCase("Paper")) {

        if (computerChoice.equalsIgnoreCase("Scissors")) {
            losses++;
            return GameOutcome.LOSE;
        } else if (computerChoice.equalsIgnoreCase("Rock")) {
            wins++;
            return GameOutcome.WIN;
        }
    } else if (userChoice.equalsIgnoreCase("Scissors")) {
        if (computerChoice.equalsIgnoreCase("Rock")) {
            losses++;
            return GameOutcome.LOSE;
        } else if (computerChoice.equalsIgnoreCase("Paper")) {
            wins++;
            return GameOutcome.WIN;
        }
    }
    ties++;
    return GameOutcome.TIE;
}

public String getRandomChoice() {
    double d = Math.random();

    if (d < .33) {
        return "Rock";
    } else if (d < .66) {
        return "Paper";
    } else {
        return "Scissors";

    }
}

public int getWins() {
    return wins;
}

public int getLosses() {
    return losses;
}

public int getTies() {
    return ties;
}

}

Answer 1

Keep the valid choices in a list, loop asking the input from the user until he enter something valid.

List<String> validChoices = Arrays.asList("rock", "paper", "scissors");
Scanner sc = new Scanner(System.in);
String choice = null;
do
{
    System.out.println("Enter a choice (rock|paper|scissors)");
    choice = sc.next().toLowerCase();//Retrieve as lower case
}
while(!validChoices.contains(choice));