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How to loop main class

I am trying to make my rock paper scissors app loop if the user wants it to and am not sure how.
I am in a programming class but I broke my code while trying to fix it last time and managed to return to what I had before.
public static void main(String[] args)
{
Scanner scan = new Scanner(System.in);
String playAgain = "Y";
while (playAgain.equals("Y") || playAgain.equals("y"))
{
String playerChoice = "";
while (!playerChoice.equals("R") && !playerChoice.equals("P") && !playerChoice.equals("S"))
{
System.out.println("Please enter a correct character\n (R)ock, (P)aper, or (S)cissors.");
playerChoice = scan.next();
//Player choice to upper case to minimize the number of wrong inputs//
playerChoice = playerChoice.toUpperCase();
}
int randNum = (int) (Math.random() * 3);
String compChoice = "";
switch (randNum)
{
case 0:
compChoice = "R";
break;
case 1:
compChoice = "P";
break;
case 2:
compChoice = "S";
break;
}
System.out.println("The computer chose: " + compChoice);
if (playerChoice.equals(compChoice))
{
System.out.println("It's a Tie!");
} else if (playerChoice.equals("R") && compChoice.equals("S") || playerChoice.equals("P") && compChoice.equals("R") || playerChoice.equals("S") && compChoice.equals("P"))
{
System.out.println("You Win!");
} else
{
System.out.println("You Lose");
}
System.out.println("Would you like to play again?\n(Y or N)");
playAgain = scan.nextLine();
if (playAgain.equals("N"))
{
break;
}
}
}

The solution is you need agreement between your scan method calls.
Either both should be nextLine or both should be next.
As an aside - I quite like the use of .equalsIgnoreCase instead of .equals on string comparisons, since it makes the code simple and easy to read. (You can skip the toUpperCase for instance, and the extra conditions)
As for why/what exactly the scanner is doing which causes that problem, that's a lot more detailed and fiddly.

Why does my flag keep getting set to false?

Hello I am writing a simple program for a user to take a 3 question test. I am trying to validate the user input however if the user enters in a loop to enter the correct data because they previously entered the wrong data they cannot get out of the loop. Even if there next answer is correct. Something is setting one of my flags to false and I cannot figure out what. I tried debugging it to no avail.
import java.util.Scanner;
public class ALittleQuiz {
private static int correct = 0;
public static void main (String[] args){
Scanner keyboard = new Scanner(System.in);
System.out.print("Are you read for the quiz? ");
keyboard.next();
System.out.println("Okay, here it comes!");
questionOne();
questionTwo();
questionThree();
System.out.println("Overall, you got "+correct+" out of 3 correct.");
System.out.println("Thanks for playing!");
}
public static void questionOne(){
System.out.println("Q1) What is the capital of Alaska?");
System.out.println(" 1) Melbourne");
System.out.println(" 2) Anchorage");
System.out.println(" 3) Juneau");
System.out.print("> ");
getUserInputForQuestionOne();
}
public static void questionTwo(){
System.out.println("Q2) Can you store the value 'cat' in a variable of type int? ");
System.out.println(" 1) Yes");
System.out.println(" 2) No");
System.out.print("> ");
getUserInputForQuestionTwo();
}
public static void questionThree(){
System.out.println("Q3) What is the result of 9+6/3?");
System.out.println(" 1) 5");
System.out.println(" 2) 11");
System.out.println(" 3) 15/3");
System.out.print("> ");
getUserInputForQuestionThree();
}
public static void getUserInputForQuestionOne(){
int testvar = 0;
try{
Scanner inputForQuestionOne = new Scanner(System.in);
testvar = inputForQuestionOne.nextInt();
validateUserInputForQuestionOne(testvar);
} catch (Exception e) {
System.out.println("Error: invalid entry please try again");
getUserInputForQuestionOne();
}
}
public static void getUserInputForQuestionTwo(){
int testvar = 0;
try{
Scanner inputForQuestionTwo = new Scanner(System.in);
testvar = inputForQuestionTwo.nextInt();
validateUserInputForQuestionTwo(testvar);
} catch (Exception e) {
System.out.println("Error: invalid entry please try again");
getUserInputForQuestionTwo();
}
}
public static void getUserInputForQuestionThree(){
int testvar = 0;
try{
Scanner inputForQuestionThree = new Scanner(System.in);
testvar = inputForQuestionThree.nextInt();
validateUserInputForQuestionThree(testvar);
} catch (Exception e) {
System.out.println("Error: invalid entry please try again");
getUserInputForQuestionThree();
}
}
public static void validateUserInputForQuestionOne(int choiceOne){
if(choiceOne >= 1 && choiceOne <= 3){
sendResponseForQuestionOneToDetermineIfCorrectOrNot(choiceOne);
}else {
System.out.println("Please enter 1, 2 or 3 for your selection");
getUserInputForQuestionOne();
}
}
public static void validateUserInputForQuestionTwo(int choiceTwo){
if(choiceTwo >= 1 && choiceTwo <= 3){
sendResponseForQuestionTwoToDetermineIfCorrectOrNot(choiceTwo);
}else {
System.out.println("Please enter 1 or 2 for your selection");
getUserInputForQuestionTwo();
}
}
public static void validateUserInputForQuestionThree(int choiceThree){
if(choiceThree >= 1 && choiceThree <= 3){
sendResponseForQuestionThreeToDetermineIfCorrectOrNot(choiceThree);
}else {
System.out.println("Please enter 1, 2 or 3 for your selection");
getUserInputForQuestionThree();
}
}
public static void sendResponseForQuestionOneToDetermineIfCorrectOrNot(int validChoiceOne){
switch (validChoiceOne){
case 1: System.out.println("Sorry, that is not correct\n");
break;
case 2: System.out.println("Sorry, that is not correct\n");
break;
case 3: System.out.println("That's right\n");
correct++;
break;
}
}
public static void sendResponseForQuestionTwoToDetermineIfCorrectOrNot(int validChoiceTwo){
switch (validChoiceTwo){
case 1: System.out.println("Sorry, 'cat' is a string. Ints can only store numbers\n");
break;
case 2: System.out.println("That's right\n");
correct++;
break;
}
}
public static void sendResponseForQuestionThreeToDetermineIfCorrectOrNot(int validChoiceThree){
switch (validChoiceThree){
case 1: System.out.println("Sorry, that is not correct\n");
break;
case 2: System.out.println("That's right\n");
correct++;
break;
case 3: System.out.println("Sorry, that is not correct\n");
break;
}
}
}
Here is what is happening in my terminal:
Are you ready for a quiz? y
Okay, here it comes!
Q1) What is the capital of Alaska?
1) Melbourne
2) Anchorage
3) Juneau
> 456
Please enter 1, 2 or 3 for your selection
Q1) What is the capital of Alaska?
1) Melbourne
2) Anchorage
3) Juneau
> 3
That's right
Please enter 1, 2 or 3 for your selection
Q1) What is the capital of Alaska?
1) Melbourne
2) Anchorage
3) Juneau
> 2
Sorry, that is not correct
Please enter 1, 2 or 3 for your selection
Q1) What is the capital of Alaska?
1) Melbourne
2) Anchorage
3) Juneau
>
Edit: So after reading the article one of you had posted I went back to the drawing board and was able to resolve my issue. I do want to thank those that took the time to help me with this. I updated my code which I believe is a lot more comprehensible and easier to look at. Above is my newly created code. If anyone has any notes on this I would much appreciate it!

Inside of questionOne, you make a call to validateQuestionOneUerInput. In validateQuestionOneUerInput, if the user enters something other than 1, 2, or 3, it sets flagForQuestionOne and then calls questionOne again. Regardless of the result of this call, flagForQuestionOne is false, so you now have an infinite loop.
That method could see if the user's response is valid or not and return true or false, or could throw an exception for invalid input, but should not re-call questionOne.

in the do while loop, instead of using the flag, you are checking the flag equals == false.
Below is incorrect because when you set the flag value to true the while condition evaluates to false.
while (flag == false);
Change it to
while(flag)

Rock, paper, scissors game Invalid entry loop

I am trying to figure out where to put the loop that when the user enters any value other than "rock", "paper" or "scissors" the program stays in the loop and displays "Invalid entry" while requesting the user to enter again.
Any help is much appreciated.
As it stands now, the program will display "Invalid entry" but then continues without asking the user to try again.
import java.util.Scanner; // Import the Scanner class
import java.util.Random; // Import the random class for game
/**
*/
public class Challenge17
{
// Method to determine the random choice of computer
public static String getComputerChoice(Random random)
{
int number;
number = random.nextInt(3) + 1;
String computerChoice;
switch (number)
{
case 1:
computerChoice = "rock";
break;
case 2:
computerChoice = "paper";
break;
case 3:
computerChoice = "scissors";
break;
default:
computerChoice = "";
}
return computerChoice;
}
// Method to display the menu for choices
public static void displayChoice( )
{
System.out.println("Game Options\n----------\n"
+ "1: rock\n2: paper\n3: scissors");
}
// Method to request and hold user choice for game
public static String getUserInput(Scanner keyboard)
{
String userInput;
System.out.println("Enter your choice: ");
userInput = keyboard.nextLine();
return userInput;
}
// Method to determine winner
public static String determineWinner(String computerChoice, String userInput)
{
String winner = "Tie Game!"; // Default display Tie game
String message = ""; // To determine the message for winner
String displayMessage; // To display the message for winner
// Custom messages below
String rockMessage = "Rock smashes scissors";
String scissorsMessage = "Scissors cuts paper";
String paperMessage = "Paper wraps rock";
boolean loop = false;
if(computerChoice.equals("rock") && userInput.equalsIgnoreCase("scissors"))
{
winner = " Computer wins!";
message = rockMessage;
loop = true;
}
else if (userInput.equalsIgnoreCase("rock") && computerChoice.equals("scissors"))
{
winner = "You win!";
message = rockMessage;
loop = true;
}
if(computerChoice.equals("scissors") && userInput.equalsIgnoreCase("paper"))
{
winner = " Computer wins!";
message = scissorsMessage;
loop = true;
}
else if (userInput.equalsIgnoreCase("scissors") && computerChoice.equals("paper"))
{
winner = "You win!";
message = scissorsMessage;
loop = true;
}
if(computerChoice.equals("paper") && userInput.equalsIgnoreCase("rock"))
{
winner = " Computer wins!";
message = paperMessage;
loop = true;
}
else if (userInput.equalsIgnoreCase("rock") && computerChoice.equals("scissors"))
{
winner = "You win!";
message = paperMessage;
loop = true;
}
else
{
System.out.println("Invalid entry.");
loop = false;
}
displayMessage = winner + " " + message;
return displayMessage;
}
// Main method to initiate and execute game
public static void main(String[] args)
{
Random random = new Random(); // To call the random class
Scanner keyboard = new Scanner(System.in); // To call the scanner class
String computerChoice; // Hold computer input
String userInput; // Hold user input
String input; // Hold input for repeat
char repeat; // Character for repeat
do
{
displayChoice(); // Call method to display the choices
computerChoice = getComputerChoice(random); // Hold the PC random choice
userInput = getUserInput(keyboard); // To get the user input
System.out.println("You chose: " + userInput + " computer chose: \n"
+ computerChoice);
System.out.println(determineWinner(computerChoice, userInput));
// Does the user want to play again
System.out.println("Would you like to play again?");
System.out.print("Enter Y for yes, or N for no: ");
input = keyboard.nextLine();
repeat = input.charAt(0);
}
while (repeat == 'Y' || repeat == 'y');
}
}

From my understanding i'm pretty sure you want this kind of a loop
public static String getUserInput(Scanner keyboard)
{
String userInput;
System.out.println("Enter your choice: ");
userInput = keyboard.nextLine();
while(!userInput.equals("rock")&&!userInput.equals("paper")&&!userInput.equals("scissors"))
{
System.out.println("Invalid Entry, Please re-enter your choice:");
userInput = keyboard.nextLine();
} //It wont go out of the loop unless it's one of the 3 choices
return userInput;
}

You could create a Boolean method.
private Boolean checkUserInput(String input) {
if(!input.equalsIgnoreCase("rock") && !input.equalsIgnoreCase("paper") && !input.equalsIgnoreCase("scissors")) return false;
return true;
Then you do the check before you determine the winner like if true then run the determineWinner(computerChoice, userInput) code else it doesn't.
That way, you can edit the code and add functionalities in the future if need be.

Java Do While isn't working properly

Sorry I am new to this site so not sure how this will show up. I am trying to make a simple Rock, Paper, Scissors game. After the while statement, if R, P, S isn't entered, the program just does nothing. I want it to loop back to the question at the beginning so a right choice can be entered. Also, how would I enter a print statement like "Invalid Choice Please Retry"?
package rps.gameapp;
import java.util.Scanner;
public class RPSGameApp
{
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
String userChoice;
String playAgain;
int randNum = (int) (Math.random() * 3);
do
{
System.out.println("Welcome to Rock, Paper, Scissors Game.");
System.out.println("Pick R, P, or S.");
userChoice = sc.nextLine();
while (!userChoice.equalsIgnoreCase("P")
&& !userChoice.equalsIgnoreCase("R")
&& !userChoice.equalsIgnoreCase("S"));
String compChoice = "";
switch (randNum)
{
case 0:
compChoice = "R";
break;
case 1:
compChoice = "P";
break;
case 2:
compChoice = "S";
break;
}
System.out.println("The computer entered \"" + compChoice + "\".");
if (compChoice.equalsIgnoreCase(userChoice))
{
System.out.println("Draw");
} else if (userChoice.equalsIgnoreCase(userChoice)
&& compChoice.equalsIgnoreCase("S")
|| userChoice.equalsIgnoreCase("P")
&& compChoice.equalsIgnoreCase("R")
|| userChoice.equalsIgnoreCase("S")
&& compChoice.equalsIgnoreCase("P"))
{
System.out.println("User Wins");
} else
{
System.out.println("User Loses");
}
System.out.print(
"Do you want to play again? (Y/N)");
playAgain = sc.nextLine();
} while (playAgain.equalsIgnoreCase("Y"));
System.out.println("Thanks for Playing!");
}
}

It looks like you forgot one do for your inner do while loop.
It should be :
do {
do {
System.out.println("Welcome to Rock, Paper, Scissors Game.");
System.out.println("Pick R, P, or S.");
userChoice = sc.nextLine();
} while (!userChoice.equalsIgnoreCase("P") && !userChoice.equalsIgnoreCase("R") && !userChoice.equalsIgnoreCase("S"));
...
} while (playAgain.equalsIgnoreCase("Y"));
Without that inner do (and the curly braces surrounding that loop's body), the inner loop becomes a while loop with an empty body.

Like Eran said, you need to wrap your do-while loop in another loop, that will keep asking user for correct input. This is fully working code. One thing that could be better is the message after user inputs wrong letter.
Edit: also make sure you draw random number for every iteration.
Edit 2: to change the message depending on user input you can introduce a new variable that will keep the track of number of times you asked user for correct input. If it is 0- it means user is asked the first time and we should print "Welcome" message. It is anything other than 0- you need to ask the user for correct input. After every round we assign zero to the variable again and the cycle repeats. I have implemented this change in the code. Note that this variable can also be a boolean.
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
String userChoice;
String playAgain;
int iterationNumber;
while (true) {
iterationNumber = 0;
do {
if (iterationNumber == 0) {
System.out.println("Welcome to Rock, Paper, Scissors Game.");
System.out.println("Pick R, P, or S.");
} else {
System.out.println("Please enter valid letter.");
System.out.println("Pick R, P, or S.");
}
iterationNumber++;
userChoice = sc.nextLine();
} while (!userChoice.equalsIgnoreCase("P")
&& !userChoice.equalsIgnoreCase("R")
&& !userChoice.equalsIgnoreCase("S"));
String compChoice = "";
int randNum = (int) (Math.random() * 3);
switch (randNum) {
case 0:
compChoice = "R";
break;
case 1:
compChoice = "P";
break;
case 2:
compChoice = "S";
break;
}
System.out.println("The computer entered \"" + compChoice + "\".");
if (compChoice.equalsIgnoreCase(userChoice)) {
System.out.println("Draw");
} else if (userChoice.equalsIgnoreCase("R")
&& compChoice.equalsIgnoreCase("S")
|| userChoice.equalsIgnoreCase("P")
&& compChoice.equalsIgnoreCase("R")
|| userChoice.equalsIgnoreCase("S")
&& compChoice.equalsIgnoreCase("P")) {
System.out.println("User Wins");
} else {
System.out.println("User Loses");
}
System.out.print(
"Do you want to play again? (Y/N)");
playAgain = sc.nextLine();
if (playAgain.equalsIgnoreCase("N")) {
break;
}
iterationNumber = 0;
}
System.out.println("Thanks for Playing!");
}

Creating menu system using int, do-while and if

I tried making a menu system to make a user select between four options. To distinguish between the selections I check the int entered. It works but somehow I feel it is not very elegant. Especially when I set the initial value of selectedMenu to 1902475424 to check for when the user entered a mismatcing value. I assumed the user wont accidentally type 1902475424.
Is there a way more simple way to make a menu system or will this do? Is this major flawed?
Yes im a beginner to Java :-)
import java.util.Scanner;
import java.util.InputMismatchException;
public class Menu {
public void printMenu() {
System.out.println(
"1. Start new game\n" +
"2. Load game\n" +
"3. Settings\n" +
"4. Exit\n"
);
}
public void selectMenu() throws InputMismatchException {
int selectedMenu = 1902475424;
Scanner aScanner = new Scanner(System.in);
do {
selectedMenu = 1902475424;
try {
System.out.println("Try block begin.");
selectedMenu = aScanner.nextInt();
} catch (InputMismatchException e) {
System.out.println("Catch blok begin.");
System.out
.println("Invalid input, please input a number between 1-4.");
aScanner.nextLine();
}
if ((selectedMenu < 1 || selectedMenu > 4)
&& (selectedMenu != 1902475424)) {
System.out.println("Input out of range \"" + selectedMenu
+ "\". Input a number between 1-4.");
}
} while (selectedMenu == 1902475424
|| (selectedMenu < 1 || selectedMenu > 4));
if (selectedMenu >= 1 && selectedMenu <= 4) {
System.out.println("A new game will now start.");
}
}
}

Your method is leaning into the overkill category :]You can do away with your random value of 1902475424 like so:
public void selectMenu() throws InputMismatchException {
int selectedMenu;
Scanner aScanner = new Scanner(System.in);
do {
try {
System.out.println("Try block begin.");
selectedMenu = aScanner.nextInt();
if(selectedMenu < 1 || selectedMenu > 4) {
System.out.println("Input out of range \"" + selectedMenu + "\". Input..");
}
} catch(InputMismatchException e) {
System.out.println("Catch blok begin.");
System.out.println("Invalid input, please input a number between 1-4.");
aScanner.nextLine();
selectedMenu = 0;
}
} while(selectedMenu < 1 || selectedMenu > 4);
System.out.println("A new game will now start.");
}

Consider the following alternative (pseudocode):
int getMenuOption() {
print(message)
read(input)
if input is valid then return input
else then return getMenuOption()
}
This is recursive, so if the user sits there and enters bad numbers long enough, you could overflow the stack. You could easily augment this to give the user a fixed number of tries:
int getMenuOption(int triesRemaining) {
if (triesRemaining == 0) throw new RetriesExceededException();
print(message)
read(input)
if input is valid then return input
else then return getMenuOption(triesRemaining - 1)
}

Try something like that (I haven't tested it)
import java.util.Scanner;
import java.util.InputMismatchException;
public class Menu {
public void printMenu() {
System.out.println("1. Start new game\n" + "2. Load game\n"
+ "3. Settings\n" + "4. Exit\n");
}
public void selectMenu() throws InputMismatchException {
int selectedMenu;
boolean validSelection = false;
Scanner aScanner = new Scanner(System.in);
while(!validSelection) {
selectedMenu = aScanner.nextInt();
validSelection = true;
switch (selectedMenu) {
case 1:
// doWhen1();
break;
case 2:
// doWhen2();
break;
case 3:
// doWhen3();
break;
case 4:
// doWhen4();
break;
default:
System.out.println("Input out of range \"" + selectedMenu
+ "\". Input a number between 1-4.");
validSelection = false;
}
}
}
}

Here is a revision to the selectMenu() method you provided that should get the job done! I tested it out and it seems to work as expected. :)
public void selectMenu() {
int selectedMenuItem = 0;
Scanner aScanner = new Scanner(System.in);
while(selectedMenuItem == 0){
String userInputMenuItemString = aScanner.nextLine();
try {
int userInputMenuItem = Integer.parseInt(userInputMenuItemString);
if(userInputMenuItem > 0 && userInputMenuItem <= 4){
selectedMenuItem = userInputMenuItem;
}else{
System.out.println("No option #" + Integer.toString(userInputMenuItem) + " exists!\nTry again:");
}
} catch(NumberFormatException ex) {
System.out.println("Please input a number!");
}
}
switch(selectedMenuItem){
case 1:
System.out.println("You chose to start a new game!");
break;
case 2:
System.out.println("You chose to load a game!");
break;
case 3:
System.out.println("You chose to access settings!");
break;
case 4:
System.out.println("You chose to exit. Bye!");
System.exit(0);
break;
}
}