Develop Reference

What is wrong with this java while loop?

New to Java and learning how to use While loops and random generator. This prints a multiplication question. Every time the user answers a question wrong, it should print the same question. Instead, it exits the program. What should I do?
while (true) {
Random multiply = new Random();
int num1 = multiply.nextInt(15);
int num2 = multiply.nextInt(15);
int output = num1 * num2;
System.out.println("What is the answer to " + num1 + " * " + num2);
Scanner input = new Scanner(System.in);
int answer = input.nextInt();
if (answer == output) {
if (answer != -1)
System.out.println("Very good!");
} else {
System.out.println("That is incorrect, please try again.");
}
}

If you want to repeat the same question when the user gets the answer wrong, you should use another while inside your main loop.
This inner loop continues to ask as long as you give a wrong answer.
I also replaced nextInt with nextLine, which reads in a whole line of text. This consumes the "Enter" key and is a safer approach at reading from the console. Since the result is now a String you need to use Integer.parseInt to convert it to an int. This throws an exception if you enter anything but a whole number so I wrapped it into a try-catch block.
If you want, you can add an additional check for validating user input. So in case the user wants to stop playing they only need to input "exit" and the whole outer loop will exit.
boolean running = true; // This flag tracks if the program should be running.
while (running) {
Random multiply = new Random();
int num1 = multiply.nextInt(15);
int num2 = multiply.nextInt(15);
int output = num1 * num2;
boolean isCorrect = false; // This flag tracks, if the answer is correct
while (!isCorrect) {
System.out.println("What is the answer to " + num1 + " * " + num2);
Scanner input = new Scanner(System.in);
try {
String userInput = input.nextLine(); // Better use nextLine to consume the "Enter" key.
// If the user wants to stop
if (userInput.equals("exit")) {
running = false; // Don't run program any more
break;
}
int answer = Integer.parseInt(userInput);
if (answer == output) {
if (answer != -1) {
System.out.println("Very good!");
isCorrect = true; // Set the flag to true, to break out of the inner loop
}
} else {
System.out.println("That is incorrect, please try again.");
}
}
catch(NumberFormatException e) {
System.out.println("Please enter only whole numbers");
}
}
}

Avoid while true. Declare a variable to true, pass the variable to the condición loop and set it to false when the answer is incorrect. You can use break too, but is easier to read the code when you use a exit condition in the while. Also read more about loops https://docs.oracle.com/javase/tutorial/java/nutsandbolts/while.html

Parsing string generating my generated error message when nothing is entered

I am currently working on an assignment to parse strings and I am running into an issue.
It appears, that if nothing is entered, it is generating my error message I have created when a comma is not inputted.
According to the assignment in zybooks, it should not be outputting anything. Below is my code.
import java.util.Scanner;
public class ParseStrings {
public static void main(String[] args) {
Scanner scnr = new Scanner(System.in); // scanner for input
//local variables
String lineString;
String firstWord;
String secondWord;
int commaLocation;
boolean inputDone;
//checks to end the program
inputDone = false;
//keeps the loop running until q is entered
while (!inputDone) {
System.out.println("Enter input string: ");
lineString = scnr.nextLine();
//checks comma
commaLocation = lineString.indexOf(',');
if (commaLocation == -1) {
System.out.println("Error: No comma in string");
}
else {
firstWord = lineString.substring(0, commaLocation);
firstWord = firstWord.replace(" ", "");
secondWord = lineString.substring(commaLocation + 1, lineString.length());
secondWord = secondWord.replace(" ", "");
System.out.println("First word: " + firstWord);
System.out.println("Second word: " + secondWord);
System.out.println();
System.out.println();
}
if (lineString.equals("q")) {
inputDone = true;
}
}
return;
}
}

It happens because when nothing is entered, comma cannot be found, therefore indexOf returns -1.
I would add something like
if (lineString.isEmpty()) {
continue;
}
right after lineString = scnr.nextLine();
EDIT:
I just noticed, that your error message will be printed in case, when your input equals 'q'. I assume this is not an expected behaviour, so I recommend to place
if (lineString.equals("q")) {
inputDone = true;
}
right after assignment of lineString or the if block I suggested above.

Loop not breaking when using user input

public static void main(String[] args) {
Scanner input = new Scanner(System.in);
Queue que = new Queue(50);
System.out.println("Noki's Calculator");
System.out.println("Choose an operation (*, /, +, -)");
String inputOp = input.nextLine();
System.out.println(" ");
System.out.println("Now please enter your numbers. When finished, simply press enter.");
boolean cont = true;
while(true){
System.out.print(" :: ");
String j = input.next();
if(j != null) {
int numb = Integer.parseInt(j);
que.insert(numb);
} else if (j==null) {
input.close();
break;
}
}
que.view(); // PROBLEM: never executes this line
}
My code won't break out of the while loop, instead still asks for user input. When I press enter, it doesn't print "::" again, but never executes que.view()

Use nextLine() instead of next() like below,
String j = input.nextLine();

According to your implementation 'j' will never be null. When you hit enter "" will be assigned to j. You may need to change your logic as follows; Use input.nextLine() instead of input.next()
if(!(j.isEmpty())) {
int numb = Integer.parseInt(j);
que.insert(numb);
} else {
input.close();
break;
}

input.next() does not return "null" when you just press "enter".
change
else if (j==null)
to
else if (j.equals(""))
Edit: this doesn't exactly answer the question but it has already been answered so i am leaving it as is.

Get some pointers/suggestions on first basic java program?

When I run the program and the user input field comes up, when I just hit enter without entering anything in the blank space another input field comes up. This happens over and over if I keep hitting enter. If there is a way to solve this please let me know. I couldn't find anything to fix it in the documentation.
import java.util.Scanner;
import java.util.Random;
class Game {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
//generate a random number
//take user input
//check the input act appropriately
boolean running = true;
boolean guessed = true;
boolean realInt = true;
int number = 0;
int input = 0;
String firstName=" ";
System.out.print("want to play the game? Yes/No: ");
firstName = sc.next();
while (running) {
if (guessed && (("yes".equals(firstName)) | ("cheater".equals(firstName)))){
System.out.println("I have chosen a random value");
System.out.println("Have a go at guessing it");
Random rand = new Random();
number = rand.nextInt(10) + 1;
guessed = false;
}
if(("yes".equals(firstName)) | ("cheater".equals(firstName)))
{
if(sc.hasNextInt()) {
realInt=true;
input = sc.nextInt();
}
else{
System.out.println("That is not an integer.");
running = false;
}
}
else
running = false;
if((((input != number) && realInt)&&(input != -1))){
System.out.println("Plsease try again");
}
//below line sets up when to stop the program, when -1 is entered
if (input == -1){
running = false;
}
else{
guessed = input == number;
}
//below lines are cheat codes
if (input == -5){
System.out.println("Answer: " + number);
}
if ("cheater".equals(firstName) && (input != number)){
System.out.println("Answer: " + number);
}
}
}
}

This is due to the way how Scanner's next() method reads input.
A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace.
So even if you hit number of enters, there will not be any token and hence Scanner is just waiting from user to have one valid token so that in can read and continue reading the next token.
public String next()
Finds and returns the next complete token from this scanner. A complete token is preceded and followed by input that matches the delimiter pattern. This method may block while waiting for input to scan, even if a previous invocation of hasNext() returned true.
Solution:
Use Scanner's hasNextLine() and nextLine() methods.It will allow you to capture empty or white-space lines. Refer to the API on java site for other methods.

Java: Using scanner to read in boolean values failing.

import java.util.Scanner;
public class Cardhelp2{
private static String[] pairArray={"A,A","K,K","Q,Q","J,J","10,10","9,9","8,8","7,7","6,6","5,5","4,4","3,3","2,2"};
public static void generateRandom(int k){
int minimum = 0;
int maximum = 13;
for(int i = 1; i <= k; i++)
{
int randomNum = minimum + (int)(Math.random()* maximum);
System.out.print("Player " + i +" , You have been dealt a pair of: ");
System.out.println(pairArray[randomNum]);
}
} //reads array and randomizes cards
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.print("How many players would you like to play with? ");
int m = scan.nextInt();
generateRandom(m);
//displays the cards
___________________________________________________
System.out.println("Would you like to play?");
Scanner scanner = new Scanner(System.in);
if(scanner.next().equalsIgnoreCase("y")||scanner.next().equalsIgnoreCase("yes")) {
System.out.println("This will be fun");
} else if(scanner.next().equalsIgnoreCase("n")||scanner.next().equalsIgnoreCase("no")) {
System.out.println("Maybe next time");
} else {
System.out.println("Invalid character");
}
}
}
Im having trouble understanding why the end part is not working, I've been told i need to change scanner.next(); to a variable but im not sure how to do it and get the code working. Is there a simple way of reading in the users answer then displaying a response to the user?
Thanks

Your conditional expression
if(scanner.next().equalsIgnoreCase("y")||scanner.next().equalsIgnoreCase("yes"))
calls scanner.next() twice, which means the second call will read/wait for more input. Instead you need to call it only once, store the result and use that in the comparison:
String tmp = scanner.next();
if(tmp.equalsIgnoreCase("y")||tmp.equalsIgnoreCase("yes"))

Let's assume the user inputs "yes".
At
if(scanner.next().equalsIgnoreCase("y")||scanner.next().equalsIgnoreCase("yes")) {
Scanner.next() produces "yes" in the first test. So the code is effectively
"yes".equalsIgnoreCase("y")
Which is false, so it moves to the next test:
scanner.next().equalsIgnoreCase("yes")
Here's where your issue is.
the "yes" entered has already been consumed by the first test. Now the Scanner has nothing in the buffer.
If you want to test the SAME input again, you must capture it, and use that in your tests.
So
String userReply= Scanner.next();
if(userReply.equalsIgnoreCase("y")||userReply.equalsIgnoreCase("yes")) {...
This is becauswe, with each call to scanner.next(), the Scanner returns the next value in the stream, and then MOVES PAST IT
If the user had entered "yes" and then "no", the tests would be performed like this:
if("yes".equalsIgnoreCase("y")||"no".equalsIgnoreCase("yes")) {...

You need change the way of Scanner's calls.
The user input \n and Scanner seems don't follow with the next token. Then you need read line by line.
:
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("How many players would you like to play with? ");
int m = Integer.parseInt(sc.nextLine()); // May thrown NumberFormatException
generateRandom(m);
//displays the cards
System.out.print("Would you like to play? ");
String input = sc.nextLine();
if (input.equalsIgnoreCase("y") || input.equalsIgnoreCase("yes")) {
System.out.println("This will be fun");
} else if (input.equalsIgnoreCase("n") || input.equalsIgnoreCase("no")) {
System.out.println("Maybe next time");
} else {
System.out.println("Invalid character");
}
}