First post/question here and very new to java. Trying to make a small text based movie database app. One part of it is to add a review. The problem is when I just hit [enter] when asked for a Score number [0-10] on the review, the prompt just drops one step down and keeps waiting for input. I want it to be impossible to leave this field blank. Here is what I have so far:
int score;
do {
while (!sc.hasNextInt()) {
sc.next();
System.out.print("\nInvalid input! ");
System.out.print("Please enter a number from 0-10: ");
}
score = sc.nextInt();
if (!(score >= 0 && score <= 10)) {
System.out.print("\nInvalid input! ");
System.out.print("Please enter a number from 0-10: ");
}
} while (!(score >= 0 && score <= 10 ));
Scanner is reading tokens, i.e. text separated by whitespaces. That means, that as long as you just press Enter, the next token hasn't even started yet.
One of the fallacies of Scanner is that it's so easy to use, but even easier to misuse, e.g. mishandling user input.
Example: What should happen if used enters 123 abc<enter>? Your code will read 123 and continue, leaving abc in the buffer for the next prompt, which might want text and hence read the abc as that text. Oops!!
Most of the time, to handle bad user input, you should never use hasNextXxx() and nextXxx() methods other than nextLine(). It's the only way to ensure you get one answer (input) for one prompt.
So, you should do something like this:
int score;
for (;;) {
System.out.print("Please enter a number from 0-10: ");
String line = sc.nextLine();
try {
score = Integer.parseInt(line.trim());
} catch (NumberFormatException e) {
System.out.println("Invalid input! Not a number.");
continue;
}
if (score < 0 || score > 10) {
System.out.println("Invalid input! Out of range.");
continue;
}
break;
}
Related
System.out.println("Enter your age here:");
setAge(sc.nextInt());
How can I validate that users' age is not a char or a negative number?
Ideally, if the users input anything but an int, the program would ask for the input again.
I have tried using a do-while, but doesn't seem to be working.
I am a beginner. Any help is super appreciated.
Thanks!
What you are doing with sc.nextInt() will only allow the user to enter an int or the program will throw an InputMismatchException (thus that part behaves the way you want). If you want to make sure the number isn't negative though, do this:
System.out.println("Enter your age here:");
while (!sc.hasNextInt()) {
System.out.println("Please enter an integer.");
sc.next();
}
int age = sc.nextInt();
if(age < 0) {
//do what you want if the number is negative
//if you're in a loop at this part of the program,
//you can use the continue keyword to jump back to the beginning of the loop and
//have the user input their age again.
//Just prompt them with a message like "invalid number entered try again" or something to that affect
}
else {
setAge(age);
//continue execution
}
The following block will do what you need:
int age;
System.out.println("Please enter an integer");
while (true) {
try{
age= scan.nextInt();
if (age<=0) throw new Exception("Negative number");
break;
} catch(Exception e){
System.out.println("Please enter a positive integer");
}
scan.nextLine();
}
// below just call
setAge(age);
I hope this helps.
I'm making a simple program that asks the user to input five numbers between 0-19. I would like to add something (like an if statement) after every number to make sure it's within that range. If not, the program should say "please read instructions again" and will then System.exit(0). This is the piece of the code that is relevant:
System.out.println("Please enter 5 numbers between 0 and 19");
System.out.print("1st Number: ");
userNum1 = scan.nextInt();
System.out.print("2nd Number: ");
userNum2 = scan.nextInt();
System.out.print("3rd Number: ");
userNum3 = scan.nextInt();
System.out.print("4th Number: ");
userNum4 = scan.nextInt();
System.out.print("5th Number: ");
userNum5 = scan.nextInt();
Any help would be greatly appreciated.
You can put this after each of your inputs, but you might want to think about putting this logic into its own method, then you can reuse the code and just call it with something like validateInput(userNum1);.
Replace val with your actual variable names.
if (val < 0 || val > 19) {
System.out.println("please read the instructions again");
System.exit(0);
}
First of all, I would create a for-loop that iterates N times, with N being the number of numbers you want to ask for (in your case, 5). Imagine your example with 50 numbers; it would be very repetitive.
Then, when you get each number with scan.nextInt() within your for-loop, you can validate however you want:
if (userNum < 0 || userNum > 19) {
// print error message, and quit here
}
Also, instead of just exiting when they input a number outside the range, you could have your logic inside a while loop so that it re-prompts them for the numbers. This way the user doesn't have to restart the application. Something like:
boolean runApplication = true;
while(runApplication) {
// do your for-loop with user input scanning
}
Then set the runApplication flag as needed based on whether or not the user put in valid numbers.
This code will do the trick for you, i added some securities :
public static void main(String[] args) {
int count = 1;
Scanner scan = new Scanner(System.in);
List<Integer> myNumbers = new ArrayList<Integer>();
System.out.println("Please enter 5 numbers between 0 and 19");
do {
System.out.println("Enter Number "+count+" ");
if(scan.hasNextInt()){
int input = scan.nextInt();
if(input >= 0 && input <= 19){
myNumbers.add(input);
count++;
}else{
System.out.println("Please read instructions again");
System.exit(0);
}
}else{
scan.nextLine();
System.out.println("Enter a valid Integer value");
}
}while(count < 6);
/* NUMBERS */
System.out.println("\n/** MY NUMBERS **/\n");
for (Integer myNumber : myNumbers) {
System.out.println(myNumber);
}
}
Hope it helps
Since you already know how many numbers you want the user to input, I suggest you use a for loop. It makes your code more elegant and you can add as many more entries as you want by changing the end condition of the loop. The only reason it looks long is because number 1, 2, 3 all end in a different format i.e firST secoND thiRD, but the rest of the numbers all end with TH. This is why I had to implement some if else statements inside the loop.
To explain the code, every time it loops it first tells the user the count of the number he/she is entering. Then numEntry is updated every time the loop loops, therefore you do not need to assign multiple inputs to multiple variables. It is more efficient to update the same variable as you go on. If the input the user inputs is less than 0 OR it is more than 19, the system exits after an error message.
System.out.println("Please enter a number between 0 and 19");
Scanner scan = new Scanner(System.in);
for(int i = 1; i <=5; i++){
if(i == 1)
System.out.println("1st Number");
else if(i == 2)
System.out.println("2nd Number");
else if(i == 3)
System.out.println("3rd Number");
else
System.out.println(i + "th Number");
int numEntry = scan.nextInt();
if(numEntry < 0 || numEntry > 19){
System.out.println("Please read instructions again.");
System.exit(1);
}
I am trying my hand a few basic do-while codes, and am running into a couple of problems.
I want the code to ask the user to input 1 of 3 options (choosing which group they would like to add a number to, or to exit and total), give an error if they input an irrelevant option, and then total all ints at the end for each group.
public static void main(String[] args) {
String answer = "default";
int grp1 = 0;
int grp2 = 0;
int input1 = 0;
int input2 = 0;
do{
System.out.println("Make a selection:\n");
System.out.println("A: Enter a number for Group 1.");
System.out.println("B: Enter a number for Group 2.");
System.out.println("X: Exit and total the numbers for each group.\n");
System.out.println("Select your option: ");
answer = keyboard.next();
if (answer.equalsIgnoreCase("A")){
System.out.println("Enter int: ");
input1 = keyboard.nextInt(); // add an int to grp1
}
else if (answer.equalsIgnoreCase("B")){
System.out.println("Enter int: ");
input2 = keyboard.nextInt(); // add an int to grp2
}
else if (answer.equalsIgnoreCase("X")){
} // exit and total
else {
System.out.println("Invalid option - Try again.");
} // Invalid input - restart
}
while (answer.equals("A") || answer.equals("B"));
grp1 += input1;
grp2 += input2;
keyboard.close();
System.out.println("Group 1's total is: + grp1);
System.out.println("Group 2's total is: + grp2);
}
I need the to add a qualifier for if the user does not input a valid option, I tried using else:
else {
System.out.println("Invalid option - Try again.")
}
but this just skips to printing the totals, and does not ask the user for another input. How would I best achieve this?
Also,
grp1 += input1;
grp2 += input2;
Only counts the lasted entered int, is there a way to have it add all the entered ints?
Any help would be greatly appreciated, even outside of the questions I asked.
I think you have two confusions.
1) The "while" line in your code applies to the "do" block above it. That means that based on where the grp1 += and grp2 += lines are, they will only ever be run once. I suggest moving those calls to the end of the loop. You could move each line inside the relevant if block so that the code is run every time the user successfully enters a number after A or B.
2) The while condition is asking if the user entered "A" or "B". It's saying if they did, continue looping by going back to "do". If they entered literally anything else (any invalid answer), it will stop and run the code after the "while" line. I think what you really want is while (!answer.equals("X")), which will continue the loop until the user correctly enters an "X" character.
You'll also want to move those grp += lines up a bit.
Just change the condition inside while And also shift the totalling logic
do{
System.out.println("Make a selection:\n");
System.out.println("A: Enter a number for Group 1.");
System.out.println("B: Enter a number for Group 2.");
System.out.println("X: Exit and total the numbers for each group.\n");
System.out.println("Select your option: ");
answer = keyboard.next();
if (answer.equalsIgnoreCase("A")){
System.out.println("Enter int: ");
input1 = keyboard.nextInt(); // add an int to grp1
grp1 += input1;
}
else if (answer.equalsIgnoreCase("B")){
System.out.println("Enter int: ");
input2 = keyboard.nextInt(); // add an int to grp2
grp2 += input2;
}
else if (answer.equalsIgnoreCase("X")){
} // exit and total
else {
System.out.println("Invalid option - Try again.");
} // Invalid input - restart
}
while (!answer.equals("X"));
keyboard.close();
This will make your do while loop running i.e showing options to user until they wishes to exit. And also group total would be updated properly. I have updated answer based on answer by #Devin Howard
Somehow I am getting some funny play here and I just cant see why.
This method is supposed to make sure that the input is either y or n, and is NOT blank.
I should note this is for school, and the 2 separate error outputs are required.
When I enter a blank line, I get exactly that at the console- a blank line.
After that point, or after a time where I purposely enter bad data, such as x, the very next time I enter a valid data, like y or n, I continue to get the bad data in an endless loop.
What have I done wrong?
public static boolean getContinue(Scanner sc)
{
boolean decision = false;
System.out.println("Continue? Y/N: ");
String userChoice = sc.next();
boolean isValid = false;
while (isValid == false)
{
if (userChoice.equalsIgnoreCase("y"))
{
decision = true;
isValid = true;
}
else if (userChoice.equalsIgnoreCase("n"))
{
decision = false;
isValid = true;
}
else if (userChoice.equals(""))
{
System.out.println("Error! This entry is required. Try again.");
userChoice = sc.next();
}
else if (!userChoice.equalsIgnoreCase("y") | (!userChoice.equalsIgnoreCase("n")))
{
System.out.println("Error! Entry must be 'Y' or 'N'. Try again.");
userChoice = sc.next();
}
}
return decision;
}
NOTE:
Code was amended to include
New console outputs(Still wrong)
FORMATTED RESULTS
Loan amount: $5.00
Yearly interest rate: 500%
Number of years: 5
Monthly Payment: $2.08
Continue? Y/N:
b
Error! Entry must be 'Y' or 'N'. Try again.
y
Error! Entry must be 'Y' or 'N'. Try again.
y
Enter loan amount:
You don't set the variable userChoice to the new value. Change your last if clause to
System.out.println("Error! Entry must be 'Y' or 'N'. Try again.");
userChoice = sc.next();
Because userChoice never changes inside the loop (which is because you don't change it).
Please look at else if part:
else if (!userChoice.equalsIgnoreCase("y") | (!userChoice.equalsIgnoreCase("n")))
here you have done 2 errors:
you have used |, which is bitwise operator, not logical
the logic requires logical AND (&&) to get the cases when the input is not either y or n
SO the correct else-if:
else if (!userChoice.equalsIgnoreCase("y") && (!userChoice.equalsIgnoreCase("n")))
{
System.out.println("Error! Entry must be 'Y' or 'N'. Try again.");
userChoice = sc.next();
}
You have to use sc.nextLine() in order to catch the Empty Blank Line. So replace all the instances of sc.next() with sc.nextLine()
In the below code, I ask the user to give an integer input and if the input is 0 or a negative number, it loops again until the positive number is given. The thing is that if the users presses a letter, my code crashes and despite the fact that I used try-catch in a lot of ways nothing really worked. Any ideas?
I used try-catch inside the loop, but it only worked for one letter input and not correctly.
System.out.print("Enter the number of people: ");
numberOfPeople = input.nextInt();
while (numberOfPeople <= 0) {
System.out.print("Wrong input! Enter the number of people again: ");
numberOfPeople = input.nextInt();
}
The problem in your current code is that you're always trying to read an int so when receiving a non-integer input you can't handle the error in the right way. Modify this to always read a String and convert it into an int:
int numberOfPeople = 0;
while (numberOfPeople <= 0) {
try {
System.out.print("Enter the number of people: ");
numberOfPeople = Integer.parseInt(input.nextLine());
} catch (Exception e) {
System.out.print("Wrong input!");
numberOfPeople = 0;
}
}
//continue with your life...