When I run the program if I type something other than "true" or "false" it throws a InputMismatchException.
do {
System.out.print("Do passengers have an individual tv screen?"
+ "(true OR false): ");
hasVideo = keyboard.nextBoolean();
bus.setIndividualVideo(hasVideo);
} while (!(hasVideo == true) && !(hasVideo == false));
Catch the error and treat it as a invalid response...
try {
System.out.print("Do passengers have an individual tv screen?"
+ "(true OR false): ");
hasVideo = keyboard.nextBoolean();
} catch (InputMismatchException exp) {
System.err.println("Please, enter only true or false");
}
Take a look at The try Block for more details
Aha, time to learn about Exception handling! Any of the Exception types that you see when java crashes can in fact be caught inside the program with a try-catch block.
try {
// code that might throw exceptions 1
// code that might throw exceptions 2
} catch (Exception e) {
// do something to fix the error
}
If any code in the try{ } part does throw an Exception then it will immediately skip to the catch( ) { } part, skipping any other statements in the try{ }.
Your code with a try-catch would look like:
boolean loopAgain = false;
do {
try {
System.out.print("Do passengers have an individual tv screen?"
+ "(true OR false): ");
hasVideo = keyboard.nextBoolean();
bus.setIndividualVideo(hasVideo);
loopAgain = false;
} catch (InputMismatchException e) {
System.err.println("Please, enter only true or false");
loopAgain = true;
}
} while (loopAgain);
Edit: I borrowed the println("Please, enter only true or false"); from #MadProgrammer's answer.
You have to prompt the user to enter a Boolean value. Because nextBoolean() can throw an exception, the best thing to do is to put that code inside a try/catch. The catch block code is only executed if anything other than true or false is entered. You can add a while() or do/while() loop to keep telling the user to try again. However, the most important thing to do in the catch block is to flush the input stream. Remember, even though there was an exception, the stream still contains stuff in it. It has to be properly consumed before using again. The code below should do exactly what you are looking for:
public static void main(String[] args)
{
Scanner input = new Scanner(System.in);
Boolean answer = null;
do
{
System.out.println("Enter either true or false");
try
{
answer = input.nextBoolean();
}
catch(InputMismatchException e)
{
System.out.println("ERROR: The input provided is not a valid boolean value. Try again...");
input.next(); // flush the stream
}
} while(answer == null);
input.close();
}
Related
This question already has answers here:
How to use Scanner to accept only valid int as input
(6 answers)
Closed 2 years ago.
I have a separate thread class and below is the run method,
#Override
public void run() {
Scanner scanner = new Scanner(System.in);
int iRequestType = 0;
int iRequestId = 0;
while (!exit){
try {
System.out.print("\nRealTime - 1\nHttpUrl - 2\nSelect one : ");
iRequestType = scanner.nextInt(); //it will wait here after the exception occurs
System.out.print("\nEnter Request ID : ");
iRequestId = scanner.nextInt();
if(iRequestType == 1 && !map_RealTimeRequests.isEmpty()){
sendRealTimeRequest(iRequestId);
} else if (iRequestType == 2 && !map_HttpUrlRequests.isEmpty()){
sendHttpUrlRequest(iRequestId);
} else if(iRequestType > 2) {
System.out.println("Invalid List Number");
}else {
System.out.println("Empty List");
}
} catch (InputMismatchException e){
System.out.println("Invalid request type or ID : " + e);
scanner.close();
scanner = new Scanner(System.in);
}
}
}
If I enter numbers, the program works fine. When I enter a character, it goes to the catch block and and execute the lines within it as expected. Once the execution of catch block completes, it will iterate again and ask for the keyboard inputs. But when I try to enter a value (a number or a character), it does not read anything and keep waiting without executing the next lines.
What I want to do is, ask the user to enter a number and if the user enter a character, print an error message in console and iterate the while loop again. How can I solve this issue? Thanks in advance!
You should not call scanner.close() because that closes System.in. You don't want to close System.in you just want to clear out the buffer. You can do that by replacing the scanner as you did (without the close() call), or you can call nextLine() if it's single-line input like this:
} catch (InputMismatchException e){
System.out.println("Invalid request type or ID : " + e);
scanner.nextLine();
}
The print statement in my exception never prints out. If I do not enter anything it will just continue to ask me to enter name of cruise ship. I need it to throw the exception print statement along with that if someone were to enter an empty string or anything other than a string.
do {
try {
System.out.println("Enter name of cruise ship: ");
cruiseShipName = scnr.nextLine();
}
catch(Exception e){
e.printStackTrace();
System.out.println("Invalid Input. Please try again.");
}
} while (cruiseShipName.equals(""));
Maybe your code will not throw any exception if the scnr.nextLine() is empty.
In your case if you want to ask the user to try again, you could do as following:
`
do {
System.out.println("Enter name of cruise ship: ");
cruiseShipName = scnr.nextLine();
if (cruiseShipName.equals("")) {
System.out.println("Invalid Input. Please try again.");
}
} while (cruiseShipName.equals(""));
`
The may help you.
do {
try {
System.out.println("Enter name of cruise ship: ");
cruiseShipName = scnr.nextLine();
if(cruiseShipName.equals("")) throw new Exception("");
}
catch(Exception e){
e.printStackTrace();
System.out.println("Invalid Input. Please try again.");
}
} while (cruiseShipName.equals(""));
This code doesn't actually throw an exception. You don't have anything that checks if the input is valid and throws an exception if it isn't.
So right now, the input is always valid.
import java.util.Scanner;
public class HelloWorld
{
public static void main(String[] args)
{
Scanner math = new Scanner(System.in);
System.out.println("What is 2 + 2?");
int num = math.nextInt();
System.out.println("Wrong."); // Displays "Wrong." no matter the answer.
{
Thread.sleep(2000); // Adds a timer for 2 seconds.
}
catch(InterruptedException e)
{
System.out.println("Please, try again :-)");
}
int num1 = math.nextInt();
System.out.println(""); //Displays some message.
}
}
The code is supposed to display "What is 2 + 2?", a user inputs an answer, the code returns "Wrong." no matter what the answer. After a 2 second pause, the code displays "Please, try again :-)" and a user inputs an integer and the code returns a message.
The errors occur on the line with the catch token. The errors are:
Syntax error on token "catch", ( expected,
Syntax error, insert "-> LambdaBody" to complete LambdaExpression,
Syntax error, insert "AssignmentOperator Expression" to complete Assignment,
Syntax error, insert ";" to complete Statement
To use a catch in java you need to have a try. It is called a try..catch block. Please read the documentation here
So, adding a try as follows should get rid of the errors you are asking about here :
System.out.println("Wrong."); // Displays "Wrong." no matter the answer.
try // Looks like you missed the try here
{
Thread.sleep(2000); // Adds a timer for 2 seconds.
} catch (InterruptedException e) {
System.out.println("Please, try again :-)");
}
I cant see try block. you can use this way also without try catch. Main method can throw InterruptedException ...
public class HelloWorld {
public static void main(String[] args) throws InterruptedException {
Scanner math = new Scanner(System.in);
System.out.println("What is 2 + 2?");
int num = math.nextInt();
System.out.println("Wrong."); // Displays "Wrong." no matter the answer.
{
Thread.sleep(2000); // Adds a timer for 2 seconds.
}
int num1 = math.nextInt();
System.out.println(""); //Displays some message.
}
}
I am trying to make a program which allows the user to enter 2 integers (marks).
In case the user doesn't enter an integer, I am creating a try and catch code.
The problem is that after I try to enter letters instead of numbers, there is an error coming out but the program carries on, saying that I didn't pass. How do I let the program stop after saying to the user that he entered a wrong mark?
Here is my code:
public void actionPerformed(ActionEvent e)
try{
myCalculator.setCWK(Integer.parseInt(courseEnter));
myCalculator.setExam(Integer.parseInt(examEnter));
}
catch (Exception a){
System.out.print("System error");
}
displayArea.setText("" + myCalculator.calculateModuleMark());
if(myCalculator.hasPasssed()==true)
{
displayArea.setText(myCalculator.getModuleCode() + "Congratulations! You have PASSED! With a score of " + myCalculator.calculateModuleMark() + "%");
getContentPane().setBackground(Color.green);
}
else
{
displayArea.setText("I am sorry");
getContentPane().setBackground(Color.red);
}
}
If you want the program to "stop" after a certain statement, System.exit(0) is your friend. So, in your catch statement, you could have
catch (Exception a){
System.out.print("System error");
System.exit(0);
}
Note that this is different than return, as System.exit(0) will completely stop your program flow, not just this specific method.
After printing out an error simply type return.
The try...catch blocks means
if an error occurs in the try instruction block, execute the catch instruction block
In your case, you're not exiting the function in your catch block, so it will carry on.
Exception handling is essentially done to prevent the code from exiting abruptly without any error message.
You can just call System.exit(1) after System.out.print("System error").
Note: System.exit(0) means program terminated as expected while any other error code within bracket means there was an error.
So, now your code will be:
public void actionPerformed(ActionEvent e){
try{
myCalculator.setCWK(Integer.parseInt(courseEnter));
myCalculator.setExam(Integer.parseInt(examEnter));
}
catch (Exception a){
System.out.print("System error");
System.exit(1);
}
displayArea.setText("" + myCalculator.calculateModuleMark());
if(myCalculator.hasPasssed()==true)
{
displayArea.setText(myCalculator.getModuleCode() + "Congratulations! You have PASSED! With a score of " + myCalculator.calculateModuleMark() + "%");
getContentPane().setBackground(Color.green);
}
else
{
displayArea.setText("I am sorry");
getContentPane().setBackground(Color.red);
}
}
I have a very simple loop that waits for a number (int) and as long as that number is not exitOption it does not leave the loop, however I get an unexpected error, and I don't know what's causing it.
Edit
Adding another snippet so you can compile
public static void main(String[] args) throws FileNotFoundException,
SecurityException,
IOException,
ClassNotFoundException {
while (controller.selectOptionMM());
/Edit
public boolean selectOptionMM() throws SecurityException,
FileNotFoundException,
IOException {
int cmd = ui.getExitOption();
ui.mainMenu();
cmd = utils.readInteger(">>> "); // this is my problem, right here
// code in next snippet
while (cmd <1 || cmd > ui.getExitOption()) {
System.out.println("Invalid command!");
cmd = utils.readInteger(">>> ");
}
switch (cmd) {
case 1:
case 2:
case 3:
case 4: this.repository.close();
return true;
case 5: return false;
}
return false;
}
Here is what fails:
public int readInteger(String cmdPrompt) {
int cmd = 0;
Scanner input = new Scanner(System.in);
System.out.printf(cmdPrompt);
try {
if (input.hasNextInt())
cmd = input.nextInt(); // first time it works
// Second time it does not allow me to input anything
// catches InputMissmatchException, does not print message
// for said catch
// infinitely prints "Invalid command" from previous snippet
} catch (InputMismatchException ime) {
System.out.println("InputMismatchException: " + ime);
} catch (NoSuchElementException nsee) {
System.out.println("NoSuchElementException: " + nsee);
} catch (IllegalStateException ise) {
} finally {
input.close(); // not sure if I should test with if (input != null) THEN close
}
return cmd;
}
First time I pass trough, it reads the number no problem. Now if the number is not 5 (in this case exitOption), it passes again trough readInteger(String cmdPrompt) except this time it jumps to catch (InputMismatchException ime) (debug) except it does not print that message and just jumps to Error, input must be number and Invalid command.
Is something stuck in my input buffer, can I flush it, why is it (input buffer) stuck (with random data)???
I'll try debugging again and see what's stuck in my input buffer, if I can figure out how to see that.
The problem is in the call to input.close() - this causes the underlying input stream to be closed. When the input stream being closed is System.in, bad things happen (namely, you can't read from stdin any more). You should be OK just eliminating this line.
input.hasNextInt()
This line throws the exception if there is no Integer, so instead of it going to else block it forward to catch block. It will never go to else block if exception get caught.