I'm new to java but I've experimented a lot with this particular program and I've hit a wall. The purpose of this program is to catch if the user inputs a value exceeding the variable limit and also to validate that the input satisfies my specified range.
If the input exceeds the variable limit without a while loop, the program exits.
If the input exceeds the variable limit with a while loop, it loops infinitely.
I've tried using a while loop and my three outcomes are this:
The user inputs a programmer-specified valid number and all is well.
The user inputs a number above 100 and is prompted to try again...
The user exceeds the variable limit and the program goes into an
infinite loop.
If I put an escape in the catch block, it does the same as if the while was not there. I want the program to print the error but allow the user to retry a value.
while(!success){
try{
sh = sc.nextShort();
while(sh < 1 || sh > 100){
System.out.print("You must enter a value between 1-100: ");
sh = sc.nextShort();
} //close try
System.out.println("Great job!");
success = true; //escape the while
}catch{Exception ex){
System.out.println("ERROR: " + ex);
success = false; //causes infinite loop... I understand
} //close catch
} //close while
The problem is, that Scanner stores the whole input string and processes it.
When your input is not a short value but a string it throws an exception. You log the exception in the catch and in the next circle it tries to read a short value from the same stored string, which throws exception again .... -> infinite loop.
If you create a new Scanner in the catch, it will read again from the console:
while (!success) {
try {
sh = sc.nextShort();
while (sh < 1 || sh > 100) {
System.out.print("You must enter a value between 1-100: ");
sh = sc.nextShort();
} //close try
System.out.println("Great job!");
success = true; //escape the while
} catch (Exception ex) {
System.out.println("ERROR: " + ex);
sc = new Scanner(System.in);
} //close catch
}
As i understand your problem , you are facing a loop of exception on passing a bigger value (something bigger like e.g "10000000000").
you have the exceptions coming as below in infinite loop:
ERROR: java.util.InputMismatchException: For input string: "10000000000"
ERROR: java.util.InputMismatchException: For input string: "10000000000"
But you want program to print a exception line and then allow user to input again.
You can do that by below a way of reading bad input (String bad_input = sc.next();) from the scanner you used.
while (!success) {
try {
sh = sc.nextShort();
while (sh < 1 || sh > 100) {
System.out.print("You must enter a value between 1-100: ");
sh = sc.nextShort();
} // close try
System.out.println("Great job!");
success = true; // escape the while
} catch (Exception ex) {
System.out.println("ERROR: " + ex);
success = false; // causes infinite loop... I understand
String bad_input = sc.next();// your bad input to allow read that exception
} // close catch
} // close while
The cause of this problem can be found here:
If the translation is successful, the scanner advances past the input
that matched
If I understand your question correctly, you can get rid of your second while loop and replace it with an if. Then you can print out the value must be 1-100 and throw and exception to cause your program to go through the catch statement and print out the error you threw.
while(!success){
try{
sh = sc.nextShort();
if(sh < 1 || sh > 100){
System.out.print("You must enter a value between 1-100");
throw Exception;
} //close try
System.out.println("Great job!");
success = true; //escape the while
}catch{Exception ex){
System.out.println("ERROR: " + ex);
success = false; //causes infinite loop... I understand
} //close catch
} //close while
Related
I want to create a class that will serve me as an exception. I will give you two examples of how I tried. I've been looking for it but I can't find an example of how to put this into practice, or it doesn't work for me.
This is my method where user choose flight by ID, I want to throw an exception if a program user enters a String.
Code:
#Override
public void choosePassenger(ArrayList<Passenger> passengerList) {
System.out.println("Choose passenger by ID: ");
try {
int pickedPassenger = scanner.nextInt();
for (Passenger tempUser : passengerList) {
if (pickedPassenger == tempUser.getId()) {
System.out.println("You picked passenger: " + tempUser.getFirstName() + ", "
+ tempUser.getLastName() + ". Balance is: " + tempUser.getBalance());
selectedPassenger = tempUser;
break;
}
}
} catch (InputMismatchException e) {
System.out.println("Wrong input! Try again");
scanner.nextLine();
}
}
I tried on that way but after program show me message Wrong input! Try again it goes to another method not giving me chance to enter valid input
Also, how I can create exception here for just Strings?
System.out.print("Add name of passenger: ");
passenger.setFirstName(scanner.nextLine());
This is part of code that enable program user to create new user, he is adding here name, I want to create exception so if program user add integer for name I want to show him an exception
EDIT:
If I delete scanner.nextLine(); program stops but first print me my own exception message then print me message from next method and then InputMissMatchException for that method on his own because I input string instead of integer
If you want the user to try again, you can keep them in an infinite loop that breaks after correct input.
Editing as per #DevilsHnd observation. You cannot use scanner.nextInt() in this manner as it will read without user being prompted for input and it results in being stuck in an infinite loop.
You can then use nextLine() and validate the input is a number like this:
while(true) {
try {
int pickedPassenger = Integer.parseInt(scanner.nextLine());
} catch (NumberFormatException e) {
System.out.println("Wrong input! Try again");
}
}
For restricting input to a non numeral value as you want for the name, you would have to check if the value entered is a number (there is no scanner method to restrict reading to non numeral values)
Most elegant way in my opinion is to use an external library that has this like Apache Commons:
while(true) {
String firstName = scanner.nextLine();
if (!NumberUtils.isCreatable(firstName )) {
passenger.setFirstName(firstName );
break;
} else {
System.out.println("Value cannot be a number!");
}
}
However, if you cannot import external libraries, simplest way to do this is by using built in Java Integer.parseInt() function(although you might want to consider regex - see link below):
while(true) {
String firstName = scanner.nextLine();
try {
Integer.parseInt(firstName);
System.out.println("Value cannot be a number!");
} catch (NumberFormatException e) {
passenger.setFirstName();
break;
}
}
For more options on checking if a value is a number (like regex) check this:
https://www.baeldung.com/java-check-string-number
} else if (selectionKey == 2) {
System.out.println("Please enter the item name");
if (s.nextLine() != "") {
item = s.nextLine();
}
try {
ZybezChecker zb = new ZybezChecker(item);
zb.getAveragePrice();
System.out.println(zb.toString());
} catch(Exception e) {
System.out.println("Something went wrong. Perhaps an invalid item name?");
}
That's my code atm. How do I return back to the if statement and continue the loop after it catches?
You could embed it in a loop like,
for (;;) { // <-- start an infinite loop
System.out.println("Please enter the item name");
if (s.nextLine() != "") {
item = s.nextLine();
}
try {
ZybezChecker zb = new ZybezChecker(item);
zb.getAveragePrice();
System.out.println(zb.toString());
break; // <-- terminate the infinite loop.
} catch(Exception e) {
System.out.println("Something went wrong. Perhaps an "
+ "invalid item name?");
e.printStackTrace(); // <-- tell them what went wrong.
}
}
I think (if I understand your question and code correctly) that what you want is a loop containing the s.nextLine(). Note that I am assuming several things here:
s is a Scanner or something equivalent that reads input from the user
an exception is thrown if the user enters invalid input
you want to keep asking the user for input until they enter something valid
If this is the case, then you should create a loop like this:
while (true) {
System.out.println("Please enter the item name");
if (s.nextLine() != "") {
item = s.nextLine();
}
try {
ZybezChecker zb = new ZybezChecker(item);
zb.getAveragePrice();
System.out.println(zb.toString());
break;
} catch(Exception e) {
System.out.println("Something went wrong. Perhaps an invalid item name?");
}
}
Also, why are you calling nextLine() twice? When you call it the first time, it will read a line from the scanner. When you call it again, it will not return the same line; it will instead wait for a new line. This means the user has to enter some random string, then enter the actual value. Finally, you should NEVER use == or != on Strings. Since they are reference types, you are essentially checking if they occupy the same location in memory, rather than if they are equal. Use s.nextLine().equals("") instead.
When I run my simple code and enter char instead of integer value which was supposed to be Entered.
Program, listed below is supposed to be terminated after printing "error please Enter integer value".
But this code, also printing the line after Occurrence of error
import java.util.InputMismatchException;
import java.util.Scanner;
public class Test {
public static void main(String[] args) {
System.out.println("enter value integer ");
Scanner sn = new Scanner(System.in);
try{
int a = sn.nextInt();
} catch (InputMismatchException ex){
System.out.println("error please enter integer value");
}
System.out.println("not terminating");
}
}
But this code, also printing the line after Occurrence of error
Because it is out side of try-catch, that is the advantage of exception handling.
Exception handling prevents the abnormal termination of program due to run time error. And that is what happened.
See also
exception handing docs
It is terminating, it just prints out the System.out first. This is as expected - it jumps into the catch block, and then continues.
System.out.println("enter value integer ");
Scanner sn = new Scanner(System.in);
try {
int a = sn.nextInt();
} catch (InputMismatchException ex) {
System.out.println("error please enter integer value");
// you are catching input mis match here
// exception will catch and program continues
}
System.out.println("not terminating"); // this is out side the try-catch
So you will get this line in your out put too.
After entering the catch block, flow continues on, so the next line to execute is the bottom print.
If you want to terminate from within the catch:
try {
int a = sn.nextInt();
} catch (InputMismatchException ex) {
System.out.println("error please enter integer value");
return; // program will end
}
If you want it to be terminated you need to re-throw the exception e.g.:
System.out.println("enter value integer ");
Scanner sn = new Scanner(System.in);
try {
int a = sn.nextInt();
} catch (InputMismatchException ex) {
System.out.println("error please enter integer value");
throw new RuntimeException(ex);
}
System.out.println("not terminating"); // this is out side the try-catch
That way the last system output would not be printed and you would get a stacktrace instead.
I'm making a game which plays until the user enters quit in the command line.
The user can enter different commands like get and go, with the get command the user can say what to get like, get baseball bat. What I do in my code is split the command.
everything is working fine but I have found a bug which I can't solve. If I enter "get" and press space and then ctrl+z it gets in a while loop which never ends.
It only happens with ctrl+z (1 time with ctrl c but after that 1 time not anymore)
private void run()
{
while (! quitCommand)
{
String input = null;
try
{
input = null;
System.out.println("Input "+ input);
System.out.println("Give a command.");
BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
input = is.readLine();
handleCommand(input);
// As long as the command isn’t to quit:
// get the next input line and handle it. (With handleCommand.)
}
catch (Exception e)
{
System.out.println("Something went wrong we are sorry try again.");
e.printStackTrace();
}
}
}
/**
* #param userInput (This is the entire input string from the user.)
*
* (Tell others to) Perform the task which belongs to the given
* command.
*/
private void handleCommand(String userInput)
{
// Split the user input string.
if (userInput != null) // user input can not be empty
{
String[] delenTekst = userInput.split(" ");
// The first word is a command. The rest is extra information
String command = delenTekst[0];
String extra = "";
for (int i = 1; i < delenTekst.length; i ++)
{
if (i == 1)
{
extra = extra + delenTekst[i];
}
else
{
extra = extra +" " + delenTekst[i];
}
}
switch (command)
{
// Check if the command is to travel between rooms. If so, handle
case "go"
:
this.checkRoomTravel(extra);
break;
// If there isn't any room travel, then check all other command
case "get"
:
System.out.println("Looking for " +extra );
this.handleGetCommand(extra);
break;
case "quit"
:
quitCommand = true;
break;
default
:
System.out.println("Command is not known try help for information");
break;
}
}
else
{
userInput = "help";
}
}
I'm new to java so it can be something really simple.
On the top of my script I have a private boolean quitCommand = false; which is to check if the user entered quit.
Ctrl+Z closes the Console and therefore your readLine() returns null as pretended to indicate that end of file was reached. So all you need to do, is to check for null returned by readLine() and handle this as you handle the "quit".
I've changed your code (just to test my thesis) and also stream lined a few things, e.g. you dont need to recreate a BufferedReader every time you read a line.
private boolean quitCommand = false;
private void runIt() {
BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
String input = null;
while(!quitCommand) {
try {
System.out.print("Give a command: ");
input = is.readLine();
// As long as the command isn’t to quit:
if(input == null || "quit".equals(input.trim())) quitCommand = true;
if(quitCommand) break;
// get the next input line and handle it. (With handleCommand.)
String[] words = input.trim().split("\\s+");
// ** This is the original handleCommand line **
System.out.println(input + ":" + Arrays.toString(words));
}
catch (Exception e) {
System.out.println("Something went wrong we are sorry try again.");
e.printStackTrace();
}
}
}
BTW: To split the input into words I'd use the regular expression as shown in my code. This works also if the user enters tabs or multiple spaces.
On DOS/Windows Ctrl+Z means end of input. This causes readLine() to return null no matter how many times you call it. This is likely to cause your code to fail as you don't appear to check for it. I suspect you are getting a NullPointerException which you are pretending didn't happen and trying again, endlessly.
So if a user puts in a postfix value like say 453-* , my method EvalPostFix() does the work, but when the user inputs something invalid like 43*+ or any invalid string want the program to repromt the user for input dont know how to implement with try catch..
'
String in;
while(true){
System.out.println("Please enter the numbers first followed by the operators, make sure to have one less operator than of numbers");
try {
in = getString();
int result = EvalPostFix(in);
System.out.println(result);
} catch (IOException e) {
// TODO Auto-generated catch block
String s = "Not a valid postfix string";
e.toString();
in = getString();
}
}
'
Looking at your code I think you just need to get rid of the in = getString(); in the catch block and add an break at the end of the try block.
I don't recommend using a while(true) or an IOException for what you are doing though, but that should get your code working.
Use a flag:
boolean flag = false;
while(!flag)
{
//make the loop break, if no exception caught
flag = true;
try{
}
catch{
//make the loop repeat
flag = false;
}
}
this should repeat the prompt every time you catch an exception. you can also use this to validate input.
how the flag is oriented depends on your preference. I like to flag true when an error occured ;)
this will also break your while loop, as soon as you get a valid input.
Something like this is can be used to get an input of desired specifications
public static void userMove() {
System.out.println("Where would you like to move? (R, L, U, D)\n");
Scanner input = new Scanner(System.in) ;
while (true){
String userInput = input.next() ;
if(userInput.length()>1){
System.out.println("Please input a valid direction");
}else{
break ;
}
}
}