Develop Reference

How to create custom exception for just Integers

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

Validating input and preventing try/catch from exiting

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

User prompt is one line off for some reason?

I'm asking for user input, but I want it to follow the enter: prompt and be on the same line.
My code produces this as a result input from 'ok'
ok
enter: ok
ok
I would like the user input to start after enter: - hoping for this as a result...
enter: ok
ok
Here's my code:
private static Scanner u = new Scanner(System.in);
try{
while(u.hasNext() && !u.equals("exit")) {
System.out.printf("enter: ");
usrInput = u.next();
System.out.printf(usrInput + "\n");
System.out.println(findClosestMatch(usrInput.toLowerCase()));
}
} catch(NullPointerException e) {
System.out.println("Error - NullPointerException");
}

u.hasNext() is blocking on input before the prompt. It's unnecessary, since calling u.next() afterwards will block anyway. And you're comparing the actual Scanner object to "exit", which will never be true. Try this:
while (true) {
System.out.print("enter: ");
if (!u.hasNext() || (usrInput = u.next()).equals("exit")) {
break;
}
System.out.println(usrInput);
System.out.println(findClosestMatch(usrInput.toLowerCase()));
}

Java bufferreader crashes on ctrl z

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.

Code works in Netbeans, but not TextPad?

Here is the code throwing the error. In TextPad, I get a NullPointerException when I try to write the contents of the array to the text file. (It doesn't see anything in the array.) Note: it works perfectly in Netbeans. I only get this in Textpad. I have scoured Google and i have no idea why it is doing this.
void enterContact(){
// test contact
contactName = nameField.getText();
if (contactName == null || contactName.equals(""))
{
JOptionPane.showMessageDialog(null, "Please enter a name.");
return;
}
//test age betweeen 0 and 120
contactAge = ageField.getText();
try{
Integer.parseInt(contactAge);
}
catch(NumberFormatException e){
JOptionPane.showMessageDialog(null, "Please enter a valid age.");
}
finally{
if (Integer.parseInt(contactAge) <= 0 || Integer.parseInt(contactAge) >= 121){
JOptionPane.showMessageDialog(null, "Please enter a valid age.");
return;
}
}
// test email
contactEmail = emailField.getText();
if ( contactEmail == null || contactEmail.equals(""))
{
JOptionPane.showMessageDialog(null, "Please Enter an Email Address.");
return;
}
//test cell number
contactPhone = phoneField.getText();
try
{
Integer.parseInt(contactPhone);
}
catch(Exception e) {
JOptionPane.showMessageDialog(null, "Please Enter a valid Phone Number.");
return;
}
String columns2[] = { contactName, contactAge, contactEmail, contactPhone };
//write data to file
try{
for (int i = 0; i < columns2.length; i++){
fw.write(columns2[i].toString() + ", ");
}
fw.write("\r\n");
fw.flush();
fw.close();

If you get NullPointerException during writing in file, there is two possibilities:
You may not open the file. For example because of forgetting calling the constructor of FileWriter.
You don't have any permission to open the file and write into it.
If you got the exception only when you are run it using TextPad, I think the second problems occurred. You don't have permission.

Try To run your TextPad in Administrator Mode
Right Click on Textpad icon -> Run as administrator
& then RUN above program
i sure your program get executed properly.