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 ;
}
}
}
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
Can someone explain and help me fix this program.
import java.util.*;
import java.io.*;
public class test {
public static void main(String[] args) {
Scanner key = new Scanner(System.in);
boolean clear;
int in = 0;
do {
clear = true;
try {
in = key.nextInt();
} catch (InputMismatchException e) {
System.out.println("Invalid");
clear = false;
}
} while (clear == false);
String stringIn = Integer.toString(in);
String[] dec = stringIn.split("");
for (int i = 1; i < (dec.length); i++) {
System.out.print(dec[i] + " ");
}
}
}
Whenever I enter a invalid input instead of an int, my program keeps looping "Invalid" instead of giving the option to enter a new value for in.
The problem is that if the scanner fails to find an input in the correct format, it will throw an exception and not read the input.
Because the scanner does not read the invalid int input, the next time nextInt is called, it will try to read the invalid input again, and miserably fails at it, printing another "invalid!"
So you need to read the input afterwards if it finds an invalid int:
// do this in the catch block:
key.next();
This makes sure that the next token is read.
Full code:
Scanner key = new Scanner(System.in);
boolean clear;
int in = 0;
do {
clear = true;
try {
in = key.nextInt();
} catch (InputMismatchException e) {
System.out.println("Invalid");
clear = false;
key.next();
}
} while (clear == false);
String stringIn = Integer.toString(in);
String[] dec = stringIn.split("");
for (int i = 1; i < (dec.length); i++) {
System.out.print(dec[i] + " ");
}
Check the API of the nextInt method:
This method will throw InputMismatchException if the next token cannot be translated into a valid int value as described below. If the translation is successful, the scanner advances past the input that matched.
Meaning, that if it's not successful - it will not advance and will try to execute nextInt over the illegal token over and over again failing every time.
Try adding next() into the exception catch clause, it should skip the token and read the next one then. next() reads a String, so it does not really care about the formatting, and will allow you to advance the position in the stream to read the next token.
The problem is that you are writing to the console inside the catch, so then when you call key.nextInt() in the try the program reads the value you print to the console, so an easy way to solve this is to add a line like: key.nextLine() inside the catch and that will solve your problem.
} 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.
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.
I developed the following application in which I needed to masking the PIN and terminate the program after the user has entered the wrong PIN thrice. However, the program terminates only if i close the stopThread at the beginning (I commented it in the code below), however the password masking does not occur for all the three channces when I do so. But, when I close the stopThread just before displaying the login successful screen, the program does not terminate. I need to use ctrl+c to end the program.
Any help is greatly appreciated.
boolean stopThread = false;
boolean hideInput = false;
boolean shortMomentGone = false;
public static double userBal=0.0D;
public void run(){
try{
sleep(500);
} catch(InterruptedException e){
}
shortMomentGone = true;
while(!stopThread){
if(hideInput){
System.out.print("\b*");
}
try{
sleep(1);
} catch(InterruptedException e){
}
}
}
public static final int NB_OF_TRIES = 3;
public void validatePin(){
BankAccount getAll=new BankAccount();
String pin="";
getAll.Login();
Login hideThread =new Login();
hideThread.start();
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
try{
do{
} while(hideThread.shortMomentGone == false );
// Now the hide thread should begin to overwrite any input with "*"
hideThread.hideInput = true; // Read the PIN
System.out.println("\nPIN:");
boolean pinMatch = false;
int i = 0;
while(!pinMatch && i < NB_OF_TRIES) {
hideThread.hideInput = true;
pin = in.readLine();
i++;
//hideThread.stopThread = true; //Program terminates after third attempt
//PIN masking is stopped, if uncommented
System.out.print("\b \b");
if(pin.equals(" ")){
System.out.println("Please do not leave unnecessary spaces!");
getAll.Login();
}else if(pin.equals("")){
System.out.println("Please do not press the enter key without entering the PIN!");
getAll.Login();
}
FileInputStream fileinputstream = new FileInputStream(".\\AccountInfo.txt");
DataInputStream datainputstream = new DataInputStream(fileinputstream);
BufferedReader bufferedreader1 = new BufferedReader(new InputStreamReader(datainputstream));
do
{
String s1;
if((s1 = bufferedreader1.readLine()) == null)
{
break;
}
if(s1.trim().charAt(0) != '#')
{
String as[] = s1.split(" ");
if(pin.equals(as[0]))
{
System.out.println("You have login!");
String s2 = as[2];
userBal = Double.parseDouble(s2);
getAll.balance = userBal;
hideThread.stopThread = true;
getAll.MainMenu();
System.exit(0);
}else if(pin != as[0]){
System.out.println("Invalid PIN!");
getAll.Login();
System.out.println("\n NOTE :- You are only allowed to enter the PIN THREE times. The number of tries remaining before your card is blacklisted are "+i + "\n Please re-enter your PIN");
}
}
} while(true);
datainputstream.close();
}//End of While Loop
}catch(Exception exception)
{
System.err.println((new StringBuilder()).append("Error: ").append(exception.getMessage()).toString());
}//End of try-catch block
}
There's a readPassword() method in java.io.Console, use that. Why do you need a separate thread at all? That makes everything way too complicated.
Regarding your question why this does not close: Java may optimize while(isTrue){} to something like if(isTrue) { while(true) { } } if you don't set isTrue volatile or synchronize the access to isTrue (getter/setter). This optimizations is called hoisting and explained in Effective Java SE, item 66.
Here is an article which explains exactly your problem: echoing * instead of blanks.
http://java.sun.com/developer/technicalArticles/Security/pwordmask/
They are going the complicated way, too but it works. I would prefer blanks over asterisks since that is the easier way to go. Not echoing * is *nix standard afaik.
Actually after I analysed it a but more i realized that the reason the system wont terminate is because it is not kept in the proper place. Therefore, the solution would be to end the program as soon as the while loop is closed and then everything would work fine.
} while(true);
datainputstream.close();
}//End of While Loop
System.exit(0); // After the system is closed the program would terminate after the third attempt
}catch(Exception exception)
{
System.err.println((new StringBuilder()).append("Error: ").append(exception.getMessage()).toString());
}//End of try-catch block