I'm having a problem handling exceptions. Honestly, I really don't understand how it works since I self study.
I'm working with a program where there would be a main menu with the following choices.
Odd/Even - asks an integer input from user and identify if it is an odd or even. Program would continuously ask for an integer input if the user keeps on giving character inputs. (I was able to do this but I keep on getting errors when I use br.readLine() in getting input. Pls see codes below. So I used the normal parsing. Since I didn't use Buffered Reader, I try to delete it but the Odd/Even program wouldn't handle the exception without it.)
Vowel/Consonant - asks the user for a character input and identify if it is a vowel or a consonant. Program should reject integer inputs. The program I made with the codes below doesn't reject integer inputs. I tried searching for answers but I can't find one.
Please ignore for now.
My problem/s involve/s the following questions.
1. Why doesn't the program Odd/Even handle the NumberFormat exception whenever I try to delete the BufferedReader line even though it wasn't used in the whole program?
How can I reject integer inputs for the Vowel/Consonant program?
Here is a video when I tried to run the program.
http://tinypic.com/r/24ou9kz/9
When I exit the program, the console shows this.
Exception in thread "main" java.lang.NumberFormatException: null at
java.lang.Integer.parseInt(Unknown Source) at
java.lang.Integer.parseInt(Unknown Source)
import javax.swing.JOptionPane;
import java.io.*;
import java.util.InputMismatchException;
public class DoWhileIf {
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String input;
int choice, num = 0;
char again = 0;
boolean err = true;
do {
input = JOptionPane.showInputDialog("Menu\n[1] Odd/Even\n[2] Vowel/Consonant\n[3] CQM\n[4] Fuel Efficiency\n[5] Scholarship\n[6] Exit program.\n\nEnter Choice.");
choice = Integer.parseInt(input);
if (choice == 1) {
do {
do {
try {
input = JOptionPane.showInputDialog("Input an integer : ");
num = Integer.parseInt(input);
err = false;
} catch (NumberFormatException o) {
JOptionPane.showMessageDialog(null,"Error!");
err = true;
}
} while (err);
if (num % 2 == 0) {
JOptionPane.showMessageDialog(null,"Even.");
}
else {
JOptionPane.showMessageDialog(null,"Odd.");
}
do {
input = JOptionPane.showInputDialog("Try again? Press Y for yes or N to go back to main menu.");
again = input.charAt(0);
} while (again != 'Y' && again != 'y' && again !='N' && again !='n');
} while (again == 'Y' || again == 'y');
}
if (choice == 2) {
char letter = 0;
do {
do {
try {
input = JOptionPane.showInputDialog("Character : ");
letter = input.charAt(0);
err = false;
} catch (InputMismatchException a) {
JOptionPane.showMessageDialog(null,"Error!");
err = true;
}
} while (err);
if (letter == 'a' || letter == 'A' || letter == 'e' || letter == 'E' || letter == 'i' || letter == 'I' || letter == 'o' || letter == 'O' || letter == 'u' || letter == 'U') {
JOptionPane.showMessageDialog(null,"Vowel");
}
else {
JOptionPane.showMessageDialog(null,"Consonant");
}
do {
input = JOptionPane.showInputDialog("Try again? Press Y for yes or N to go back to main menu.");
again = input.charAt(0);
} while (again != 'Y' && again != 'y' && again !='N' && again !='n');
} while (again == 'Y' || again == 'y');
}
} while (choice <= 0 || choice > 6 || again == 'N' || again == 'n');
}
Why doesn't the program Odd/Even handle the NumberFormat exception whenever I try to delete the BufferedReader line even though it wasn't
used in the whole program?
I am not able to duplicate this problem. I removed the BufferedReader and option #1 works the same as it did before. I entered integer values, special characters, letters, spaces and it works fine.
How can I reject integer inputs for the Vowel/Consonant program?
You could modify your else condition from this:
else {
JOptionPane.showMessageDialog(null,"Consonant");
}
to this:
else if(Character.isLetter(letter)){
JOptionPane.showMessageDialog(null,"Consonant");
}
else{
JOptionPane.showMessageDialog(null,"Error! You must enter a valid letter.");
}
When I exit the program, the console shows this.
Exception in thread "main" java.lang.NumberFormatException: null at ...
Regarding the NumberFormatException you're seeing, I'm guessing you're pressing the Cancel button on the dialog. When you press cancel the variable input receives the value null. When you try to parse null as an integer it fails and throws the exception:
Exception in thread "main" java.lang.NumberFormatException: null
at java.lang.Integer.parseInt(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at exception.DoWhileIf.main(DoWhileIf.java:18)
Line 18 is this line: choice = Integer.parseInt(input);
Notice how the exception told us - java.lang.NumberFormatException: null which tells us that the parameter being passed to the parseInt method is null.
Lastly some additional thoughts for you to consider:
Whenever you get input from the user you must account for all the possibilities somehow. For example when you have code like this:
letter = input.charAt(0);
you're not accounting for the possibility that the input could be null or empty in which case this logic will throw an exception.
A concrete example is when the user clicks Cancel on the dialog that asks whether they want to try again:
input = JOptionPane.showInputDialog("Try again? Press Y for yes or N to go back to main menu.");
When the user clicks Cancel on this dialog the same thing happens that I described above regarding the NumberFormatException - input becomes null. If you try to use input like this:
again = input.charAt(0);
it will fail with the exception:
Exception in thread "main" java.lang.NullPointerException
because you can't invoke a method on a null.
Another example is when the user enters nothing at the main menu but simply presses OK. The result is this exception:
Exception in thread "main" java.lang.NumberFormatException: For input string: ""
This happens because input was set to an empty string and parseInt does not know how to convert empty string into an integer value.
Another item I want to bring up is that you're using this same piece of code over and over again. Whenever you have code you want to reuse you should not copy and paste it but instead create a method, object, or other construct so that you can refer to it.
do {
input = JOptionPane.showInputDialog("Try again? Press Y for yes or N to go back to main menu.");
again = input.charAt(0);
} while (again != 'Y' && again != 'y' && again !='N' && again !='n');
Breaking up your logic into smaller more manageable pieces will help you to debug, test, and maintain your code more easily.
Another point I want to touch on regarding this same block of logic is that you're using the same kind of dialog to ask for many different kinds of input. Since you're using a GUI dialog, you could use a dialog that is better suited to your task such as one that asks the user to press either a Yes button or No button.
You can learn more about different kinds of dialogs by reading the How to Make Dialogs Tutorial
Here is an example of how you could create a more friendly dialog:
/**
* Asks the user if they want to try something again and
* returns a boolean representing the user's response.
* #return true if the user answers Yes, false otherwise.
*/
private static boolean promptToRepeatSelectedOption(){
int n = JOptionPane.showOptionDialog(null,
"Try again?",
"Repeat Selection",
JOptionPane.YES_NO_OPTION,
JOptionPane.QUESTION_MESSAGE,
null,
null,
null);
return n == JOptionPane.YES_OPTION;
}
The above method, when invoked, will create and display a dialog with two buttons - Yes and No - and the user will have to select one of them or close the dialog. The logic simply looks for if the user said Yes (by clicking the Yes button) and returns true when that is the case. If the user closes the dialog or chooses the No option the method returns false simply because either of those two scenarios will cause the n == JOptionPane.YES_OPTION comparison to result in a value of false.
You can replace your entire loop with a call to this method like this:
First, define a variable to hold the user's response.
boolean repeat = false;
Then invoke the method and set the variable to its result:
repeat = promptToRepeatSelectedOption();
Now replace the outer loop condition
while (again == 'Y' || again == 'y');
with this: while (repeat);
and finally replace part of the outermost loop condition
again == 'N' || again == 'n'
with this: !repeat
One final thought is that you're using very general error messages when the user enters something incorrect or invalid:
JOptionPane.showMessageDialog(null, "Error!");
It's always better to explain to the user a little bit about what they did wrong so that they know how to avoid the error next time. You should probably consider adding more detail to your error messages.
Hope this helps!
Related
So i need help, i am trying to input a Y/N program but it is not accepting a big 'Y' or 'N'. Also another thing that i am trying to do is after pressing 'Y'/'y' i am trying to get the program to loop back to the code written above. Example a program that displays '123' and do i need to continue? Y/N, if entered yes it goes back up to restart the program from scratch. Please help me.
System.out.println("continue? Yes or no ");
char check = s.next().charAt(0);
while (check != 'y' && response != 'n')// corrected this part, however need help with restarting the loop back to the first line of code in a loop {
System.out.println("\nInvalid response. Try again.");
check = s.next().charAt(0);
} if ((check == 'n') || (check == 'N')) {
// I tried (check == 'n' || check == 'N')
System.out.println("Program terminated goodbye.");
System.exit(0);
} else if (check == 'y') {
//need help with restarting the loop back to the first line of code in a loop
}
I think this is what you are looking for.
char check;
Scanner scanner = new Scanner(System.in);
do
{
//your piece of code in here e.g.
System.out.println("Printed 123");
System.out.println("Do you wish to continue?[Y/y] or [N/n]");
choice = scanner.next().charAt(0);
}while (check =='Y' || check == 'y');
System.out.println("Program terminated goodbye.");
A do-while loop runs at least once before the condition is checked and so when a user enters either Y or y, then the condition will be true, meaning that they wish for the loop to run again. If the user enters any other value, then the condition will become false since choice is neither Y nor y and the loop will terminate.
Use String.equals() to compare the value of strings, == compares the strings in memory.
If you want to check without case-sensitive, you should convert the char to a String, then do s1.equalsIgnoreCase(s2);
So
while(true) {
System.out.println("Continue? [Y/N]");
char check_char = s.next().charAt(0);
String check = Character.toString(check_char);
while(check.equalsIgnoreCase("y") && !response.equalsIgnoreCase("n")) {
System.out.println("\nInvalid response. Try again.");
check = s.next().charAt(0);
}
if (check.equalsIgnoreCase("n")) {
System.out.println("Program terminated goodbye.");
System.exit(0);
}
}
For returning to the first line, I used a while loop that loops forever.
To the end if it is n then exits, otherwise it returns back to the first line of the loop.
I'm a student currently taking basic Java classes.
I'm working on a code that asks the user an input for a game to "start" and to "quit". Sooo I chose the string "S" and "Q" respectively. If the user enters S, the game proceeds. If the user enters Q, the program displays "Thanks for playing" or something. If the user enters something other than S and Q, the program asks again until it gets a valid input. I almost got everything correctly except for the error checking part. Any possible suggestions to fix my code?
Thank you in advance! :)
(partial code)
Scanner scan = new Scanner(System.in);
String userInput;
boolean game = false;
System.out.println("Welcome to the Game! ");
System.out.println("Press S to Start or Q to Quit");
userInput = scan.nextLine();
if (userInput.equals("S")){
game = true;
} else if (userInput.equals("Q")){
game = false;
} else {
do {
System.out.println("Invalid input! Enter a valid input: ");
userInput = scan.nextLine();
} while (!"S".equals(userInput)) || (!"Q".equals(userInput)); // I'm not sure if this is valid???
}
if (userInput.equals("S")){
///// Insert main code for the game here////
} else if (userInput.equals("Q")){
System.out.println("Thank you for playing!");
}
You're creating an infinite loop:
while (!"S".equals(userInput)) || (!"Q".equals(userInput)); // always true
For this condition to not hold you'll need an input that is equal to "S" and to "Q" together. It's easy to see applying De-Morgan's law:
while (!("S".equals(userInput)) && "Q".equals(userInput))); // always true
Obviously, it won't happen.
You probably want:
while (!"S".equals(userInput)) && (!"Q".equals(userInput));
I can't vote on answers yet, but the prior one is correct.
Breaking out the logic:
input = "Z"
while( !(S==Z) || !(Q==Z) ) -> while( !(F) || !(F) ) -> while( T || T ) -> repeat
input = "Q"
while( !(S==Q) || !(Q==Q) ) -> while( !(F) || !(T) ) -> while( T || F ) -> repeat
Switching to "and" makes case #2 work.
Do you do anything with your "game" boolean? If the user enters the while loop then the boolean will always be false.
Here's my code:
public boolean isConsonant(char x){
if (!Character.isLetter(x)){
System.out.print ("What you have entered cannot be a consonant or vowel.");
return false;
}
return (x != 'a' && x != 'e' && x != 'i' && x != 'o' && x != 'u');
}
The problem I'm having is the first if statement. I call the isConsonant method multiple times in the code after this and depending on the return calue (true or false) the code does some action.
The problem is that I don't want the method to continue at all if the char isn't a letter. I want the program to end. What I tried to do is write another method that looked like this:
public voidisNotLetter(char x)
if (!Character.isLetter(x){
System.out.println("What you have entered cannot be a consonant or vowel.");
}
This is where I'm stuck. I don't know what I can put in that method that will stop the program from running and just print that statement to the user. I thought about throwing an IllegalArgumentException, but that's not technically true since the argument is valid but just isn't what I want.
If you want to "stop the program from running and just print that statement to the user", this might help :
if (!Character.isLetter(x)){
System.out.print ("What you have entered cannot be a consonant or vowel.");
System.exit(0); //This would terminate the execution if the condition is met
}
More details here. Hope it helps.
You can try nesting if statements.
if(isLetter(input)){
if(isConsonant(input)
//input is consonant
else
//input is not consonant
}else{
//input is not letter
}
So basically, I had to write a little java programm that assigns student scores.
It works fine, I can enter H (Higher) grades fine, but when I choose S(standart) it throws me back to the editor and I just don't udnerstand whats wrong.
boolean isHigher = getInput().charAt(0) == 'H' || getInput().charAt(0) == 'h';
The only reason I can think of is that your getInput() removes the input, and doesn't actually ask it again.
In that case the following scenario occurs: the input isn't equal to "H" so you go to the or, you have no input left so you return an empty String of which you ask the char at position 0 => StringIndexOutOfBoundException.
You should use:
Character enteredCharacter = getInput().charAt(0);
boolean isHigher = enteredCharacter == 'H' || enteredCharacter == 'h';
Here is my code:
public static void nameset(){
int no = 1;
name = JOptionPane.showInputDialog(frame, "The last people who still had cake had to defend it with heir lives, No matter the cost.\nOne of those last people, was you. What is your name?", "",1);
if(name.equals("") || name.equals(JOptionPane.CANCEL_OPTION));{
JOptionPane.showMessageDialog(frame,"Please tell me your name. I don't wanna have to exit out of the game about you.","Hey!",1);
no++;
}if (name.equals("") || name.equals(JOptionPane.CANCEL_OPTION)){
if (no == 2){
JOptionPane.showMessageDialog(frame, "Seriously? Again?! that's it..");
if (name.equals(JOptionPane.CANCEL_OPTION)){
System.exit(0);
}else{
System.exit(0);
}
}
}
}
I want it so if you press the cancel option it tell you to restart. But if you press cancel, it shows an error in the console. I think it's the name.equals(JOptionPane.CANCEL_OPTION), But I'm not sure. Is there any reason for it not to work? Thanks in advance for any help.
The cancel button will always result in null being returned. See official JavaDoc:
Returns: user's input, or null meaning the user canceled the input
So your condition should be changed to:
if(name == null || name.equals(""))
and you also need to remove the semicolon after your first if statement! Otherwise the following block will always be executed.
Once that's fixed, your "exit after 3 times no" will not work because you're not actually looping your input dialog.
Try this
int no = 1;
String name = JOptionPane.showInputDialog(null, "The last people who still had cake had to defend it with heir lives, No matter the cost.\nOne of those last people, was you. What is your name?", "",1);
if(name == null || name.equals(""));{
JOptionPane.showMessageDialog(null,"Please tell me your name. I don't wanna have to exit out of the game about you.","Hey!",1);
no++;
}if (name == null || name.equals("")){
if (no == 2){
JOptionPane.showMessageDialog(null, "Seriously? Again?! that's it.."+name);
if (name == null || name.equals("")){
System.exit(0);
}else{
System.exit(0);
}
}
}