I'm new to java and I'm trying to account for possible errors that might come up in my program. I'm creating a calculator and converting the inputs from infix to postfix to calculate the result. I want to try accounting for mismatched parenthesis in the input, but I'm having trouble. For example, when converting from infix to postfix, when a ) is reached, it will pop numbers from the stack and put them onto the new postfix list. In one case where there may not be a matching left parenthesis present (the while loop reaches the end of the stack without coming across a (, it should throw an exception. I've implemented the following code, but it doesn't seem to be working:
else if(tok.equals(")")){
while(stack.peek().equals("(") == false){
try{
Operator o = stack.pop();
nlist.add(o);
} catch(EmptyStackException e){
System.out.println(e.getMessage());
}
stack.pop();
}
Then in the other file which constructs the GUI and processes the inputs, I entered:
try{
java.util.List<Token> postfix = ExpressionManager.infixToPostfix(infix);
// build the expression
Expression exp = ExpressionManager.buildExpression(postfix);
// display the results
}catch(ArithmeticException e){
entryField.setText(e.getMessage())
}
Any suggestions?
stack.peek() throws EmptyStackException before you enter inside try-catch block (I assume you expect that pop will throw the exception).
Second block doesn't show how the ArithmeticException is thrown, so, not sure what you expecting here.
If you want to re-throw an exception you can do
catch(EmptyStackException e){
throw new ArithmeticException("empty stack, bad mathematical expression.", e);
}
Related
I have this code:
public static void main(String[] args) {
List<Valtozas> lista = new ArrayList<Valtozas>();
try {
File fajl = new File("c://data//uzemanyag.txt");
Scanner szkenner = new Scanner(fajl, "UTF8");
while (szkenner.hasNext()) {
String sor = szkenner.nextLine();
String [] darabok = sor.split(";");
String szoveg = darabok[0];
Valtozas valtozas = new Valtozas(Integer.valueOf(darabok[0]), Integer.valueOf(darabok[1]), Integer.valueOf(darabok[2]));
lista.add(valtozas);
}
}
catch (Exception e) {
}
//3.FELADAT
System.out.println("3.Feladat: Változások száma: "+lista.size());
}
}
Here I want to convert the String to int, but I cant. I tried the Integer.Valueof(darabok[0]), but its not working. And nothing is happening, so the code is build, but quit from the while.
Based on the source code you have shown us, the problem is that there is a format mismatch between the input file and the program you are trying to read.
The program reads the file a line at a time, and splits it into fields using a single semicolon (with no whitespace!) as the file separator. Then it tries to parse the first three fields of each split line as integers.
For this to work the following must be true:
Every line must have at least three fields. (Otherwise you will get a ArrayIndexOutOfBound exception.)
The three fields must match the following regex: -?[0-9]+ i.e. an optional minus signed followed by one or more decimal digits.
The resulting number must be between Integer.MIN_VALUE and Integer.MAX_VALUE.
Elaborating on the above:
Leading and trailing whitespace characters are not allowed.
Embedded decimals markers and grouping characters are not allowed.
Scientific notation is not allowed.
Numbers that are too large or too small are not allowed.
Note that if any of the above constraints is not met, then the runtime system will throw an exception. Unfortunately, you surround your code with this:
try {
...
}
catch (Exception e) {
}
That basically ignores all exceptions by catching them and doing nothing in the handler block. You are saying to Java "if anything goes wrong, don't tell me about it!". So, naturally, Java doesn't tell you what is going wrong.
DON'T EVER DO THIS. This is called exception squashing, and it is a really bad idea1.
There are two ways to address this:
Print or log the stacktrace; e.g.
catch (Exception e) {
e.printStackTrace();
}
Remove the try / catch, and add throws IOException to the signature of your main method.
(You can't just remove the try / catch because new Scanner(fajl, "UTF8") throws IOException. That's a checked exception so must be handled or declared in the method signature.)
Once you have dealt with the exception properly you will get an exception message and stacktrace that tells you what is actually going wrong. Read it, understand it, and fix your program.
1 - It is like taping over the "annoying" red light that indicates that your car's oil level is low!
Language: Java, IDE: eclipse mars
The program is supposed to prompt the user (using JOptionPane) for a positive value. I'm trying to catch the invalid entries. My while statement catches the negative numbers but not the strings. When a negative number is entered, the prompt is shown again, but when a string value is entered, the exception is caught and the program moves on (when it should re prompt the user).
Once a positive value has been entered, the program assigns it to a value in another class. (We're learning the MVC OOP design pattern).
Double.isNaN(Double.parseDouble(h)) ---> can anyone help me find what am I missing?
// prompt to get bandwidth from user
// check for validity
// if invalid, prompt again
try{
h = JOptionPane.showInputDialog("Enter bandwidth as a positive number");
// loop until parsed string is a valid double
while (Double.isNaN(Double.parseDouble(h)) || Double.parseDouble(h) <=0) {
h = JOptionPane.showInputDialog("Enter bandwidth as a positive number");
}
// h has been set to valid double, set it to bandwidth
model.setBandwidth(Double.parseDouble(h));
}catch(NumberFormatException|NullPointerException NFE){
System.err.println("Caught exception: " + NFE.getMessage());
}
This is because of how parseDouble() works.
Throws:
NumberFormatException - if the string does not contain a parsable double.
(See here)
So if the String is not a double parseDouble() will not return NaN but throw an exception, which means your catch clause will be called.
To solve this problem maybe use recursively algorithm which will call your method again if an exception is thrown.
As 4castle already stated, you need to move your try/catch block inside your while loop.
However, for validating user input you can basically stick to the following pattern:
public Foo getUserInput() {
Foo result;
do {
try {
String s = requestUserInput(); // something like Scanner.nextLine()
result = parseUserInput(s); // something like Double.parseDouble(String)
}
catch(Exception exc) {
// maybe you want to tell the user what's happened here, too
continue;
}
}
while(!isValid(result)); // something like (0 < result)
return result;
}
First post so my apologies if this was done incorrectly (and am also relatively new to programming, so any extraneous tips are also appreciated).
So I have written up a basic calculator program in Java. It works well currently, but I am having a particular issue with NumberFormatException. Here's the code:
private static double spaceTestAndConvert(String numInput){
Scanner input= new Scanner(System.in);
if (numInput.equalsIgnoreCase("quit")){
System.exit(1);
}
else if(numInput.equalsIgnoreCase("C/E")){
Restart();
}
try{
return Double.parseDouble(numInput.trim());
}
catch(NumberFormatException nfe){
numInput = "";
System.out.println("Please enter only one number without any spaces or letters: ");
numInput = input.nextLine();
spaceTestAndConvert(numInput.trim());
return Double.parseDouble(numInput.trim());
}
}
The issue is that after trying to force an error by entering in several inputs which would cause NumberFormatException and then entering in a valid input, the program will crash from a NumberFormatException citing the previous invalid input.
I.E. -
"1 2 3"
loops back
"1 2 q 3"
loops back
"12q3 3 sqw 1"
loops back
"12"
crash - Exception in thread "main" java.lang.NumberFormatException: For input string: "12q3 3 sqw 1"
It only occurs after several occurrences of the exception. I'm curious why it is doing this. Any advice on how to fix this or explanation of what is happening? If you need any other part of the code, please let me know! Thanks!
I don't follow everything that you're saying, but these 2 lines (from within your catch block) look problematic...
spaceTestAndConvert(numInput.trim());
return Double.parseDouble(numInput.trim());
You are calling the spaceTestAndConvert function recursively, but throwing away the value. I don't understand why you would call it and not be interested in the value.
The second line is also a mess. You so carefully surround the first call to Double.parseDouble() with try/catch, but then you call it again within your catch block. If the second Double.parseDouble() generates a NumberFormatException, it will not be caught.
removing the return in catch will solve your problem. because if you have return on it, you are going to return an invalid Number format since you are in a catch. What you want to do is to return a value when it is now valid, you are now actually doing it inside the try. Don't force your program to return the value with error (since it is in a catch) because it will really give you an error.
returning to previous method after you had the right value (because of recursion) will still have the stack of error value aside from success value you gained from the end part because they are different variables.
private static double spaceTestAndConvert(String numInput){
Scanner input= new Scanner(System.in);
if (numInput.equalsIgnoreCase("quit")){
System.exit(1);
}
else if(numInput.equalsIgnoreCase("C/E")){
Restart();
}
try{
return Double.parseDouble(numInput.trim());
}
catch(NumberFormatException nfe){
numInput = "";
System.out.println("Please enter only one number without any spaces or letters: ");
numInput = input.nextLine();
spaceTestAndConvert(numInput.trim());
}
}
I'm looking for an exception on how to catch an invalid String that is user input. I have the following code for a exception on an integer input:
try {
price = Integer.parseInt(priceField.getText());
}
catch (NumberFormatException exception) {
System.out.println("price error");
priceField.setText("");
break;
But I'm not aware of a specific exception for strings, the input is a simple JTextBox so the only incorrect input I can think of is if the user enters nothing into the box, which is what I'm looking to catch.
if (textField.getText().isEmpty())
is all you need.
Or maybe
if (textField.getText().trim().isEmpty())
if you also want to test for blank inputs, containing only white spaces/tabs.
You generally don't use exceptions to test values. Testing if a string represents an integer is an exception to the rule, because there is no available isInt() method in String.
You can do like
if (priceField.getText().isEmpty())
throw new Exception("priceField is not entered.");
You could check if priceField contains a string by using this:
JTextField priceField;
int price;
try {
// Check whether priceField.getText()'s length equals 0
if(priceField.getText().getLength()==0) {
throw new Exception();
}
// If not, check if it is a number and if so set price
price = Integer.parseInt(priceField.getText());
} catch(Exception e) {
// Either priceField's value's length equals 0 or
// priceField's value is not a number
// Output error, reset priceField and break the code
System.err.println("Price error, is the field a number and not empty?");
priceField.setText("");
break;
}
When the if-statement is true (If the length of priceField.getText() is 0) an exception gets thrown, which will trigger the catch-block, give an error, reset priceField and break the code.
If the if-statement is false though (If the length of priceField.getText() is greater or lower than 0) it will check if priceField.getText() is a number and if so it sets price to that value. If it not a number, a NumberFormatException gets thrown, which will trigger the catch-block etc.
Let me know if it works.
Happy coding :) -Charlie
if you want your exception to be thrown during the normal operation of the Java Virtual Machine, then you can use this
if (priceField.getText().isEmpty())
throw new RunTimeException("priceField is not entered.");
I implemented a circular list in Java. The code asks for the int values to be entered, and I wish to terminate the input list with an "END". The code works but throws a runtime exception: NumberFormatException.
try{
while(true){
newnode=new Node();
oldnode.next=newnode;
newnode.prev=oldnode;
System.out.print("Enter value:");
try{
ctrlstr=bfr.readLine();
}
catch(Exception ex){
ex.printStackTrace();
}
if (ctrlstr=="END") break;
newnode.val=Integer.parseInt(ctrlstr);
oldnode=newnode;
i++;
}
}
catch(Exception ex){
ex.printStackTrace();
}
Here:
if (ctrlstr=="END")
you're comparing strings using ==, which always checks for reference identity. Instead, you should use equals:
if (ctrlstr.equals("END"))
or perhaps (if you want just a false result when ctrlstr is null)
if ("END".equals(ctrlstr))
A few extra notes:
Catching Exception is usually a bad idea - you should catch more specific exceptions
Catching an exception and then continuing anyway after printing it is usually a bad idea
Your code will be more readable if you indent appropriately
Your code will be more readable if you always use braces for if statements etc
Try
ctrlstr.equals("END") instead.