I am checking if number the user entered is Zeckendorf and I want to display an exception if it is not, how do i do that? Also how do I convert the Zeckondorf to its decimal equivalent?
import java.util.Scanner;
public class IZeckendorfNumberConvertor {
static String number;
int zeckonderEquivalent;
static Scanner scanner = new Scanner(System.in);
public static void main(String[] args) {
convertToZeckonderNumber();
isTrueZeckonderNumber();
}
private static boolean isTrueZeckonderNumber() {
System.out.println("Enter a Zeckonder Number:");
number = scanner.nextLine();
if (number.equals("10100"))
{
return true; }
else if (number.equals("010011") || number.equals("010100"))
{
return false; }
return false;
}
private static void convertToZeckonderNumber() {
}}
I advise you not to display an exception (i.e. trace and such) as it is very user Unfriendly.
You can use the throw syntax to throw a proper exception :
throw new Exception("Given number is not a Zeckendorf number");
but be sure to catch it and display a nice and clean message :
try {
// Your input code goes here
} catch (Exception e) {
System.out.println(e.getMessage());
}
Another easier option will be to just check the return value of the method and print the results accordingly.
I will recommend to use the latter solution as exceptions are used when something bad and unexpected happens in your program and you want to handle it gracefully. In your case the behavior is expected (i.e. user giving a wrong number) so checking the return value will be much clean and easier.
Use try catch block for catch an exception
try {
} catch (Exception e) {
e.printStackTrace();
}
Also use throw for throw a new exception
Assuming to really do want to display the exception, and not a more user friendly message, the first step is probably to get the exception as a string. Then you can do what you like with that string (echo to console, place in a javax.swing.JTextArea, email it, etc).
If you just want the message from the exception, then getMessage() will do:
try { ... }
catch(FooException e) {
String msg = e.getMessage();
}
However, if you want the whole exception, stack trace and all, you'll want something like this:
public static String stackTrace(Exception e) {
StringWriter w = new StringWriter();
e.printStackTrace(new PrintWriter(w));
return w.toString();
}
// ...
try { ... }
catch(FooException e) {
String st = stackTrace(e);
}
If you just want to echo the full stack trace to the console, there is the printStackTrace(), no-arg method:
try { ... }
catch(FooException e) {
e.printStackTrace();
}
If you want to take more control of the presentation of the stack trace you can get the details with:
try { ... }
catch(FooException e) {
StackTraceElement[] stes = e.getStackTrace();
// Do something pretty with 'stes' here...
}
You can just print a error message to the user saying that the input is wrong using a simple if.
if(yourCondition){
// Happy scenario
// Go shead
}else{
// Error Scenario
System.out.println("Error. Invalid Input.");
// If you persist to throw an exception, then you can do something like this
// throw new Exception("Exception Explanation"); // I've commented this, but you can uncomment it if needed
// But the advice is to display an error message, than throw a exception.
}
And regarding the conversion, you can convert binary to decimal like this
int i = Integer.parseInt(binaryString, 2); // 2 indicates the radix here, since you want to convert from binary.
With this code snippet you can convert the String into an integer :
int numberAsInt;
try {
numberAsInt = Integer.parseInt(number);
} catch (NumberFormatException e) {
//Will throw an Exception
}
If you want to create your own Exception class, you can do it like shown here or just throw a RuntimeException with
throw new RuntimeException("Your Message");
My opinion, you can try some thing like following
public static void main(String[] args) {
if(!isTrueZeckonderNumber()){
// your message should goes here
System.out.println("Your message");
}
}
If you really want to throws an exception do following
private static boolean isTrueZeckonderNumber() throws Exception{
System.out.println("Enter a Zeckonder Number:");
number = scanner.nextLine();
if (number.equals("10100")) {
return true;
} else{
throw new Exception("your message");
}
}
What do you mean you want to display an exception?
I would suggest just giving the user feedback instead, as exceptions are used more commonly for EXCEPTIONAL actions that are not supposed to happen.
However if you do want to, you can print a message explaining what happened.
try {
} catch (Exception e) {
System.out.println(e.getMessage());
}
Related
I am trying to write some kind of a stack calculator.
Here is a part of my code where I am handling a push command. I want to push integers only, so I have to get rid of any invalid strings like foobar (which cannot be parsed into integer) or 999999999999 (which exceeds the integer range).
strings in my code is a table of strings containing commands like POP or PUSH, numbers, and random clutter already split by white characters.
Main problem:
I've got difficulties with throwing an exception for long parseNumber = Long.parseLong(strings[i]); - I don't know how to handle the case, when strings[i] cannot be parsed into a long and subsequently into an integer.
while (i < strings.length) {
try {
if (strings[i].equals("PUSH")) {
// PUSH
i++;
if (strings[i].length() > 10)
throw new OverflowException(strings[i]);
// How to throw an exception when it is not possible to parse
// the string?
long parseNumber = Long.parseLong(strings[i]);
if (parseNumber > Integer.MAX_VALUE)
throw new OverflowException(strings[i]);
if (parseNumber < Integer.MIN_VALUE)
throw new UnderflowException(strings[i]);
number = (int)parseNumber;
stack.push(number);
}
// Some options like POP, ADD, etc. are omitted here
// because they are of little importance.
}
catch (InvalidInputException e)
System.out.println(e.getMessage());
catch (OverflowException e)
System.out.println(e.getMessage());
catch (UnderflowException e)
System.out.println(e.getMessage());
finally {
i++;
continue;
}
}
Long.parseLong(String str) throws a NumberFormatException if the string cannot be parsed by any reason. You can catch the same by adding a catch block for your try, as below:
catch ( NumberFormatException e) {
System.out.println(e.getMessage());
}
No need to worry. Long.parseLong() throws a NumberFormatException if it got other than a Number.
After reading your comments and answers I was able to come up with such a solution (this code is embedded inside the outside try-catch.)
if (strings[i].equals("PUSH")) {
// PUSH
i++;
if (strings[i].length() > 10) {
throw new OverflowException(strings[i]);
}
try{
parseNumber = Long.parseLong(strings[i]);
if (parseNumber > Integer.MAX_VALUE) {
throw new OverflowException(strings[i]);
}
if (parseNumber < Integer.MIN_VALUE) {
throw new UnderflowException(strings[i]);
}
number = (int)parseNumber;
stack.push(number);
}
catch (NumberFormatException n){
throw new InvalidInputException(strings[i]);
}
}
I try to make client sending some data to the server. In some reason it's always enter exception region...
I just want server accepting the data. And after that it needs to do this simple function.
The initialize of the socket, reader and writer is ok.
Client side code:
public void SendPlayer(String Name, float Score,int place) throws NullPointerException
{
out.println("New High Score");
try
{
while (!in.readLine().equals("ACK"));
out.println(Name);
out.println(Score);
out.println(place);
}
catch (IOException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
}
Server side code:
while(true)
{
try
{
if (in.ready())
{
option = in.readLine();
while(option == null)
{
option = in.readLine();
}
switch(option)
{
case ("New High Score"):
{
out.println("ACK");
System.out.println("ack has been sent");
this.setHighScore(in.readLine(),Integer.parseInt(in.readLine()),
Integer.parseInt(in.readLine()));
break;
}
default:
{
System.out.println("nothing");
break;
}
}
}
}
catch (Exception e)
{
// TODO Auto-generated catch block
System.out.println("Exception e");
}
}
Your problem is that you cannot compare null with a String.
The best is that you surround with try and catch the function call.
try {
SendPlayer (/*Here the params*/);
//Continue, since no null pointer exception has been thrown
} catch (NullPointerException) {
//Your handling code here...
}
As per your comment - The full exception is NullPointerException on the line of if(in.ready)
Its clear from Exception that variable in is not initialized. Please check it again.
A best practice to avoid NullPoinerException while comparing String is compare in reverse order as shown below:
while (!"ACK".equals(in.readLine()));
Still there is one more issue in your code.
Client sending three values to server as shown below:
out.println(Name); // String
out.println(Score);// float
out.println(place); // int
Now at server side your are converting float to int using Integer.parseInt(in.readLine()) as shown below that will result in NumberFormatException
this.setHighScore(in.readLine(),Integer.parseInt(in.readLine()),
Integer.parseInt(in.readLine()));// converting Score to int
For example
Integer.parseInt("2.0");
will result into
java.lang.NumberFormatException: For input string: "2.0"
One more sample code
float val = 2;
String str = String.valueOf(val);
Integer.parseInt(str);
will result into
java.lang.NumberFormatException: For input string: "2.0"
I am encountering an error when user doesn't type anything into input statement. I thought of using Try/Catch blocks to instead throw exception to set boolAskRepeat to true which should skip to the end of the code and repeat the loop.
This doesn't work, and I believe I'm missing something but I'm not sure what... It still throws exception saying:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.lang.String.charAt(Unknown Source)
at ITSLab03.main(ITSLab03.java:34)
Which is this line of code: inputStatus = input.readLine().toLowerCase().charAt(0);
What am I doing wrong here?
while (boolAskStatus == true)
{
System.out.print("Employment Status (F or P): ");
try
{
inputStatus = input.readLine().toLowerCase().charAt(0);
if (inputStatus == "f".charAt(0))
{
boolAskStatus = false;
String stringCheckSalary = null;
boolean boolCheckSalary = true;
while (boolCheckSalary == true)
{
// some code
}
outputData(inputName, inputStatus, calculateFullTimePay(inputSalary));
}
else if (inputStatus == "p".charAt(0))
{
// some code
outputData(inputName, inputStatus, calculatePartTimePay(inputRate, inputHours));
}
else boolAskStatus = true;
}
catch (IOException e) { boolAskStatus = true; }
}
You need to catch StringIndexOutOfBoundsException as well (If you observe the stack trace properly this is the exception you are getting)
catch (StringIndexOutOfBoundsException e) {
boolAskStatus = true;
}
(or)
catch Exception which catches all runtime exceptions
catch (Exception e) {
boolAskStatus = true;
}
The normal try catch pattern should look like this:
try
{
// code that is vulnerable to crash
}
catch (Specific-Exception1 e1)
{
// perform action pertaining to this exception
}
catch (Specific-Exception2 e2)
{
// perform action pertaining to this exception
}
....
....
catch (Exception exp) // general exception, all exceptions will be caught
{
// Handle general exceptions. Normally i would end the program or
// inform the user that something unexpected occurred.
}
By using .charAt(0), you are assuming that the String has a length > 0.
You could simplify this a bunch by just doing:
String entry = input.readLine().toLowerCase();
if (entry.startsWith("f")) {
...
}
else if ("entry".startsWith("p")) {
...
}
Your code doesn't work the way you want because input.readLine().toLowerCase().charAt(0) throws a StringIndexOutOfBoundsException, which is not an IOException, so the catch block never gets hit. You can make it work by changing the catch to
catch (StringIndexOutOfBoundsExceptione e) { boolAskStatus = true; }
But...
It's generally not a good idea to base your program's normal behaviour on exception handling. Think of exception throwing as something that could happen, but usually won't. Why not use something like:
final String STATUS_F = "f";
final String STATUS_P = "p";
String fromUser = null;
do {
String rawInput = input.readLine().toLowerCase();
if (rawInput.startsWith(STATUS_F)) {
fromUser = STATUS_F;
} else if (rawInput.startsWith(STATUS_P)) {
fromUser = STATUS_P;
}
} while (fromUser == null);
if (STATUS_F.equals(fromUser)) {
// do something
} else if (STATUS_P.equals(fromUser)) {
// do something else
} else {
// Shouldn't be able to get here!
throw new Exception("WTF!?");
}
It much easier for another person reading this to understand why the program loops and how the loop is controlled, in part because the code that figures out what the user is inputting and the code that decides what to do with that information are separated. Plus, you don't need to deal with exceptions.
I have a program developed and it has a single entry point. A Try catch block is surrounding it.
try {
Runner runner = new Runner();
// Adhoc code
UIManager.setLookAndFeel(new NimbusLookAndFeel());
runner.setupVariables();
runner.setLookAndFeel();
runner.startSessionFactory();
runner.setupApplicationVariables();
runner.setupDirectories();
// This will be used to test out frames in development mode
if (Runner.isProduction == true) {
execute();
} else {
test();
}
} catch (Exception e) {
SwingHelper.showErrorMessageMainFrame(e.getMessage());
Logger.getRootLogger().error(e);
e.printStackTrace();
}
But suppose a null pointer exception is thrown, the message box is empty since the Exception doesn't contain a message. For this I added a logic-
if(e instanceof NullPointerException){
NullPointerException n =(NullPointerException) e;
SwingHelper.showErrorMessageMainFrame("Unexpected Exception due at ");
}else{
SwingHelper.showErrorMessageMainFrame(e.getMessage());
}
This works all fine but I also want the line number to be displayed. How can I get it done. How can I get the line number of the exception?
Among the answer to this question, you can use this snippet:
public static int getLineNumber() {
return Thread.currentThread().getStackTrace()[2].getLineNumber();
}
Althought is recommended to use a logging library such as log4j.
The metadata for the exception is stored in StackTraceElement class, which you can get from your exception by calling getStackTrace().
Example of using it is:
if (e instanceof NullPointerException) {
NullPointerException n = (NullPointerException) e;
StackTraceElement stackTrace = n.getStackTrace()[0];
SwingHelper.showErrorMessageMainFrame("Unexpected Exception due at " + stactTrace.getLineNumber());
}
if(e instanceof NullPointerException){
NullPointerException n =(NullPointerException) e;
SwingHelper.showErrorMessageMainFrame("Unexpected Exception due at line" + e.getStackTrace()[0].getLineNumber());
} else {
SwingHelper.showErrorMessageMainFrame(e.getMessage());
}
Wow I was ninja'd by those above...
EDIT: Forgot to indent
I'm trying to throw an exception (without using a try catch block) and my program finishes right after the exception is thrown. Is there a way that after I throw the exception, to then continue execution of my program? I throw the InvalidEmployeeTypeException which I've defined in another class but I'd like the program to continue after this is thrown.
private void getData() throws InvalidEmployeeTypeException{
System.out.println("Enter filename: ");
Scanner prompt = new Scanner(System.in);
inp = prompt.nextLine();
File inFile = new File(inp);
try {
input = new Scanner(inFile);
} catch (FileNotFoundException ex) {
ex.printStackTrace();
System.exit(1);
}
String type, name;
int year, salary, hours;
double wage;
Employee e = null;
while(input.hasNext()) {
try{
type = input.next();
name = input.next();
year = input.nextInt();
if (type.equalsIgnoreCase("manager") || type.equalsIgnoreCase("staff")) {
salary = input.nextInt();
if (type.equalsIgnoreCase("manager")) {
e = new Manager(name, year, salary);
}
else {
e = new Staff(name, year, salary);
}
}
else if (type.equalsIgnoreCase("fulltime") || type.equalsIgnoreCase("parttime")) {
hours = input.nextInt();
wage = input.nextDouble();
if (type.equalsIgnoreCase("fulltime")) {
e = new FullTime(name, year, hours, wage);
}
else {
e = new PartTime(name, year, hours, wage);
}
}
else {
throw new InvalidEmployeeTypeException();
input.nextLine();
continue;
}
} catch(InputMismatchException ex)
{
System.out.println("** Error: Invalid input **");
input.nextLine();
continue;
}
//catch(InvalidEmployeeTypeException ex)
//{
//}
employees.add(e);
}
}
If you throw the exception, the method execution will stop and the exception is thrown to the caller method. throw always interrupt the execution flow of the current method. a try/catch block is something you could write when you call a method that may throw an exception, but throwing an exception just means that method execution is terminated due to an abnormal condition, and the exception notifies the caller method of that condition.
Find this tutorial about exception and how they work - http://docs.oracle.com/javase/tutorial/essential/exceptions/
Try this:
try
{
throw new InvalidEmployeeTypeException();
input.nextLine();
}
catch(InvalidEmployeeTypeException ex)
{
//do error handling
}
continue;
If you have a method that you want to throw an error but you want to do some cleanup in your method beforehand you can put the code that will throw the exception inside a try block, then put the cleanup in the catch block, then throw the error.
try {
//Dangerous code: could throw an error
} catch (Exception e) {
//Cleanup: make sure that this methods variables and such are in the desired state
throw e;
}
This way the try/catch block is not actually handling the error but it gives you time to do stuff before the method terminates and still ensures that the error is passed on to the caller.
An example of this would be if a variable changed in the method then that variable was the cause of an error. It may be desirable to revert the variable.