This question seems pretty simple. How do I keep running my application on an exception.
For instance how would I go about doing this.
Server application tries to run on port 1234, but it's not available. Normally it would just crash. But how can I make it keep running and say; Would you like to try another port?
Or if we're trying to load a file which doesn't exists. How do I avoid the program from crashing, but just display a little message saying the it was unable to load the file.
How is this done?
Use a try catch block.
try{
//Where exception may happen
}catch(Exception e){//Exception type. Exception covers it all.
//Print error if you would like or do something else
}finally{//Finally is optional, as the code in here will run regardless of an exception.
}
//program continues
Most try-catch blocks do not have finally at the end. You will use finally if you need code to be run whether or not there was an exception. More information about the finally block
Here's an example where a catch would fail, but the finally does execute:
int number = 0;
try {
number = 1 / 0;
} catch (IndexOutOfBoundsException e) {
System.out.println("Nooooo!");
} finally {
System.out.println("What just happened?");
}
System.out.println(number);
This outputs:
What just happened?
Exception in thread "main" java.lang.ArithmeticException: / by zero
at ...
The catch failed to execute because it only catches IndexOutOfBoundsExceptions and not ArithmeticExceptions, which is what you get when you try to divide by zero.
I was suffering from same problem but my case was the exception thrown from binary tree class which terminated the program and I wanted it to continue execution.So, your problem can be solved by below written code.
do{
try{
//get the value for port
//it may throw exception
}
catch(Exception e){
// handle exception
}
//ask for yes or no to continue
}while(choice);
Well, this will keep asking for port values as long as you want.Use loops to ask values for ports continuously.
Remember:You can add code after catch block but not in between try and catch block.
Related
we specify the exception in try and catch.If we know what exception is gonna be generated why go for exception handling rather than just debug that part of the code?
According to Oracle definition of Exception
An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions.
CONCLUSION:
If you put a try {} catch block you know will always trow an Exception, your code is wrong.
For example, this code compiles? YES, but is wrong
String s = "throwMe!";
try {
int number = Integer.parseInt(s);
} catch (NumberFormatException) {
}
CORRECT EXCEPTION USE
System.out.println("Enter a number");
String s = Scanner..... // user enters something
try {
int number = Integer.parseInt(s);
} catch (NumberFormatException) {
// TELL USER THERE IS A PROBLEM
}
// TELL USER IS A GENIUS
This will have 2 execution flows, the correct one (user is a genius) but in the moment the user enters a value disrupting the flow (Integer.parseInt(s);) the Exception is thrown...
No, exceptions refer to run-time conditions that we cannot foresee
For example, the divide-by-zero error happens due to user inputting wrong data.
So you catch with try-catch
try {
}
catch(ArithmeticException){
}
You don't have to do exception handling or debugging, you can do both and good exception handling helps you debug your code later on.
If nothing else a catch block should print the Stack Trace which gives you information regarding where things went wrong with your code and it's much better than failing silently and then manually debug your whole code to search for the problem.
There are many other advantages to using exceptions for error handling as well.
Try / catch blocks are for errors that you cannot foresee. Things like null pointers and divide by 0 errors don't need a try catch blocks. Those things are typically errors on the part of programmers and should be debugged by the programmers. But things like IOException or SQLException, where you are interfacing with some other system that could fail or give invalid input that the programmer cannot control, those things need a try / catch block.
Can we write any implementation code in catch block? What are the rules to be used to implement in catch block?
try{
resultado = (T) mensaje.getBody(clase);
}
catch(Exception ex){
resultado = null;
this.destruye();
throw ex;
}
You can write all the code you want in your catch block.
Exception handlers can do more than just print error messages or halt
the program. They can do error recovery, prompt the user to make a
decision, or propagate the error up to a higher-level handler using
chained exception
Remember, this code will only be executed if an exception is thrown.
Yes you can write any code you want in the catch block, as you can see in the catch Blocks Documentation:
The catch block contains code that is executed if and when the exception handler is invoked.
So here the catch block is only executed if the code inside the try block raises an exception, so in that case you can handle the Exception and write whatever code you want.
For example you can write:
Int input = 0;
try {
System.out.println("Enter a number :");
input = scanner.nextInt();
} catch (IOException e) {
System.err.println("Caught IOException: " + e.getMessage());
System.out.println("Rewrite the number please");
}
And to answer you question about "What are the rules to be used to implement in catch block?" and if it's a bad practice to write code in the catch block:
You can see in the documentation that:
Exception handlers can do more than just print error messages or halt the program. They can do error recovery, prompt the user to make a decision, or propagate the error up to a higher-level handler using chained exceptions, as described in the Chained Exceptions section.
So it's fine to write whatever code you need to write in the catch block, you can take a look at Exception Chaining to see that you can write code there, but keep in mind that it's made to write code that handles the given Exception.
Can we write any implementation code in catch block?
You can write whatever code you want in your catch block. The code will only be executed only when the exception is thrown.
What are the rules to be used to implement in catch block?
Ideally catch blocks contain code which deal with the exceptions for an instance printing stack trace, log the exception, forwarding the flow to a jsp or a method, wrapping the exception & rethrowing it, exit gracefully. Besides this you can write whatever code as per your requirement you don't have to follow any specific rules.
Yes, you can write whatever you want....
If you think that your code porting may be arise any error
for that reason the program have terminated in that case to handle that exception you have to used catch block so that the program will not terminate.
try{
/h/ere code arise exception
}
catch(this is place where Exeption class which hold the exception type throw object){
//here for handling the exception
}
In my program I have to constantly access the hard drive thousands of times to view images (no way around it), occasionally my program gets tripped up on a "file not found IO Exception" most likely because of the many modifications I'm making to the images and re saving quickly. How do I continue my program even if this error occurs, because currently it causes my program to stop?
Code:
filepattern=imageLocation+temp+"image.jpg";
File outputfile = new File(filepattern);
BufferedImage img = null;
try {
img = ImageIO.read(outputfile);
} catch (IOException e) {
}
Note: I have fixed the problem by making sure the file exists first. Thanks for all your help!
Catch the exception and handle it as needed.
try {
// your code
} catch (<Expected exception> e) {
// handle the exception
}
Surround the statement with a try catch.
This will stop the code from crashing and in the catch block you can write code to deal with the failure.
There are 3 key words
try {}
executes the block that can cause problems
catch (Exception ex) {}
executes code to handle specific exceptions. Instead of Exception you can handle specific exception types
finally {}
executes cleanup code. Even if exception occurs and breaks the execution flow in try block, this code will always execute.
You might try something like the fragment below. Of course, you will want to encapsulate the actual reading in a try / catch / finally block to make sure you close the file
try {
filesTried++;
[you code]
} catch (IOException e) {
fileErrors++;
In VB, for error catching there is
Public Sub MySub()
On Error GoTo Errr
'do stuff
Errr:
'handle error
Resume Next
End Sub
which uses the magnificent Resume Next command.
In Java, you have a try catch block
try
{
//some code
}
catch (Exception e)
{
//handle error
}
which seems to be equivalent to the VB error catching, but specifically without the Resume Next option, so Java just quits the entire code block after the error, instead of trying to run the rest of the code after the error. Is there any way to get the power of Resume Next in Java? Thanks!
Just put the code that you want to run regardless of any error after the catch block.
try {
// stuff that could cause error
} catch(Exception e) {
// handle error
}
// do stuff
If you're going to throw an exception from the catch block but you still want the "do stuff" code to run, you can put it in a finally block like this:
try {
// stuff that could cause error
} catch(Exception e) {
// throw exception here
} finally {
// do stuff that will run even when the exception is thrown
}
There is no equivalent in Java, to VB resume statements; in VB depending on the error case, you can choose to resume at a particular label within your code, in order to re-run the code after fixing the error, similar to a goto statement; this is not possible in java, except when you're inside a loop, then you can use the continue to a defined label block.
I am designing a program in JAVA that captures results in about 10 iterations. At the end of these iterations all the results must be written into a log file.
If any exception occurs then it should be written on my text file and secondly the program must not stop, it must go on till the last iteration is completed...
That is to say - if some error occur on any part of any iteration the program must not stop here. The error must be mentioned within my results by the name of error and it must go on and update my log file.
My code till now is bit lengthy...used try-catch, the try block is doing my calculations and writing my text file, but I need if incase some exception occurs my program must not stop and that exception must be updated in my log file.
You're looking for the try-catch block. See, for example, this tutorial.
OutputStream os = ....;
PrintStream ps = new PrintStream(os);
while(notDone) {
try {
doStuff();
}
catch(Throwable t) {
t.printStackTrace(ps);
}
ps.print(results);
}
the case is, in this kind of a question, you should better provide us a sample code, then only we can identify the problem without any issue.
If you just need to view the error, then "e.printStackTrace" will help you. The "e" is an instance of class "Exception".
However, if you need to LOG, then "Logger" class will help you, with Exception class.For an example,
try {
f = location.createNewFile();
} catch (IOException ex) {
Logger.getLogger(TestForm.class.getName()).log(Level.SEVERE, null, ex);
}
To do all of these, it is better to surround your code with try catch block