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
}
Related
This question already has answers here:
Can I catch multiple Java exceptions in the same catch clause?
(10 answers)
Closed 2 years ago.
If I have a try/catch block which catches an IndexOutOfBounds exception, but in the same code, I want to access a file, is it possible to do a general excepction which could catch an IOException too?
Normally when I use try/catch I just put the exception type, but I have never tried it before. Is it wrong to do so or just bad practice?
try {
// code to access a File which does not exist and inserts into an array index which does not exist
} catch (Exception eitherException){
// prints "An exception happened"
}
Would I need nested blocks, multiple catch clauses or is this ok? I have researched online for a while but cannot find an answer. Thank you!
The code you have written will catch IOException too since Exception class is the parent class of all exceptions. It would catch all types of exception. If you do not want to catch all exceptions and just these two exceptions , you can write like this.
try {
// code to access a File which does not exist and inserts into an array index which does not exist
} catch (IndexOutOfBounds | IOException eitherException){
// prints "An exception happened"
}
It depends on how you want to handle the different exception thrown. You may want different functional flow during different exceptions thrown (case 1) or have a same function flow during exception (case 2) or just ignore the exception altogether and log it (case 2).
Ex: Say I have an application that loads a file externally. In case an IO exception is thrown I want to use the default properties defined in the code itself. So I can use case 1 and do the required when IOException is thrown etc.. and when IndexOutOfBounds is thrown some other logic get executed.
Case 1:
try {
// business logic
} catch (IndexOutOfBounds exceptionOne) {
// print log as index out of bounds exception thrown
// execute some business logic
} catch (IOException excpetionTwo) {
// print log as IO exception thrown
// execute some other business logic
}
Case 2:
try {
// business logic
} catch (IndexOutOfBounds | IOException exception) {
// print log as exception thrown
// execute business logic
}
I want to write a code like this:
try {
try {
someStuffThatCausesBusinessExceptions();
} finally {
try {
cleanUp();
} catch (Exception e) {
// I don't really care
}
}
} catch (BusinessLogicException e) {
// work with exception
// cleaning up must be done by that point (or at least tried to)
}
Will exceptions from business logic survive the possible hiatus during cleanUp? Is there a better way to ignore all the possible exceptions from cleanUp?
A catch block will only catch Throwables that were thrown in its corresponding try block. Thus, your exceptions thrown in the surrounding try block will be preserved and caught in the outer catch block.
Yes. Your exception will reach the last catch. However, this structure seems weird an non-idiomatic to me, i guess i would even prefer having cleanUp() more than once in that code over having 3 tries.
There are two cases-:
1. if excetpion occurs from mehtod someStuffThatCausesBusinessExceptions only then it will be caught in your outer catch block.
2. if the methods someStuffThatCausesBusinessExceptions and cleanUp both throw exceptions then the exception thrown from try block is suppressed.
Yes!! there is better way.You can use try-with-resources statement.
Please refere to this link.
https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html
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.
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++;
This question already has answers here:
Why do we use finally blocks? [duplicate]
(11 answers)
Closed 5 years ago.
Why do this
} catch (SQLException sqle) {
sqle.printStackTrace();
} finally {
cs.close();
rs.close();
}
Instead of this
} catch (SQLException sqle) {
sqle.printStackTrace();
}
rs.close();
cs.close();
Because if an exception gets thrown no code after the try block is executed unless the exception is caught. A finally block is always executed no matter what happens inside your try block.
Look at your catch block - it's going to throw DAOException. So the statements after your catch block aren't going to be executed even in the sample you've given. What you've shown (wrapping one exception in another) is one common pattern - but another possibility is that the catch block "accidentally" throws an exception, e.g. because one of the calls it makes fails.
Additionally, there may be other exceptions you don't catch - either because you've declared that the method throws them, or because they're unchecked exceptions. Do you really want to leak resources because an IllegalArgumentException got thrown somewhere?
Because if an exception is thrown,
Code in the finally clause will execute as the exception propagates outward, even if the exception aborts the rest of the method execution;
Code after the try/catch block will not get executed unless the exception is caught by a catch block and not rethrown.
According to HeadFirst Java, a finally block will run even if the try or catch block has a return statement. Flow jumps to finally and then back to return.
Because it ensures that the stuff in the finally block gets executed. Stuff after catch might not get executed, say, for example, there is another exception in the catch block, which is very possible. Or you just do what you did, and throw an exception wrapping the original exception.
The finally keyword guarantees that the code is executed. In your bottom example, the close statements are NOT executed. In the top example, they are executed (what you want!)
Your second approach won't do the 'close' statements because it is already left the method.
This is the way to avoid resource leaks
If you catch all errors, there should no difference, otherwise, only code inside finally block is executed because the code execution sequence is:
finally code -> error throw -> code after catch
hence, once your code throw any unhandled error, only finally code block works as expected.
The code in the finally block will get called before the exception is rethrown from the catch block. This ensures that any cleanup code that you've put in the finally block gets called. Code outside of the finally block will not be run.
this might clarify: http://www.java2s.com/Code/Java/Language-Basics/Finallyisalwaysexecuted.htm
consider the catch can throw an exception to the higher level functions in the call stack. This will lead calling final before throwing the exception to the upper level.
In http://docs.oracle.com/javase/tutorial/essential/exceptions/finally.html this is misleading (and may have originated the question):
The try block of the writeList method that you've been working with here opens a PrintWriter. The program should close that stream before exiting the writeList method. This poses a somewhat complicated problem because writeList's try block can exit in one of three ways.
1. The new FileWriter statement fails and throws an IOException.
2. The list.get(i) statement fails and throws an IndexOutOfBoundsException.
3. Everything succeeds and the try block exits normally.
The 4th way (an exception other than IOException and IndexOutOfBoundsException is thrown) is missing. The code depicted in the preceding page only catches (1) and (2) before resorting to finally.
I'm also new to Java and had the same questioning before finding this article. Latent memory tend to attach itself more to examples than to theory, in general.
The finally block may not always run, consider the following code.
public class Tester {
public static void main(String[] args) {
try {
System.out.println("The main method has run");
System.exit(1);
} catch (Exception e) {
e.printStackTrace();
} finally {
System.out.println("The finally block has run");
}
}
}
In your case, I would suggest to wrap the code inside finally block into try/catch, as this code apparently may throw an exception.
} catch (SQLException sqle) {
sqle.printStackTrace();
} finally {
try {
cs.close();
rs.close();
} catch (Exception e) {
//handle new exception here
}