I've got a doubt about using the try/catch/finally clauses,let me explain:
I know that every method which declares an exception specification in its declaration has to be surrounded (in a calling method) with a try block,followed by a catch block that can capture that exception (unless I don't declare the calling method throwing an exception as well). If the calling method has other statements after the try-catch, those are executed regardless of what happens (exception thrown or not). So if I have the following code:
public class ExceptionCall {
Throwing t = new Throwing();
public void methodTry(){
while(true){
try {
if (t.flag++==0)
t.throwing();
System.out.println("no exception");
}
catch (MyException e) {
e.printStackTrace(System.err);
System.out.println("working on it!");
}
finally{
System.out.println("finally clause");
}
System.out.println("out");
if (t.flag==2)
break;
}
}
}
Here Throwing has a method called (guess what :) ) throwing() which is declared to throw a MyException exception, and has a public field called flag containing an int initialized to 0 to provide a sort of condition checking.
So for a couple of times the code outside the guarded region is executed no matter what happens in the try block.
So my question is, what is the finally block for?? I mean, I know it comes in handy when the calling method returns from the try or the catch block (in this case I could have a break statement inside the catch and the finally would be executed), but in a case like this what's the difference??
The finally block always executes when the try block exits. This ensures that the finally block is executed even if an unexpected exception occurs. But finally is useful for more than just exception handling — it allows the programmer to avoid having cleanup code accidentally bypassed by a return, continue, or break. Putting cleanup code in a finally block is always a good practice, even when no exceptions are anticipated.
See : https://docs.oracle.com/javase/tutorial/essential/exceptions/finally.html
Finally block is used to handle those resources that need you to make some action at the end of the statement no matter what happens in the code.
For example: readers or streams are resources that must be closed after it's use, but exceptions can happen during it's usage, so where you close them???
Here is where finally clause becomes really handy:
FileReader reader;
try {
reader = new FileReader("someNonExistingFile");
} catch (FileNotFoundException e) {
// handle FileNotFoundException
e.printMessage();
} finally {
// reader MUST BE CLOSED ALWAYS, so FINALLY, CLOSE IT
reader.close();
}
In this example, you need to close reader always (if file exists and if file doesn't) so, instead of closing it twice (in try and in catch) you use finally.
Further info and examples of finally here. Also check try with resources (Java 7+).
Related
This question already has answers here:
Multiple returns: Which one sets the final return value?
(7 answers)
Closed 7 years ago.
I found this piece of code in a java certification website
public class Test1{
public static void main(String args[]){
System.out.println(method());
}
public static int method(){
try{
return 1;
}
catch(Exception e){
return 2;
}
finally{
return 3;
}
}
}
So the output of this piece of code is shown as 3.
How is this possible..since it is returning 1 in the try block itself?
The code will never reach finally right??
The code will never reach finally right?
No, If there is a finally block control will go to finally after try or/and catch
Because in any case finally will always execute. Except as Mike Kobit said System.exit
Thanks Jason C, From JLS 14.17. The return Statement
It can be seen, then, that a return statement always completes abruptly.
The preceding descriptions say "attempts to transfer control" rather than just "transfers control" because if there are any try statements (§14.20) within the method or constructor whose try blocks or catch clauses contain the return statement, then any finally clauses of those try statements will be executed, in order, innermost to outermost, before control is transferred to the invoker of the method or constructor. Abrupt completion of a finally clause can disrupt the transfer of control initiated by a return statement.
From JLS 14.20.2. Execution of try-finally and try-catch-finally
If execution of the try block completes normally, then the finally block is executed, and then there is a choice:
If the finally block completes normally, then the try statement completes normally.
If the finally block completes abruptly for reason S, then the try statement completes abruptly for reason S.
So In you code try returns 1, then control goes to finally which returns 3. so return value of function will be 3.
As Sumit Singh mentioned in his answer, this happens because return causes the method to end abruptly.
You can observe the same behaviour, when you throw new Exceptions inside your try-catch-finally block, since throw is also abrupt.
See the following code:
public class FinallyTest {
public static void main(String[] args) {
abrupt();
}
#SuppressWarnings("finally")
public static void abrupt() {
try {
throw new IllegalArgumentException("In Try");
}
catch(Exception x) {
throw new IllegalArgumentException("In Catch");
}
finally {
throw new NullPointerException("In Finally");
}
}
}
When running the code, the console shows
Exception in thread "main" java.lang.NullPointerException: In Finally
at FinallyTest.abrupt(FinallyTest.java:15)
at FinallyTest.main(FinallyTest.java:3)
Even though you throw a new IllegalArgumentException inside the try block, only the NullPointerException from the finally block is thrown, since this block ended abruptly.
Finally block will always get executed just before return statement get executed. (There are some exceptions like JVM itself crash or system.exit() is called)
Here is the reason:
We normally use finally to free up our resources or to cleanup. Now if developer will write return statement accidentally in try block then logically finally would never get executed. To overcome this problem, JVM by itself handle this scenario and execute finally block just before returning from try block.
Remember the finally block always executes when the try block exits. And above explained scenario is also applied to return, continue, or break.
Because if we think about priority finally has more than try or catch, to make it clear what if the exception haven't caught even then the finally executes and here it is working inside a method so logically when a method is suppose to return a single output int value then obviously it will always return values from finally.
Is it makes sense?
Which exception handler going to handle the exception if there is no catch block ?
public class Doubt {
public static void main(String arg[])
{
System.out.println("hi");
int i=10;
int j;
try
{
j=i/0;//Arithmatic exception type object is created but no catch block so who will catch d exception..
}
finally
{
System.out.println("shit");
}
System.out.println("program still running");
}
}
None. The try..finally block doesn't catch exceptions at all.
It is used to ensure that the code in the finally block gets executed (even when an exception in the try block occurs). The finally block will always get executed, even when there is a return statement in try.
Short from cutting power from your computer or suddenly shutting JVM down (or in some cases interrupting a thread in which the try code was executed) there is no way to skip the finally block.
Add a catch block to catch an exception:
try {
j=i/0;
} catch(Excecption e) {
e.printStackTrace();
} finally {
System.out.println("shit");
}
There are three types of Exceptions in Java:
Checked - you are forced to use try catch block, if you do not do that - your program will not compile
Unchecked (Runtime Exceptions) - these exception are thrown by jre. As in your example you cannot determine everything at compile time. Instead of your i/0 there could be some number divided by user input. When user passes 0 - you will get runtime exception. Unhandled will crash application.
Error - Generally this is almost the same as above, but this is connected to jvm failures, like memory corruption etc.
The JVM system will catch the exception and crash ungracefully. Add an except statement to either handle the exception or exit gracefully.
Try block
The try block contains set of statements where an exception can occur. A try block is always followed by a catch block, which handles the exception that occurs in associated try block. A try block must be followed by catch blocks or finally block or both.
Catch block
A catch block is where you handle the exceptions, this block must follow the try block. A single try block can have several catch blocks associated with it. You can catch different exceptions in different catch blocks. When an exception occurs in try block, the corresponding catch block that handles that particular exception executes. For example if an arithmetic exception occurs in try block then the statements enclosed in catch block for arithmetic exception executes.
https://beginnersbook.com/2013/04/try-catch-in-java/ getsome idea from this..
What is the difference between "finally" and write after "catch"?
For example:
public boolean example() {
try {
// Code
} catch (RuntimeException e) {
// Code
} finally {
return true;
}
}
public boolean example() {
try {
// Code
} catch (RuntimeException e) {
// Code
}
return true;
}
First of all, the only idiom that can even be considered a candidate is this:
try {
// stuff
} catch (Throwable t) {
// handle
}
// finally stuff
Note the caught type. Only if you catch any possible exception, including such dark monsters as ThreadDeath and VirtualMachineError can you hope to unconditionally reach the code below the try-catch.
But, that's only where it begins. What if the handling code itself throws an exception? So you need at least
try {
// main stuff
} catch (Throwable t) {
try {
// handle
} catch (Throwable t) {
// no code allowed here! Otherwise we descend into
// infinite recursion
}
}
// finally stuff
Now you may be beginning to realize the benefits of finally, but that's not all. Consider a quite typical scenario:
try {
// main stuff, may throw an exception. I want the exception to
// break what I'm doing here and propagate to the caller.
return true;
} finally {
// clean up, even if about to propagate the exception
}
How do you rewrite this? Without code duplication, impossible.
In the code you have provided there is no difference. But finally is used to run some piece of code after the try block no matter whether there is an exception or not.
Interesting point to make here is, we should avoid to return from the finally block because it can create confusion in scenarion when we return something from try block as well. Consider this piece of code:
try {
return true;
}
finally {
return false;
}
Now this code will return false no matter what happens in try. In many ways this behaviour is exactly consistent with the colloquial understanding of what finally means - "no matter what happens beforehand in the try block, always run this code." Hence if you return true from a finally block, the overall effect must always to be to return true, no?
In general, this is seldom a good idiom, and you should use finally
blocks liberally for cleaning up/closing resources but rarely if ever
return a value from them.
No difference in Your case. but
The runtime system always executes the statements within the finally block regardless of what happens within the try block.
So it's the perfect place to perform cleanup.
Note that if you're using JDK 7+, then most uses of the finally block can be eliminated, simply by using a try-with-resources statement.
We use try block, catch block and finally block to handle Exception in our program. A program may be have check or unchecked Exception. so this block are are used to handle those Exception. In the try block we mention or write those code which may be cause of an Exception and if we want that our code will be run if Exception occurred Then we write those code in the catch block. finally is a very special block which give us a special feature that if our catch block doesn't run then before the program goes to terminate that finally block code definitely execute. mostly this is use to save our data during unwanted program termination. if we use try block then there must be a catch block after the try block but finally is an optional not compulsory.
First thing to understand in this question is "why we use try{ } catch{ } block" Ok.
The answer is,when there is a possibility of our code of throwing an exception.
These kind of code we put in try{... } block &
catch {...} block contain the code to catch the exception generated by code in try{ } block.
Finally{...} block contain the code executed immediately after try{} catch{} block when try{ } block throws an exception.
e.g When you visit some website,but do to some server side problem it couldn't and page display some kind of message like "404 error or server is updating",these kind of code is written in finally block.
this simple catch only is for catch eXception and process it, but finally, execute if Exception or not , for example for close connections.
Finally block is useful to cleanup your code like closing open streams.
E.g:
File f = new File("\file.txt");
FileStream fs;
try{
fs = new FileStream(f);
} catch (IOException ioe){
ioe.printStackTrace();
} finally {
if (fs != null) {
fs.close() // close the stream
}
}
As your code only returning true/false , so it won't impact much . But think for any other code/logic where some mandatory/cleanup code is written to execute.
Also, it will difficult to catch all the exceptions via our code or in other words, we should catch specific exceptions which we can handle. At that point , finally will be real savior, as it will always execute(other than System.exit() or in some recursion case).
Also, for bringing the consistency in our code, one should always use finally block to perform any clean up code.
You may refer the below posts too for more clarification:
Using finally block vs writing code after the try/catch block
Try-catch-finally-return clarification
First Case:
If exception occurred in try block then catch block would be executed, and while serving the same catch block if there is again exception occurred then finally block would execute.
Second Case:
If exception occurred in try block then catch block would be executed, and if there is further exception in the same catch block then "return true;" would execute.
finally() is used whenever we want that some line of code should be executed even if exception occurs
But if u write line of code after catch it will not execute if exception occurs..
This question already has answers here:
Does a finally block always get executed in Java?
(51 answers)
Closed 9 years ago.
I have this java code with nested try:
try
{
try
{
[ ... ]
{
catch (Exception ex)
{
showLogMessage(ex);
return;
}
while (condition == true)
{
try
{
[ ... ]
{
catch (Exception ex)
{
showLogMessage(ex);
continue;
}
[ ... ]
}
}
catch (NumberFormatException e)
{
showLogMessage(e);
}
finally
{
doSomeThingVeryImportant();
}
I want to know if finally is always executed when I get an exception. I ask this because catch blocks have return or continue statements.
When is doSomeThingVeryImportant() executed? When I get an Exception on when I get a NumberFormatException?
I only want if after any catch block is executed, the finally block is executed also.
The finally block, if used, is placed after a try block and the catch blocks that follow it. The finally block contains code that will be run whether or not an exception is thrown in a try block. The general syntax looks like this:
public void someMethod{
Try {
// some code
}
Catch(Exception x) {
// some code
}
Catch(ExceptionClass y) {
// some code
}
Finally{
//this code will be executed whether or not an exception
//is thrown or caught
}
}
There are 4 potential scenarios here:
The try block runs to the end, and no exception is
thrown. In this scenario, the finally block will be
executed after the try block.
An exception is thrown in the try block, which is
then caught in one of the catch blocks. In this scenario,
the finally block will execute right after the catch block
executes.
An exception is thrown in the try block and there's
no matching catch block in the method that can catch
the exception. In this scenario, the call to the method
ends, and the exception object is thrown to the enclosing
method - as in the method in which the try-catch-finally
blocks reside. But, before the method ends, the finally
block is executed.
Before the try block runs to completion it returns to
wherever the method was invoked. But, before it returns
to the invoking method, the code in the finally block is still
executed. So, remember that the code in the finally block
willstill be executed even if there is a return
statement somewhere in the try block.
Update: Finally ALWAYS gets executed, no matter what happens in the try or catch block (fail, return, exception, finish etc.).
The finally block always executes when the try block exits (click).
Yes, Finally always executes. With exception and with NO exception.
It's the way to be sure some portion of code get always executed.
Used for example, to dispose objects, to close opened server connections and that kind of stuff.
Check this link from oracle:
http://docs.oracle.com/javase/tutorial/essential/exceptions/finally.html
Yes, finally blocks are always executed.
Source: http://docs.oracle.com/javase/tutorial/essential/exceptions/finally.html
Yes; or, at least, as close to "always" as possible. (So, even if you have a return or another throw.)
If your process is killed, or your program gets stuck in a deadlock or infinite loop, or your device is struck by a meteor, then program flow will not reach the finally block.
Say your code block is something like this :
try
{
try
{
System.out.println("first try block");
}
catch (Exception ex)
{
System.out.println("first cathc block");
return;
}
while (true)
{
try
{
System.out.println("second try block...");
}
catch (Exception ex)
{
System.out.println("second catch block");
continue;
}
System.out.println("end of loop");
break;
}
}
catch (NumberFormatException e)
{
System.out.println("last catch block");
}
finally
{
System.out.println("finally block");
}
If you run this then the output that you will get is :
first try block
second try block...
end of loop
finally block
So finally block gets executed anyway.
The finally block always executes when the try block exits.
This ensures that the finally block is executed even if an unexpected exception occurs
Taken from here: http://docs.oracle.com/javase/tutorial/essential/exceptions/finally.html
Further, the page explains that the finally block may not be executed if the JVM exists while the try or catch code is being executed, or if the thread executing the try/catch is interrupted or killed.
So unless you may kill the JVM or the try/catch is beeing executed in a thread, the finally block will always be executed
Yes, finally blocks are always executed. But terms and conditions apply .
Untill unless you call System.exit();
Yes finally always gets executed and its because one can program to close/handle resources in case of exceptions or no exception without fail.
http://docs.oracle.com/javase/tutorial/essential/exceptions/finally.html
Update to answer the question in comment about System.exit()
The finally block will not execute in case of System.exit() unless it fails with some SecurityException.
Update after question changed. No matter what the exception is finally block will always execute.
If its Exception then your catch block will not execute as it catches only NumberFormatException
No, finally won't get execute always. There are two situations where Finally won't get execute.
Note: If the JVM exits while the try or catch code is being executed, then the finally block may not execute. Likewise, if the thread executing the try or catch code is interrupted or killed, the finally block may not execute even though the application as a whole continues.
java reference
For your question, if these two things are not there, whatever inside your finally will get executed, that's sure.
This question already has answers here:
Why use finally
(9 answers)
Closed 1 year ago.
As far as I can tell, both of the following code snippets will serve the same purpose. Why have finally blocks at all?
Code A:
try { /* Some code */ }
catch { /* Exception handling code */ }
finally { /* Cleanup code */ }
Code B:
try { /* Some code */ }
catch { /* Exception handling code */ }
// Cleanup code
What happens if an exception you're not handling gets thrown? (I hope you're not catching Throwable...)
What happens if you return from inside the try block?
What happens if the catch block throws an exception?
A finally block makes sure that however you exit that block (modulo a few ways of aborting the whole process explicitly), it will get executed. That's important for deterministic cleanup of resources.
Note that (in Java at least, probably also in C#) it's also possible to have a try block without a catch, but with a finally. When an exception happens in the try block, the code in the finally block is run before the exception is thrown higher up:
InputStream in = new FileInputStream("somefile.xyz");
try {
somethingThatMightThrowAnException();
}
finally {
// cleanup here
in.close();
}
You may want to put the code that you want to anyway get executed irrespective of what happens in your try or catch block.
Also if you are using multiple catch and if you want to put some code which is common for all the catch blocks this would be a place to put- but you cannot be sure that the entire code in try has been executed.
For example:
conn c1 = new connection();
try {
c1.dosomething();
} catch (ExceptionA exa) {
handleexA();
//c1.close();
} catch (ExceptionB exb) {
handleexB();
//c1.close();
} finally {
c1.close();
}
Finally always gets executed, where as your code after the catch may not.
Even though our application is closed forcefully there will be some tasks, which we must execute (like memory release, closing database, release lock, etc), if you write these lines of code in the finally block it will execute whether an exception is thrown or not...
Your application may be a collection of threads, Exception terminates the thread but not the whole application, in this case finally is more useful.
In some cases finally won't execute such as JVM Fail, Thread terminate, etc.
Still scrolling down? Here you go!
This question gave me tough time back a while.
try
{
int a=1;
int b=0;
int c=a/b;
}
catch(Exception ex)
{
console.writeline(ex.Message);
}
finally
{
console.writeline("Finally block");
}
console.writeline("After finally");
what would be printed in the above scenario?
Yes guessed it right:
ex.Message--whatever it is (probably attempted division by zero)
Finally block
After finally
try
{
int a=1;
int b=0;
int c=a/b;
}
catch(Exception ex)
{
throw(ex);
}
finally
{
console.writeline("Finally block");
}
console.writeline("After finally");
What would this print? Nothing! It throws an error since the catch block raised an error.
In a good programming structure, your exceptions would be funneled, in the sense that this code will be handled from another layer. To stimulate such a case i'll nested try this code.
try
{
try
{
int a=1;
int b=0;
int c=a/b;
}
catch(Exception ex)
{
throw(ex);
}
finally
{
console.writeline("Finally block")
}
console.writeline("After finally");
}
catch(Exception ex)
{
console.writeline(ex.Message);
}
In this case the output would be:
Finally block
ex.Message--whatever it is.
It is clear that when you catch an exception and throw it again into other layers(Funneling), the code after throw does not get executed. It acts similar to just how a return inside a function works.
You now know why not to close your resources on codes after the catch block.Place them in finally block.
Because you need that code to execute regardless of any exceptions that may be thrown. For example, you may need to clean up some unmanaged resource (the 'using' construct compiles to a try/finally block).
There may be times when you want to execute a piece of code no matter what. Whether an exception is thrown or not. Then one uses finally.
finally ALWAYS executes, unless the JVM was shut down, finally just provides a method to put the cleanup code in one place.
It would be too tedious if you had to put the clean up code in each of the catch blocks.
If catch block throws any exception then remaining code will not executed hence we have to write finaly block.
finally block in java can be used to put "cleanup" code such as closing a file, closing connection etc.
The finally block will not be executed if program exits(either by calling System.exit() or by causing a fatal error that causes the process to abort).