I have the following Java code (that I thought was OK) but the IDE is warning me that "IOExcpetion e is immediately rethrown."
I'm new to exception handling in Java, and I wasn't aware there was anything wrong with doing that.
From what I'm reading, it looks like my other options is to simply delete the catch block and replace it with a finally block... but then it warns that the finally block is empty.
I don't have any ideas on what I should do differently.
private InputStream getFlutterAssetAsInputStream(String fromAsset) throws IOException {
String assetPath = binding
.getFlutterAssets()
.getAssetFilePathBySubpath(fromAsset, PACKAGE_NAME);
try {
return binding.getApplicationContext().getAssets().open(assetPath);
} catch (IOException e) {
throw e;
}
}
This
try {
return binding.getApplicationContext().getAssets().open(assetPath);
}
catch (IOException e) {
throw e;
}
is functionally equivalent to this:
return binding.getApplicationContext().getAssets().open(assetPath);
Your IDE is telling you that your try-catch is pointless. It does nothing.
So what do you want to do?
Pass the exception on to your caller? In which case get rid of the try-catch
Signal a different exception on to the caller - maybe one with a more appropriate error message for the specific circumstances? Then make and throw a new exception object with a better error message.
Do something to handle the situation - at the very least, maybe display an error message? Log the event to enable debugging? Then write code in the catch-block to do that. And then you have to decide what happens next.
Fundamentally, this is a design issue. IO errors happen. So how does this method that you're writing want to handle IO errors?
This question already has an answer here:
What does "error: unreported exception <XXX>; must be caught or declared to be thrown" mean and how do I fix it?
(1 answer)
Closed 8 months ago.
While learning Java I stumble upon this error quite often. It goes like this:
Unreported exception java.io.FileNotFound exception; must be caught or declared to be thrown.
java.io.FileNotFound is just an example, I've seen many different ones. In this particular case, code causing the error is:
OutputStream out = new BufferedOutputStream(new FileOutputStream(new File("myfile.pdf")));
Error always disappears and code compiles & runs successfully once I put the statement inside try/catch block. Sometimes it's good enough for me, but sometimes not.
First, examples I'm learning from do not always use try/catch and should work nevertheless, apparently.
Whats more important, sometimes when I put whole code inside try/catch it cannot work at all. E.g. in this particular case I need to out.close(); in finally{ } block; but if the statement above itself is inside the try{ }, finally{} doesnt "see" out and thus cannot close it.
My first idea was to import java.io.FileNotFound; or another relevant exception, but it didnt help.
What you're referring to are checked exceptions, meaning they must be declared or handled. The standard construct for dealing with files in Java looks something like this:
InputStream in = null;
try {
in = new InputStream(...);
// do stuff
} catch (IOException e) {
// do whatever
} finally {
if (in != null) {
try {
in.close();
} catch (Exception e) {
}
}
}
Is it ugly? Sure. Is it verbose? Sure. Java 7 will make it a little better with ARM blocks but until then you're stuck with the above.
You can also let the caller handle exceptions:
public void doStuff() throws IOException {
InputStream in = new InputStream(...);
// do stuff
in.close();
}
although even then the close() should probably be wrapped in a finally block.
But the above function declaration says that this method can throw an IOException. Since that's a checked exception the caller of this function will need to catch it (or declare it so its caller can deal with it and so on).
Java's checked exceptions make programmers address issues like this. (That's a good thing in my opinion, even if sweeping bugs under the carpet is easier.)
You should take some appropriate action if a failure occurs. Typically the handling should be at a different layer from where the exception was thrown.
Resource should be handled correctly, which takes the form:
acquire();
try {
use();
} finally {
release();
}
Never put the acquire() within the try block. Never put anything between the acquire() and try (other than a simple assign). Do not attempt to release multiple resources in a single finally block.
So, we have two different issues. Unfortunately the Java syntax mixes up the two. The correct way to write such code is:
try {
final FileOutputStream rawOut = new FileOutputStream(file);
try {
OutputStream out = new BufferedOutputStream(rawOut);
...
out.flush();
} finally {
rawOut.close();
}
} catch (FileNotFoundException exc) {
...do something not being able to create file...
} catch (IOException exc) {
...handle create file but borked - oops...
}
I'm making a sort of chat program in Java. Specifically, if I ask "can you open chrome?", the program will reply with "yes..." and then opens Google Chrome (Windows).
I have created the path to the Chrome as a string:
Runtime rt = Runtime.getRuntime()
String file="C:\\Program Files (x86)\\Google\\Chrome\\Application\\Chrome.exe";
I try to call the String, but says to either "Surround Statement with try/catch" or "Surround block with try/catch". Or the "Add throws clause to the "java.io.IOException" ".
myVocab.addPhrase("Can you open Chrome?", "Yes, one moment..." + rt.exec(file));
Whenever I do either of these, Chrome just opens automatically.
I'm somewhat new to Java so please tell me if there's an easier way to do this, or if I'm doing this completely wrong.
Some java functions need to be implemented with the try catch statements because it is possible to get an exception inside that function. An exception is defined as "An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions" more info
So, to manage an exception, for your case, you could:
try { code1 } catch (ExceptionType name) { code2 }
where ExceptionType should correspond to the possible error type your code1 could give you.
Ex:
try { //code to open google } catch (InterruptedException e) { e.printStackTrace(); }
e.printStackTrace(); will print error details
I am working in Linux/Ubuntu. I want to run a process in through my java code, which looks like below
ProcessBuilder pb = new ProcessBuilder("/usr/lib/flume-ng/bin/flume-ng",
"agent",
"-f",
"/home/c4/Flume/New/ClientAgent.config",
"-n",
"clientAgent");
pb.start();
But i get unreported exception java.io.IOException; must be caught or declared to be thrown pb.start(); as error output. Please tell me how i can run my process. Thanks.
It's telling you the start() method could throw an Exception, and you have to deal with it. You can either:
catch it and log it or otherwise handle it, or
declare your method as possibly throwing this exception, and let a method higher up the stack handle it (using these two options)
The Exception object is checked, which means the compiler is concerned with it, and you need to be too (however much of a pain that is). Other exceptions are unchecked, and this means you don't have to worry. The compiler won't worry either (e.g. OutOfMemoryError - be aware that I'm mixing some exception terminology here, since it's a little convoluted).
Since, IOException is a checked exception you need to either catch it
try {
pb.start();
} catch (IOException e) {
e.printStackTrace();
}
or throw it with the enclosing method declared to do so.
public void yourMethod() throws IOException {
I was trying to understand why to use exceptions.
Suppose if I have an program,
(without using try/catch)
public class ExceptionExample {
private static String str;
public static void main(String[] args) {
System.out.println(str.length());
}
I got exception
Exception in thread "main" java.lang.NullPointerException
at com.Hello.ExceptionExample.ExceptionExample.main(ExceptionExample.java:22)
Now using try/catch,
public class ExceptionExample {
private static String str;
public static void main(String[] args) {
try {
System.out.println(str.length());
} catch(NullPointerException npe) {
npe.printStackTrace();
}
}
}
I got Exception,
java.lang.NullPointerException
at com.Hello.ExceptionExample.ExceptionExample.main(ExceptionExample.java:9)
Now my question is,
In both the cases I have got the same message printed. So what is the use of using try/catch? and
What can we do after catching exception, in this case I have printed the stack trace. Is catch used only for printing the trace or for finding exception details using getMessage() or getClass()?
The difference is pretty big, actually.
Take the first one and add a line after the print:
public class ExceptionExample {
private static String str;
public static void main(String[] args) {
System.out.println(str.length());
System.out.println("Does this execute?");
}
}
You'll see that Does this execute? isn't printed because the exception interrupts the flow of the code and stops it when it isn't caught.
On the other hand:
public class ExceptionExample {
private static String str;
public static void main(String[] args) {
try {
System.out.println(str.length());
} catch(NullPointerException npe) {
npe.printStackTrace();
}
System.out.println("Does this execute?");
}
}
Will print both the stack trace and Does this execute?. That's because catching the exception is like saying, "We'll handle this here and continue executing."
One other remark, the catch block is where error recovery should happen, so if an error occurs but we can recover from it, we put the recovery code there.
Edit:
Here's an example of some error recovery. Let's say we have a non-existent file at C:\nonexistentfile.txt. We want to try and open it, and if we can't find it, show the user a message saying it's missing. This could be done by catching the FileNotFoundException produced here:
// Here, we declare "throws IOException" to say someone else needs to handle it
// In this particular case, IOException will only be thrown if an error occurs while reading the file
public static void printFileToConsole() throws IOException {
File nonExistent = new File("C:/nonexistentfile.txt");
Scanner scanner = null;
try {
Scanner scanner = new Scanner(nonExistent);
while (scanner.hasNextLine()) {
System.out.println(scanner.nextLine());
}
} catch (FileNotFoundException ex) {
// The file wasn't found, show the user a message
// Note use of "err" instead of "out", this is the error output
System.err.println("File not found: " + nonExistent);
// Here, we could recover by creating the file, for example
} finally {
if (scanner != null) {
scanner.close();
}
}
}
So there's a few things to note here:
We catch the FileNotFoundException and use a custom error message instead of printing the stack trace. Our error message is cleaner and more user-friendly than printing a stack trace. In GUI applications, the console may not even be visible to the user, so this may be code to show an error dialog to the user instead. Just because the file didn't exist doesn't mean we have to stop executing our code.
We declare throws IOException in the method signature instead of catching it alongside the FileNotFoundException. In this particular case, the IOException will be thrown here if we fail to read the file even though it exists. For this method, we're saying that handling errors we encounter while reading the file isn't our responsibility. This is an example of how you can declare an irrecoverable error (by irrecoverable, I mean irrecoverable here, it may be recoverable somewhere further up, such as in the method that called printFileToConsole).
I accidentally introduced the finally block here, so I'll explain what it does. It guarantees that if the Scanner was opened and an error occurs while we're reading the file, the Scanner will be closed. This is important for many reasons, most notably that if you don't close it, Java will still have the lock on the file, and so you can't open the file again without exiting the application.
There are two cases when you should throw an exception:
When you detect an error caused by incorrect use of your class (i.e. a programming error) throw an instance of unchecked exception, i.e. a subclass of RuntimeException
When you detect an error that is caused by something other than a programming error (invalid data, missing network connectivity, and so on) throw an instance of Exception that does not subclass RuntimeException
You should catch exceptions of the second kind, and not of the first kind. Moreover, you should catch exceptions if your program has a course of action to correct the exceptional situation; for example, if you detect a loss of connectivity, your program could offer the user to re-connect to the network and retry the operation. In situations when your code cannot adequately deal with the exception, let it propagate to a layer that could deal with it.
try/catch will prevent your application from crashing or to be precise- the execution will not stop if an unintentional condition is met. You can wrap your "risky" code in try block and in catch block you can handle that exception. By handling, it means that do something about that condition and move on with execution.
Without try/catch the execution stopped at the error-making-line and any code after that will not be executed.
In your case, you could have printed "This was not what I expected, whatever, lets move on!"
Let's say you are connected to database but while reading the records, it throws some exception. Now in this particular case, you can close the connection in Finally block. You just avoided memory leak here.
What I meant to say is , you can perform your task even if exception is thrown by catching and handling it.
In the example you've given, you're right, there is no benefit.
You should only catch an exception if either
You can do something about it (report, add information, fix the situation), or
You have to, because a checked exception forces you to
Usual "handling" of an exception is logging the situation to a log file of your choosing, adding any relevant context-sesitive information, and letting the flow go on. Adding contextual information benefits greatly in resolving the issue. So, in your example, you could have done
public static void main(String[] args) {
try {
System.out.println(str.length());
} catch(NullPointerException npe) {
System.err.println(
"Tried looking up str.length from internal str variable,"
+" but we got an exception with message: "
+ npe.getMessage());
npe.printStackTrace(System.err);
}
}
when looking a message like that, someone will know based on the message what went wrong and maybe even what might be done to fix it.
If you are using Exception, don't
catch(NullPointerException npe) {
npe.printStackTrace();
}
simply
catch(NullPointerException npe) {
//error handling code
}
You are menat to remove error printing. And anyways catch general exception not just specific ones.
If you look at the two exceptions, they are actually different. The first one is referring to line 22, while the second one is referring to line 9. It sounds like adding the try/catch caught the first exception, but another line of code also threw an exception.
Consequently, the exception is being thrown because you never created a new String, or set a value to the string, unless it was done in a part of the code that is not shown.
Adding a try/catch block can be very helpful with objects that you have little to no control over, so if these objects are other than expected (such as null), you can handle the issue properly.
A string is normally something that you would instantiate first, so you shouldn't normally have to worry about using a try/catch.
Hope this helps.
To answer your original question Che, "when to use an exception?"
In Java - I'm sure you've already found out... There are certain methods in Java that REQUIRE the try / catch. These methods "throw" exceptions, and are meant to. There is no way around it.
For example,
FileUtils.readFileToString(new File("myfile.txt"));
won't let you compile until you add the try/catch.
On the other hand, exceptions are very useful because of what you can get from them.
Take Java Reflection for example...
try { Class.forName("MyClass").getConstructor().newInstance(); }
catch ( ClassNotFoundException x ) { // oh it doesnt exist.. do something else with it.
So to answer your question fully -
Use Try/Catch sparingly, as it's typically "frowned on" to EXPECT errors in your application.. on the contrary, use them when your methods require them.