Develop Reference

How to handle an exception in Java thrown by a method into another method?

Let's suppose I have this class:
public class Obj1{
...
public void do_Something(int someParameter) throws SomeException {
if(...) throw new SomeException();
...
}
...
}
then, somewhere
public class Obj2{
...
public void do_SomeOtherThing(Obj1 obj1){
obj1.do_Something();
//apparently the only solution is try-catching it directly, even if I'm not in the main...
...
}
I've learned that exceptions should only be thrown by METHOD, and catched by MAIN, so, my question is: is try-catch the unique way to handle sub-method exceptions, or the most external method (do_SomeOtherThing) will throw it, so that I can try-catch it directly in main, deleting the try-catch in Object2 class?
Basically, can I do as follows?
public static void main(String[] args){
Object1 obj1 = new Object1();
Object2 obj2 = new Object2();
try{
obj2.do_SomeOtherThing(obj1);
}
catch(SomeException e){
...
}
}
or not?

A checked exception is part of the contract that a method has with its caller, and a thrown exception will always need to be handled one way or another.
The correct answer depends on the exact situation:
The caller can handle the exception:
String getStringFromRemoteServer() throws IOException { ... }
String getConfigString() {
try {
return getStringFromRemoteServer();
} catch (IOException e) {
LOG.warn("Failed to contact server, using local version.", e);
return getLocalString();
}
}
In this case we have an alternative source of the data we need, so if the preferred method fails we catch the exception, log it (so that we know a problem exists with our network) and call the alternative.
The exception is fatal, and we don't want any function higher in the call tree to try to handle it.
Configuration parseConfiguration(String configString) throws ParseException { ... }
void loadConfiguration() {
try {
this.globalConfig = parseConfiguration(getConfigString());
} catch (ParseException e) {
throw new RuntimeException("Corrupted config", e);
}
}
In this case an exception means that the configuration of our application is fatally broken. There is no point in trying to handle this error, and no point in any of our callers trying to handle it, so declaring throws on loadConfiguration() would just be confusing clutter. We wrap the exception in a RuntimeException and rethrow it. Note that we don't log it -- there will be some top level reporting of uncaught exceptions, so logging it here would be repetition.
It is still valuable to have parseConfiguration() throw a checked exception, because when we are calling it from the interactive configuration editor we catch the exception and display an error message to the user.
Maybe our caller can handle the exception.
int stringToInteger(String s) throws BadNumberException { ... }
String decimalStringToHexString(String s) throws BadNumberException {
return intToHex(stringToInteger(s));
}
In this case we are not changing the meaning of the exception -- decimalStringToHexString is converting a number from a string, and one possible outcome is that the string is illegal. Our caller needs to be aware of that as a possible outcome, just as callers of stringToInteger() are, so we simply declare the exception and let our caller handle it. Our caller knows the context they are using the number in, so they can decide how to handle the exception.
A couple of rules:
Never completely ignore an exception (OK, maybe InterruptedException). If you write try { ... } catch (Exception e) {} the empty catch clause will make it hard to spot why your code doesn't work.
When you wrap an exception, always include the original exception as the cause.

Sonarqube - avoiding catch generic Exception

Sonar complains when catching the generic type Exception, but sometimes we need to do some general exception handling for ALL (even not yet recognized) exception types. What is the solution to pass this sonar check?

Unless you are invoking a method which throws Exception, there is no need to catch Exception: catch the exceptions you know about, and the compiler will tell you when you have to start handling another one.
The problem with catching "not yet recognized" Exceptions is that you lose the signal that you have to handle a new exception in a special way.
For example:
void someMethod() {
// Do stuff.
}
void callIt() {
try {
someMethod();
} catch (Exception e) {
// ...
}
}
If someMethod is now changed so that it throws, say, an InterruptedException:
void someMethod() throws InterruptedException {
Thread.sleep(1000);
// Do stuff.
}
you aren't told by the compiler that you need to add handling for the InterruptedException in callIt(), so you will silently swallow interruptions, which may be a source of problems.
If, instead, you had caught RuntimeException, or RuntimeException | IOException | OtherExceptionYouAlreadyKnowAbout, the compiler would flag that you had to change your code to handle that InterruptedException as well; or, that you can't change the signature of someMethod(), and the checked exception has to be handled there.

Intercept and rethrow a Java unhandled exception

I've coded a method with a catch-all handler, but I need to rethrow the exception as if it were unhandled, so that a caller (much) further up the call stack can handle it. The trivial way to do this is simply:
try {
...
} catch (Exception ex) {
// do something here...
// and rethrow
throw ex;
}
But the problem is that, because of the throw statement, Java requires this method to declare itself as throws Exception, which in turn, requires all the callers to handle the exception or declare themselves as throws Exception. And so on up the call chain...
Is there any simple way to rethrow the exception as if the current method did not handle it?

You have exactly two options with (checked) exceptions:
Handle them in the method via a try/catch (which may include rethrowing as a different exception type)
Declare that the method throws the exception.
If you want to rethrow the exception as if this method did not catch it, your only option is 2.
Note: you only want to catch (Exception e) if a method in the try block actually throws Exception. Otherwise, catch the specific exception types.

You could do what #radoh has said and just wrap into a RuntimeException, but one downside of this is your stacktrace is now polluted and will show the offending line to be where you declare throw new RuntimeException(ex).
An alternative is to use Lomboks SneakyThrows mechanism, like this:
public static void main(String[] args) {
methodWithException();
}
private static void methodWithException() {
try {
throw new Exception("Hello");
} catch (Exception e) {
Lombok.sneakyThrow(e);
}
}
Your stacktrace will remain intact, but you no longer need to declare throws Exception.
It's worth reading the documentation on why you should/shouldn't do this

Exception is never thrown in body of corresponding try statement

I have a problem with exception handling in Java, here's my code. I got compiler error when I try to run this line: throw new MojException("Bledne dane");. The error is:
exception MojException is never thrown in body of corresponding try statement
Here is the code:
public class Test {
public static void main(String[] args) throws MojException {
// TODO Auto-generated method stub
for(int i=1;i<args.length;i++){
try{
Integer.parseInt(args[i-1]);
}
catch(MojException e){
throw new MojException("Bledne dane");
}
try{
WierszTrojkataPascala a = new WierszTrojkataPascala(Integer.parseInt(args[0]));
System.out.println(args[i]+" : "+a.wspolczynnik(Integer.parseInt(args[i])));
}
catch(MojException e){
throw new MojException(args[i]+" "+e.getMessage());
}
}
}
}
And here is a code of MojException:
public class MojException extends Exception{
MojException(String s){
super(s);
}
}
Can anyone help me with this?

A catch-block in a try statement needs to catch exactly the exception that the code inside the try {}-block can throw (or a super class of that).
try {
//do something that throws ExceptionA, e.g.
throw new ExceptionA("I am Exception Alpha!");
}
catch(ExceptionA e) {
//do something to handle the exception, e.g.
System.out.println("Message: " + e.getMessage());
}
What you are trying to do is this:
try {
throw new ExceptionB("I am Exception Bravo!");
}
catch(ExceptionA e) {
System.out.println("Message: " + e.getMessage());
}
This will lead to an compiler error, because your java knows that you are trying to catch an exception that will NEVER EVER EVER occur. Thus you would get: exception ExceptionA is never thrown in body of corresponding try statement.

As pointed out in the comments, you cannot catch an exception that's not thrown by the code within your try block. Try changing your code to:
try{
Integer.parseInt(args[i-1]); // this only throws a NumberFormatException
}
catch(NumberFormatException e){
throw new MojException("Bledne dane");
}
Always check the documentation to see what exceptions are thrown by each method. You may also wish to read up on the subject of checked vs unchecked exceptions before that causes you any confusion in the future.

Any class which extends Exception class will be a user defined Checked exception class where as any class which extends RuntimeException will be Unchecked exception class.
as mentioned in User defined exception are checked or unchecked exceptions
So, not throwing the checked exception(be it user-defined or built-in exception) gives compile time error.
Checked exception are the exceptions that are checked at compile time.
Unchecked exception are the exceptions that are not checked at compiled time

Always remember that in case of checked exception you can catch only after throwing the exception(either you throw or any inbuilt method used in your code can throw) ,but in case of unchecked exception You an catch even when you have not thrown that exception.

Why try/catch around throwable?

In trying to refactor some I code I attempted to throw the exception in the catch clause like so -
try {
....
}
catch(Exception exception){
.....
throw exception
}
However when I attempted to throw the exception on line "throw exception" the compiler complained with a message that I needed to surround my throw clause in a new try/catch like so -
try
{
....
}
catch (Exception exception)
{
.....
try
{
throw exception
}
catch (Exception e2)
{
...
}
}
Why does the compiler require this and what use does it provide ?
Thanks

The exception java.lang.Exception is a checked exception. This means that it must either be declared in the throws clause of the enclosing method or caught and handled withing the method body.
However, what you are doing in your "fixed" version is to catch the exception, rethrow it and then immediately catch it again. That doesn't make much sense.
Without seeing the real code, it is not clear what the real solution should be, but I expect that the problem is in the original try { ... } catch handler:
If possible, you should catch a more specific exception at that point, so that when you rethrow it, it is covered by the method's existing throws list.
Alternatively, you could wrap the exception in an unchecked exception and throw that instead.
As a last resort, you could change the signature of the method to include Exception in the throws list. But that's a really bad idea, because it just pushes the problem off to the caller ... and leaves the developer / reader in the position of not knowing what exceptions to expect.

In Java, there is a distinction between checked and unchecked exceptions. An unchecked exception can essentially be thrown at any place in code and, if it's not caught somewhere, it will propagate up to the entry point of your application and then stop the process (usually with an error message and stack trace). A checked exception is different: The compiler won't let you just let it propagate, you need to either surround any code which might throw a checked exception with try-catch blocks (and "throw exception" is the simplest case if exception is an instance of a checked exception class) or you must mark the method which contains the call to code that might throw a checked exception with a "throws" declaration. If the desired behaviour is to throw an unchecked exception, then you'll need to wrap the exception in a RuntimeException. If the desired behaviour is to keep the exception checked, then you'll need to add a throws declaration to your current method.

In your original code, nothing catches the thrown exception. I would imagine you either have to specify that your function throws an exception or have another try/catch block as the compiler suggests to catch it.
Instead of
public void yourFunction(){
try {
....
}
catch(Exception exception){
.....
throw exception
}
}
try
public void yourFunction() throws Exception{
try {
....
}
catch(Exception exception){
.....
throw exception
}
}

My guess is that your trying to throw an exception sub class that isn't declared by the method as an exception type it can throw.
The following example works
package test.example;
public class ExceptionTest {
public static void main(String[] args) throws Exception{
try {
int value = 1/0;
} catch (Exception e) {
System.out.println("woops the world is going to end");
throw e;
}
}
}
However this example will give an error.
package test.example;
public class ExceptionTest {
public static void main(String[] args) throws RuntimeException{
try {
int value = 1/0;
} catch (Exception e) {
System.out.println("woops the world is going to end");
throw e;
}
}
}
Note in the second example I'm simply catching Exception not RuntimeException, it won't compile as I throw Exception which is an undeclared throws, even though I do declare RuntimeException.
Yes the exception is a RuntimeException but the compiler doesn't know that.
Just thought of a third working example to show you. This one also works because your throwing the same type as you declare. (note the only change is the catch block)
package test.example;
public class ExceptionTest {
public static void main(String[] args) throws RuntimeException{
try {
int value = 1/0;
} catch (RuntimeException e) {
System.out.println("woops the world is going to end");
throw e;
}
}
}
You need to understand the differences between all three of these answers