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JDK 1.7 onwards, throwing an exception object from catch block does not require a throws clause!!! Why is this so? [duplicate]

This question already has answers here:
Rethrowing an Exception: Why does the method compile without a throws clause?
(5 answers)
Closed 6 years ago.
I came across a weird scenario in java today while coding around. I have a try..catch block in my method which does not have any throws clause and I am able to throw the exception object caught in the catch block. It is an object of the Exception class, hence it is not an unchecked exception. Also, It is not printing the stacktrace if exception arises instead the exception is just getting swallowed.
Below is my code example,
public class ExceptionTest {
public void test() {
try
{
// Some code which may throw exception.
}
catch(Exception ex)
{
// Compiler should ask me to have a *throws Exception* in the signature, when I am throwing an exception object.
throw ex;
}
}
}
However, if I am throwing a new exception object instead of the caught exception object, compiler is asking me to have a throws clause in the method signature.
N.B: I am facing this situation when running in Java 7 or 8.
I am wondering, where is the thrown object going to? Anyone with any idea on this please...

You'll see this if the code in the try block can't throw any checked exception. At that point, the compiler knows that the only kind of exception caught by the catch block has to be an unchecked exception, and so it can therefore be rethrown. Note that if you assigned a different value to ex within the catch block, the compiler would no longer be able to have that assurance. At the moment, ex is effectively final.
If you try to call something that is declared to throw a checked exception within the try block, the code will fail to compile as expected.
For example:
public class ExceptionTest {
public void test() {
try {
foo();
} catch(Exception ex) {
throw ex;
}
}
public void foo() throws java.io.IOException {
}
}
Gives an error of:
ExceptionTest.java:12: error: unreported exception IOException; must be caught or declared to be thrown
throw ex;
^
As for where the exception "goes" - if the code in the try block throws an unchecked exception, it gets propagated as normal. Try this:
public class ExceptionTest {
public static void main(String[] args) {
test();
}
public static void test() {
try {
String x = null;
x.length();
} catch(Exception ex) {
throw ex;
}
}
}
Running that gives the following output, as expected:
Exception in thread "main" java.lang.NullPointerException
at ExceptionTest.test(ExceptionTest.java:10)
at ExceptionTest.main(ExceptionTest.java:4)
JLS 11.2.2 documents what exceptions a statement can throw - your code will only compile if there are no checked exceptions that can be thrown.

please check that you are really throwing an exception in the code. Otherwise the compiler will not care about catching the exception.
public class ExceptionTest {
public void test() {
try
{
throw new Exception("Error");
}
catch(Exception ex)
{
// My Compiler says that I don't catch the exception
throw ex;
}
}
}
Compiler: Error:(14, 13) java: unreported exception java.lang.Exception; must be caught or declared to be thrown

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.

Throw nested exception through java Throwable

I trying to throw inner exception in another exception through java Throwable but IDE told my that you must surround it with try/cath, What should I do to avoid from this problem?
try
{
//Some code
}
catch (IOException e)
{
Throwable cause = new Throwable();
cause.initCause(e);
throw cause.getCause();
}

Change your method signature to this:
public void someMethod() throws IOException
{
//some code
}
Have a look at this site for some useful information on checked exceptions and a little on the difference between checked and unchecked exceptions

Declare IOException as a checked exception in your function's signature.

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

at compile time, how JVM can find unnecessary catching of CloneNotSupportedException but not Exception?

public class MyClass {
public static void method() {
try {
// there is no compile time error for unnecessary catching 'Exception'
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
try {
// why compile time error for unnecessary catching 'MyException' or
// 'CloneNotSupportedException' etc..
// ultimately Exception, MyException & CloneNotSupportedException all
// are checked exception
} catch (MyException e) {
e.printStackTrace();
}
}
}
class MyException extends Exception {
}
second scenario
---------------
public class MyClass {
public static void method() throws Exception {
}
public static void main(String[] args) {
// if Exception itself is not checked ,
// why compile time error occured for calling method(); ??
method();
}
}

Because RuntimeExceptions are subclasses of Exception class. Therefore, compiler can't determine if some code throws any runtime exception, as they might be thrown by jvm.
On the other hand - checked exceptions must be declared that are thrown by some method, so the compiler knows which exceptions could be thrown and can determine unnecessary catch blocks.

Methods declare what exceptions they throw. If you're catching anything that is not a superclass of any known exception types then the catching isn't necessary.

We have checked exceptions and unchecked Exceptions. CloneNotSupportedException is a checked exception: if a methods throws it, the caller needs to catch this type of exception. And in those cases, the jvm can detect, if it is catched while no method inside the try block ever throws it.
Exception itself is not checked.