CODEWITHSUNDEEP
javaexceptiontry-catch
noMAD
noMAD

Reputation: 7844

How does throw new exception inside a catch work?

I have a snippet as follows:

try
{
   //blah!!
} 
catch(IOException e)
{
   throw new RuntimeException(e);
}

I do not understand how the above works? Will it catch an IOException and when it does that will it throw a RuntimeException? In that case the IOException will not have any meaning right? Some example would help.

Upvotes: 3

Views: 6152

Answers (6)

axcdnt
axcdnt

Reputation: 14634

You're right about some assumptions, but you should go a bit deeper. Inside your try block, suppose one of your methods of your class throws this kind of exception IOException. So catch will work as way for you to treat this exceptional case. It's basically this. If you just throw your exception away using a RuntimeException, but as you did, wrapping your IOException in RuntimeException you won't lose it at all.

A normal use is in a higher level treat your exception. Here's a good tutorial about Exception Handling, please check: Best Practices for Exception Handling

Upvotes: 2

Martijn Courteaux
Martijn Courteaux

Reputation: 68907

No. There is something special to RuntimeException. When you want to throw a RuntimeException, you don't need a throws RuntimeException in the method signature. This is called an unchecked exception. This code wraps the IOException into a RuntimeException and rethrows it to the caller, which is IMHO a bad approach, in most cases.

All subclasses of RuntimeException are unchecked, like IllegalArgumentException, NullPointerException, etc...

Upvotes: 1

Garrett Hall
Garrett Hall

Reputation: 30042

Checked Exceptions have to be caught or declared as thrown. RuntimeExceptions do not, so catching and rethrowing as a RuntimeException is way to avoid having to declare:

public void myMethod() throws IOException

When throw new RuntimeException(e); is called a new exception is created an thrown, but the original exception is wrapped inside it. So the stack trace will look like this:

Exception in thread "main" java.lang.RuntimeException: java.io.IOException: Some error.
    at com...main(SomeClass.java:36)
Caused by: java.io.IOException: Some error.

So the actual exception that goes up the call stack was a RuntimeException but the original IOException is kept in the message as the root cause.

Upvotes: 1

templatetypedef
templatetypedef

Reputation: 373462

Your understanding is correct. If an IOException is thrown, the catch handler will catch it, then immediately throw its own RuntimeException. This exception might be caught elsewhere in the program, in which case control will pick up at the handler, or it will be uncaught and will terminate the current thread.

One way to think about what's going on here is the following - an IOException is checked exception, which means that it must be caught. If it's not caught, then the program won't compile. The above code says that whenever it catches an IOException, it will throw a RuntimeException, which is an unchecked exception. This exception doesn't have to be caught if the programmer doesn't want to catch it. Notice that this RuntimeException is constructed with the caught IOException as a parameter. This means that if the RuntimeException is later caught, whoever catches it can notice that the underlying reason was an IOException and can handle it accordingly.

Hope this helps!

Upvotes: 1

kosa
kosa

Reputation: 66677

Yes your understanding is correct. This may be useful in cases where you want to display application specific exceptions instead of java provided exceptions (IOException may not be perfect case to understand this).

Upvotes: 1

Reimeus
Reimeus

Reputation: 159874

In this code the checked IOException becomes effectively unchecked. By passing the IOException into the RuntimeException you are chaining the 2 together.

Upvotes: 1

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