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Catching Exceptions - why is it necessary?

Suppose I have this trivial piece of code, and at runtime list size is 4.
try{
for(int i = 0; i < list.size(); i++){
list.get(10);
}
}catch(NullPointerException e){
System.out.println(" exception to string :" + e.toString());
System.out.println(" exception get class" + e.getClass());
e.printStackTrace();
}
So the JVM will throw an index out of bounds exception, without having to implement this in the catch block.
Similarly, if the list is null, the JVM would throw a null pointer exception.
So with that said, why bother to declare an IndexOutOfBoundsException or NullPointerException at all? Why not just declare a generic Exception at the catch block ? What is the advantage and disadvantage to this approach?

Usually, you shouldn't catch a generic exception somewhere down in the code because it often indicates a bug in your code which you have to fix.
Some checked exceptions (like IOException) which are related to external resources are not within your control, so you should catch them and react to them, but NullPointerExceptions or IndexOutOfBoundException should be avoided by checking the object beforehand.

As you surely know there are basically two types of exceptions: checked and unchecked. You need to declare or catch checked exceptions but don't have to do that for unchecked ones.
So what's the difference? It often depends on who uses them but generally it's like this:
Checked exceptions are thrown for things that are expected to go wrong eventually, e.g. IOException etc. - a file not being available (not existing, locked etc.) or writable isn't something that never happens and that the developer can try to prevent. Since you have to expect those things to go wrong you'll use a checked exception to either force the method signature to declare those things can go wrong or to catch the exception and handle the case.
Unchecked exceptions are mostly used for things that can but should not go wrong. In many cases those are due to programming errors (such as NullPointerExceptions, IndexOutOfBoundExceptions etc.) or unexpected system failures (database not available etc.). You normally don't catch them explicitly since they're not expected anyways.
That being said there are situations where checked or unchecked exceptions are used for something else (e.g. in libraries that need to rely on being able to bubble up a multitude of exceptions and thus wrap checked exceptions un unchecked ones).
As for the catching/declaring in the signature: you'll normally want to be as explicit as possible with checked exceptions, i.e. you might want to handle an IOException differently than let's say an SQLException. Unchecked exceptions might be just caught and reported (e.g. by logging or rethrowing them) but even in those cases you might want to catch some explicitly, e.g. EJBExceptions which are thrown by EJBs and which might just be a wrapper for an actual checked exception and thus you might want to unwrap those.
A final word on NullPointerException: you'll probably never want to catch those explicitly since they shouldn't occur in the first place (either make sure nothing is null or check before accessing those things) and often there are so many things that could be null in a try-block (directly and in the methods that are called) that you'd have a hard time to implement a reasonable reaction. In those cases it's best to just catch any unhandled exception (as is done for threads anyways) and report them to the developer so they can have a look and fix the problem.

In order to understand why you may want to use more specific exceptions in your catch block instead of just the generic Throwable (which means EVERYTHING that can possibly go wrong), you may want to consider what it is that is going on here.
When creating software that is to be run in production for years, you need it to be very, very robust. This mean that you want your code to be able to handle any situation gracefully, and keep running even though something went wrong.
In the particular code you are showing it might very well be that it is perfectly acceptable that the list is too short (a library that occasionally misbehaves) but it is a fatal and unforeseen error if list is null.
The simplest way to differentiate is to use the way that the exception hierarchy is designed, and have different catch-blocks.
Also, if you use Java 7 or later, you can create a combined catch block using a pipe sign which allows you to retain the original exception even if the handling code is the same.

Any Work around for not using a broader Exception, the "System.Exception"?

I am a fresher, and after successfully completing my project I scanned it through fortify. it gave me a list of issues, out of which just one categories remain. (Fortify a code quality scanning tool)
it says, "Not to use Broader Exception i.e. System.Exception" to catch exceptions unless in some conditions.
But I have a few methods that have 25-30 lines of code with different types of operations, in such case how to figure out which all specific exceptions to catch.
Should we also throw all these exceptions to be caught at a higher level catch, as i read "throw first catch late".
Please suggest me a clean way to do this.
e.g.
public SomeMethod(arg) throws IOException,SQLException,beanNotFoundException {
try {
someCode
} catch (IOException|SQLException|beanNotFoundException ex) {
logger.log(ex);
throw ex;
}
}
But also if i don't use Exception Class altogether till the end, i also have to make sure that I am not missing any exception to handle.
Is there a better approach.

Static Analysis
First and foremost, let me start with a little fallacy that most people fall subject to (and I see a lot in the corporate world): Static analysis tools are not infallible. They make mistakes. There are some warning classes that, with all the computing power known to man and with the remaining lifespan of the universe, the tool may not be able to exhaustively analyze a piece of code related to a particular warning class. Moreover, there are some warning classes that can complete before the end of time, but would be unreasonable to expect you to wait 7 days for an answer for one warning on one section of code for one execution path. That is, static analysis tools have to make guesses sometimes. The better tools are better at making guesses (you pay for what you get), but in the end, they are all capable of guessing right (true positive/negative) or wrong (false positive/negative).
Additionally, some warning classes may be applicable and some may not be. Some other warning classes may be important to you and some may not be. For example, PMD has a warning class to the effect of "Unused Import". An unused import has no effect on runtime, and while it may not be ideal, you can leave it in your code and it will not affect you unless your codebase is large and requires time to compile, and you have lots of these (unused imports make it longer to build your project). I particularly mention static analysis, because it sounds like you ran Fortify on your codebase and fixed anything and everything without questioning the validity. Most of the time it will probably be right, but just don't take it as fact because it told you so. You need to question whether these tools are always right.
Your example
So your example is not a false positive. If you do
throw new Exception("");
that's generally not what you want to do. It's fine for debugging and just quickly throwing code together. But, as your codebase gets larger, handling Exceptions will get more difficult.
This leads me right into my next point. You make the statement
But I have a few methods that have 25-30 lines of code with different types of operations, in such case how to figure out which all specific exceptions to catch.
...
But also if i don't use Exception Class altogether till the end, i also have to make sure that I am not missing any exception to handle.
Which seems to indicate to me that you either have something to the effect of
try{
//25-30 lines of code
} catch (Exception ex) { /*ex.printStackTrace(); Logger.getLogger(...).log(...); etc etc...whatever it is you do here, or maybe nothing at all*/
This is pretty bad - in most cases.
To get back to answering your question before I explain why, yes, you should absolutely catch each individual exception. The reason for this, and why your catch all is bad, is because if there is something specific that goes wrong, you can't have a specific type of error handling.
For example, take a function Foo#bar(java.lang.String), which throws an IOException when a disk access fails because you tried to write to a bad file, a BooException if you pass in a String without a special character in it, or a BazException for some other arbitrary reason. Now let's go back to your example from above: what if I wanted to realize that the file I was writing to was bad, and maybe prompt the user for some clarification before I moved on? What if I knew that if a BooException was thrown, that the user should have never been here in the first place, and I need to redirect them to some location? What if I knew that when a BazException that the system was out of sync with some other system and that this is a fatal problem, and now I need to do resource cleanup before I forcefully crash the JVM?
The above reasons are why you should do individual try/catch blocks for each statement and each exception.
That being said, there are reasons to not do this. Imagine, with the above example, that all you want to do for a IOException, BooException and BazException (and also any runtime exceptions, i.e. NullPointerException) that I just want to log the exception and move on. In this case, I would say it's OK to do a try/catch around a block of code - so long as it's just around the code that this applies to.
EDIT: You made a point about missing an exception, but putting a response to that in the comments seemed unruly to look at. In any case, let me start off by saying if it is not a runtime exception, then you will not even be able to compile without handling it. Catching Exception catches everything, runtime or not, which is how you compile now. From my example above, if there are runtime exceptions you are worried about missing, instead of just starting off with catching Exception:
Foo myFooInstance = new Foo();
String someValue = "value";
try {
myFooInstance.bar(someValue);
} catch (IOException ioe) {
/*handle file access problem*/
} catch (BooException boe) {
/*handle user in wrong spot*/
} catch (BazException bze) {
/*handle out-of-sync fatal error*/
} catch (Exception ex) {
LogRecord lr = new LogRecord(Level.SEVERE, "Unhandled exception!! returning immediately!!");
lr.setThrown(ex);
lr.setParameters(new Object[]{someValue});
Logger.getLogger(MyClass.class.getName()).log(lr);
return;
}
Here you end with your catch-all rather than starting with it. Think of it as your last-ditch effort to try to handle everything. It may not mean the exception is fatal to your program, but you probably shouldn't continue either (hence why my example uses return;)
Another small thing to consider, is that it becomes exponentially more difficult for the JVM to catch the exception properly the larger the try block gets (if the exception is never thrown, there is no overhead). This is more trivial to powerful machines, but something to keep in mind. With that in mind, I also don't have any sources for performance about exceptions being thrown in large try blocks, so take that with a grain of salt unless somebody finds something and mentions it.

In general, you only need to worry about handling/throwing checked exceptions. A clean way to write the method depends on the code base and structure of your application. Here are some alternatives:
If you want the client code (code calling someMethod) to handle the exception, you can do this:
public void someMethod(String arg) throws IOException, SQLException,
BeanNotFoundException {
doSomething(arg);
}
If you can handle the exception locally in someMethod, do so, but do not rethrow the same exception up in the call stack:
public void someMethod(String arg) {
try {
doSomething(arg);
} catch (IOException | SQLException | BeanNotFoundException ex) {
logger.log(ex);
handleException(ex);
}
}
If the exception is something you cannot recover from, and you don't want to enforce client code to handle the exception, you can wrap it as a RuntimeException. This will potentially terminate the program (which is usually what you want when a fatal error occurs):
public void someMethod(String arg) {
try {
doSomething(arg);
} catch (IOException | SQLException | BeanNotFoundException ex) {
logger.log(ex);
throw new RuntimeException(ex);
}
}
For a good explanation about checked vs. unchecked exceptions, you can refer to Java: checked vs unchecked exception explanation.

My advice would be to ignore what Fortify is saying. Or better still, figure out how to selectively suppress these warnings.
The alternative to your current version that Fortify is suggesting is objectively bad in most contexts. Your current version is not bad, only (a bit) verbose.
Another alternative would be to wrap the exceptions in a custom exception or exceptions that have a common custom checked exception as the base-class (NOT Exception or Throwable or RuntimeException). That could make code like you example neater, but it means that you need to add code (somewhere) to do the wrapping and (maybe) unwrapping. In short, that's no idea too.
OK so what is the problem with catching Exception? Well basically, it catches everything ... including any unexpected uncheck exceptions that are evidence of bugs. If you then declare the method as throws Exception it only gets worse.
I have seen an example (no names) where someone did this in a large Java product. The result is that the product's Java APIs are horrible to program against. Every non-trivial API method is declared as throws Exception, so someone programming against the APIs has no idea what to expect. And the code-base is proprietary and obfuscated. (And the worst thing is that the lead developer still thinks it was a good idea!)
What about wrapping the exceptions in RuntimeException? That's not quite as bad1 as Exception but the problem is that the programmer still can't tell what exceptions to expect.
1 - It is not quite as bad in the sense that throws Exception is effectively "noise" once you have gone done the path that anything can throw anything.

The general rule of thumb is that checked exceptions are thrown for errors that are external or unpredictable, and which the caller can reasonably be expected to do something about, even if that only means displaying a message to the user. Whether you catch them or pass them up the call stack depends on where it makes the most sense to handle them.
Unchecked exceptions are thrown for unsatisfied preconditions or invalid arguments. In other words, the programmer screwed up. That's why you're not forced to catch them; they're typically things that "shouldn't happen." You generally don't need to worry about them if you're using the API correctly. The only time you should catch unchecked exceptions is when an API doesn't follow these conventions and throws unchecked exceptions for external or unpredictable reasons.

Is it okay to handle Runtime exceptions?

It's generally a good practice to not handle runtime exceptions.
I have this scenario :
/**boolean returns false if the method execution fails,
so that an error can be shown to user*/
boolean saveData()
{
try
{
//Some database calls that throw only Runtime exceptions
}
catch(Exception e)
{
//Log the exception and return false
return false;
}
return true;
}
So from the Servlet calling it will get a boolean value. If it's false, we show a message 'Save Not sucessful'. Is this okay or is there a better way of doing it ?

Is it okay to handle Runtime exceptions?
As long as you can do something intelligent at that point, yes. This 'never catch run-time exceptions' is pure nonsense. You'll probably want to log them and do other things, as well.

It's generally a good practice to not handle runtime exceptions.
This sentence implies a false premise: the question is not whether, but when to catch runtime exceptions.
At some point in your code you should definitely have a "catch-all" exception handler. This point is called the exception barrier, and it is high enough in the call stack to encompass a complete unit of work (basically, the scope of one database transaction).
This should be your governing principle:
if an exception must cause the current unit of work to be aborted (rolled back), then do not catch that exception until the exception barrier;
if an exception is of local character only, and its appearance only means taking a different route to the handling of the request, then do catch and handle it early.
Note that the above doesn't use the fruitless checked/unchecked distinction. Most modern frameworks completely avoid checked exceptions and go out of their way to shield the programmer from such exceptions being thrown from lower-level code.
In practice, more than 90% of all exceptions are of the first, aborting kind. This is what is meant by the term "unrecoverable"—it means you can't continue what you are doing right now, but the server can stay up and process all other requests. I see many people confusing this with "the JVM is about to die", but exceptions which have that meaning are exceedingly rare and usually there is no special handling code for them.
Lastly, note that the "catch-all" handler should catch not only RuntimeExceptions but also Errors because you want every exception logged the same way. If you let an exception escape your application code, there is no knowing what behavior it will cause. Note that there are some common errors, like StackOverflow and OutOfMemory, which can in principle cause unrecoverable damage and therefore rightfully belong to the Error hierarchy, yet in most real-life cases do no damage beyond aborting the current request. You definitely do not want to shut down your service at the first sight of such an error.

No, catching a general Exception will mask any other problems you don't want to catch. Better catching specific exceptions making it harder for unexpected ones to go missing.

I guess your question has two aspects: 1) Handling RuntimeExceptionas the heading suggested and 2) handling generic Exception these are two different things.
The Javadoc of RuntimeExcepton list a bunch of its subclasses (NullPointerException..etc). If you look carefully, these are nothing but the programming errors and instead of handling them in catch block, should be corrected the the code itself so that they do not occur.
Handling generic Exception is generally referred as a poor practice and rightly so because it hides the specific exceptions which might need different handling.
But in case similar to yours, if there is a Servlet giving a call to method there has to be a graceful way to tell the user that there was some problem with the application while processing the request instead if showing the stacktrace.
So what you are doing could be one way of handling and as a good practice keep track of errors/exceptions in the log and correct any RuntimeExceptions

boolean returns false if the method execution fails,
so that an error can be shown to user
This is a clear reason for the method to throw an exception. It is why the exception mechanism exists in the first place. Best practice is to use exceptions for these types of circumstances, not boolean signals.
Anytime you follow this kind of pattern, you suffer in several ways
The caller cannot be sure what really happened. Was there a communication failure? Was the data corrupted? There is no way to tell when you return a simple "FAILED" signal.
Because the caller cannot know what happened, you take away options for the caller. Maybe the caller could do something in certain error cases.
You increase the potential for data errors. Again, since the caller cannot know why it failed, the caller may make assumptions about why it failed and continue in cases when the entire program should halt. If the program continues, it may decide to process invalid data when it really should stop.
I highly recommend reading the sections on exception handling in Effective Java. Basically, if you can't do anything meaningful with an Exception in your method, you should allow it to pass up the stack. In the worst case scenario, the API you reference throws a generic checked exception, giving you one of three choices (EDIT: Changed to void return type because the boolean return type no longer has any meaning -- you either succeed or throw an exception)
-Create your own.
// "MyFatalException" would be a checked Exception
public void saveData() throws MyFatalException{
try{
// ... do stuff that throws Exception
}catch(Exception e){
throw new MyFatalException("Unable to save Data due to a general exception"+
" thrown by foo.bar()", e);
}
}
-Throw an existing type (my preference is for subclasses of RuntimeException)
public void saveData() throws IllegalStateException{
try{
// ... do stuff that throws Exception
}catch(Exception e){
throw new IllegalStateException("Unable to save Data due to a general exception"+
" thrown by foo.bar()", e);
}
-Declare to throw a generic Exception and let it pass up (I do NOT recommend this, but you'll see many developers who do this)
public void saveData() throws Exception{
// no try/catch needed. Just make the call
}

The general concept is that frameworks or APIs would throw a RuntimeException only in situations when the error is not recoverable in code. e.g: your Db is down, or your server socket is down and things like that.
Though its not bad to catch such exceptions but whats actually thoughtful is what do you intend to do after catching them. e.g: people prefer to catch DB level connection problems and errors which are runtime exception subclasses and show error pages on their websites. Checked exceptions are usually for code level situational cases and catching them and handling is what code is expected to do.

In case you try something like this, since you don't want your users to error page you can do this, provided you caught all other exceptions before this.
boolean saveData()
{
try
{
//Some database calls that throw only Runtime exceptions
}catch(db_Connectivity_Exception e){
//exception like credentials invalid
}catch(db_table_Exception e){
//table desc. changed
}
catch(Exception e)
{
//Log the exception and return false
//Since you are going to log your error I hope is not harmfu
return false;
}
return true;
}
Still this is not best practice

Re-throwing RuntimeExceptions after catching Checked Exceptions [duplicate]

This question already has answers here:
What issues may ensue by throwing a checked exception as a RuntimeException?
(6 answers)
Closed 9 years ago.
In Java it is observed that there is a convention of re-throwing a RuntimeException just after handling a Checked Exception.
This way has both good and bad consequences. When the compiler forces something to be handled via a Checked Exception, the developer can just get rid of it by catching it and re-throwing it as a RuntimeException.
Can someone explain if this scenario can be considered as a good practice? If so, would this approach be less error prone or would it make the code base unstable?

Actually it is the incompetent attempts at handling checked exceptions which result in an unstable code base. Typically, you'll have this:
try {
//stuff
} catch (IOException e) {
log.error("Failed to do stuff", e);
throw e;
}
and then next level up you'll have to deal with it again, typically logging it all over and making a mess of the log files. It will be even worse if you don't rethrow:
try {
// do stuff
} catch (IOException e) {
return null;
}
Now the calling code has no idea something went wrong, let alone what. Compared to those attempts, this actually accomplishes exactly what the application logic needs:
try {
// do stuff
} catch (IOException e) {
throw new RuntimeException(e);
}
Now the exception can freely propagate up the call stack until it reaches the well-defined exception barrier, where it:
aborts the current unit of work;
gets logged at a single, unified spot.
In a nutshell, to decide whether to catch-and-handle or catch-and-rethrow, just ask yourself this question:
Must the occurrence of this exception abort the current unit of work?
if yes: rethrow an unchecked exception;
if no: provide meaningful recovery code in the catch-block. (No, logging is not recovery).
From many years of real-life experience I can tell you that more than 90% of all possible checked exceptions are of the "aborting" type and need no handling at the place of occurrence.
Argument against the language feature of checked exceptions
Today, checked exceptions are widely recognized as a failed experiment in language design, and here's the key argument in a nutshell:
It is not up to the API creator to decide on the semantics of its exceptions in the client code.
Java's reasoning is that exceptions can be divided into
exceptions resulting from programming errors (unchecked);
exceptions due to circumstances outside of programmer's control (checked).
While this division may be real to some extent, it can be defined only from the perspective of client code. More to the point, it is not a very relevant division in practice: what truly matters is at what point the exception must be handled. If it is to be handled late, at the exception barrier, nothing is gained by the exception being checked. If handled early, then only sometimes there is a mild gain from checked exceptions.
Practice has confirmed that any gains afforded by checked exceptions are dwarfed by real-life damage done to real-life projects, as witnessed by every Java professional. Eclipse and other IDEs are to blame as well, suggesting inexperienced developers to wrap code in try-catch and then wonder what to write in the catch-block.
Whenever you encounter a method which throws Exception, you have found yet another living proof of the deficiency of checked exceptions.

The idea of checked exceptions is "Java only" - as far as I know, no language after Java adopted this idea.
There are too many checked exceptions which are caught ... and silently ignored.
If you look at Scala, they dropped it as well - it's only there for Java compatibility.
In this tutorial on Oracle's web site, you will find this definition:
If a client can reasonably be expected to recover from an exception, make it a checked exception.
If a client cannot do anything to recover from the exception, make it an unchecked exception.
This notion has been adopted in Scala as well, and it works fine.
Technically speaking your proposal works. Discipline and code reviews are required in either way.

The term "can just get rid of it" is not totally correct in this case. This is getting rid of exceptions:
try {
} catch (Exception e){
e.printStacktrace();
}
This is the most common bad practice among the try-catch use. You are catching the exception and then, just printing it. In this case, the catch block catches the exception and just prints it, while the program continues after the catch block, as if nothing had happened.
When you decide to catch a block instead of throwing an exception, you must be able to manage the exception. Sometimes exceptions are not manageable and they must be thrown.
This is something you should remember:
If the client can take some alternate action to recover from the
exception, make it a checked exception. If the client cannot do
anything useful, then make the exception unchecked. By useful, I mean
taking steps to recover from the exception and not just logging the
exception.
If you are not going to do something useful, then don't catch the exception. Re-throwing it as a RuntimeException has a reason: as stated before, the program just cannot continue as nothing happened. This way, a good practice would be:
try {
} catch (Exception e){
//try to do something useful
throw new RuntimeException(e);
}
This means: you just caught an Exception (like an SQLException) from which you can't recover without stopping and resetting the thread. You catch it, you try to make something in between (like resetting something, closing open sockets, etc...) and then you throw a RuntimeException().
The RuntimeException will suspend the whole thread, avoiding the program continue as if nothing have happened. Furthermore, you were able to manage the other exception without just printing it.

It may or may not be okay, depending on the context, but it probably is not.
As a rule of thumb RuntimeExceptions should only be used to indicate programming errors (examples include IllegalArgumentException and IllegalStateException). They don't have to be checked exceptions because you generally assume your program is correct until proven otherwise and you cannot handle these exceptions in a meaningful manner (you have to release an updated version of the program).
Another valid use of runtime exceptions is when you use a framework that will catch and handle the exception for you. In such a scenario it would only be burdensome to having to declare the exception in every method when you are not going to handle it anyway.
So generally speaking I would say re-throwing a checked exception as a runtime exception is very bad practice unless you have a framework that will handle it properly.

The general rule is: you throw a checked exception when the caller might be able to do some kind of recovery when informed about it. Otherwise, throw an unchecked exception.
This rule applies when an exception is first thrown.
But this also applies when you catch an exception and are wondering whether to throw a checked or unchecked exception. So there is no convention to throw a RunTimeException after catching a checked one. It is decided in a case-by-case basis.
One small tip: if you are going to just re-throw an checked exception after catching one and do nothing else, most of the time it is alright to just not catch the exception and add it to the exceptions thrown by the method.

Why catch Exceptions in Java, when you can catch Throwables?

We recently had a problem with a Java server application where the application was throwing Errors which were not caught because Error is a separate subclass of Throwable and we were only catching Exceptions.
We solved the immediate problem by catching Throwables rather than Exceptions, but this got me thinking as to why you would ever want to catch Exceptions, rather than Throwables, because you would then miss the Errors.
So, why would you want to catch Exceptions, when you can catch Throwables?

From the Java API documentation:
The class Exception and its subclasses are a form of Throwable that indicates conditions that a reasonable application might want to catch.
An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch.
Errors usually are low-level (eg., raised by the virtual machine) and should not be caught by the application since reasonable continuation might not be possible.

It all depends a bit on what you're going to do with an Error once you've caught it. In general, catching Errors probably shouldn't be seen as part of your "normal" exception flow. If you do catch one, you shouldn't be thinking about "carrying on as though nothing has happened", because the JVM (and various libraries) will use Errors as a way of signalling that "something really serious has happened and we need to shut down as soon as possible". In general, it's best to listen to them when they're telling you the end is nigh.
Another issue is that the recoverability or not from an Error may depend on the particular virtual machine, which is something you may or not have control over.
That said, there are a few corner cases where it is safe and/or desirable to catch Errors, or at least certain subclasses:
There are cases where you really do want to stop the normal course of flow: e.g. if you're in a Servlet, you might not want the Servlet runner's default exception handler to announce to the world that you've had an OutOfMemoryError, whether or not you can recover from it.
Occasionally, an Error will be thrown in cases where the JVM can cleanly recover from the cause of the error. For example, if an OutOfMemoryError occurs while attempting to allocate an array, in Hotspot at least, it seems you can safely recover from this. (There are of course other cases where an OutOfMemoryError could be thrown where it isn't safe to try and plough on.)
So the bottom line is: if you do catch Throwable/Error rather than Exception, it should be a well-defined case where you know you're "doing something special".
Edit: Possibly this is obvious, but I forgot to say that in practice, the JVM might not actually invoke your catch clause on an Error. I've definitely seen Hotspot glibly gloss over attempts to catch certain OutOfMemoryErrors and NoClassDefFoundError.

A lot of the other answers are looking at things too narrowly.
As they say, if you are writing application code, you should not catch Throwable. You can't do anything about it, so allowing the surrounding system (JVM or framework) to handle these issues is best.
However, if you are writing "system code", like a framework or other low-level code then you may very well want to catch Throwable. The reason is to attempt to report the exception, perhaps in a log file. In some cases your logging will fail, but in most cases it will succeed and you will have the information you need to resolve the issue. Once you have made your logging attempt you should then either rethrow, kill the current thread, or exit the entire JVM.

Usually Errors are problems you cannot possibly recover from, like OutOfMemoryError. There's nothing to do by catching them, so you should usually let them escape, and bring down the virtual machine.

I'll go a slightly different route from others.
There are many cases where you would want to catch Throwable (mainly to log/report that something evil happened).
However, you need to be careful and rethrow anything that you cannot deal with.
This is especially true of ThreadDeath.
If you ever catch Throwable, be sure to do the following:
try {
...
} catch (SomeExceptionYouCanDoSomethingWith e) {
// handle it
} catch (ThreadDeath t) {
throw t;
} catch (Throwable t) {
// log & rethrow
}

There's at least one case when I think you may have to catch a throwable or a generic exception - if you're running a separate thread to perform a task, you may want to know if the "run" method of the thread has catched some exception or not. In that case, you probably will do something like this:
public void run() {
try {
...
}
catch(Throwable t) {
threadCompletionError = t;
}
}
I am really not sure if it's the best approach, but it works. And I was having a "ClassNotFound" error being raised by the JVM, and it's an error and not an exception. If I let the exception be thrown, I am not sure how to catch it in the calling thread (probably there's a method but I don't know about it - yet).
As for the ThreadDeath method, don't call the "Thread.stop()" method. Call Thread.interrupt and have your thread to check if it was interrupted by someone.

This post won't make the "checked exceptions are bad" people happy. However, what I am basing my answer on is how Java exceptions are intended to be used as defined by the people that created the language.
Quick reference chart:
Throwable - never catch this
Error - indicates a VM error - never catch this
RuntimeException - indicated a programmer error - never catch this
Exception - never catch this
The reason you should not catch Exception is that it catches all of the subclasses, including RuntimeException.
The reason you should not catch Throwable is that it catches all of the subclasses, including Error and Exception.
There are exceptions (no pun intended) to the above "rules":
Code you are working with (from a 3rd party) throws Throwable or Exception
You are running untrusted code that could cause your program to crash if it thew an exception.
For the second one usually it is enough to wrap main, event handling code, and threads with the catch to Throwable and then check the actual type of the exception and deal with it as appropriate.

Do NOT ever catch Throwable or Error and you should generally not simply catch a generic Exception either. Errors are generally things that most reasonable programs cannot possibly recover from. If you know what is going on, you might be able to recover from one specific error, but in that case, you should catch only that one particular error and not all errors in general.
A good reason not to catch Error is because of ThreadDeath. ThreadDeath is a fairly normal occurrence that can theoretically be thrown from anywhere (other processes like the JVM itself can generate it), and the whole point of it is to kill your thread. ThreadDeath is explicitly an Error rather than an Exception because too many people catch all Exceptions. If you ever were to catch ThreadDeath, you must rethrow it so that your thread actually dies.
If you have control over the source, it should probably be restructured to throw an Exception rather than an Error. If you don't, you should probably call to the vendor and complain. Errors should be reserved for only things that are terminal with no possible way to recover from them.

Normally when programming, you should only catch a specific exception (such as IOException). In a lot of programs you can see a very toplevel
try {
...
} catch(Exception e) {
...
}
That catches all errors which could be recoverable and all those which indicate a bug in your code, e.g. InvalidArgumentException, NullPointerException. You can then automatically send an eMail, display a message box or whatever you like, since the JavaVM itself is still working fine.
Everything derived from Error is something very bad, you can't do anything against. The question is, if it makes sense to catch a OutOfMemoryError or a VirtualMachineError. (It is a error in the JavaVM itself, probably you can't even display a message box or send an eMail then)
You should probably not a class derived from Error, you should derive from Exception or RuntimeException.

I know it might be counter-intuitive, but just because you can catch all sorts of Exceptions and Throwables and Errors does not mean you should.
Over-aggressive catching of java.lang.Exception can lead to some serious bugs in applications - because unexpected Exceptions never bubble up, are never caught during development/testing, etc.
Best practice: only catch
Exceptions that you can handle
Exceptions that are necessary to catch

In general it would be reasonable to try to catch Errors if only so that can be properly reported.
However, I believe there are cases when it would be appropriate to catch an Error and not report it. I'm referring to UnsatisfiedLinkError. In JAI the library uses some native libraries to implement most of the operators for performance reasons, however if the library fails to load (doesnt exist, wrong format, unsupported platform) the library will still function because it will fall back into a java only mode.

Slightly off topic, but you may also want to look at this very good article about exceptions.

Why not catch them all? Then log them, at least you know you have an error. So better catch Throwable/s than Exception/s only.

There is no point in catching Error.
Errors are used to indicate something went really wrong in your application and it should be restarted.
For instance one common error is
java.lang.OutOfMemoryError
There is NOTHING you can do when that happens. Is already too late, the JVM has exhausted all its options to get more memory but it is impossible.
See this other answer to understand more about the three kinds of exceptions.