It interrupts the method. Anything after the throw statement would not be executed, unless the thrown exception is handled. The exception object is not returned from the method, it is thrown from the method.
That means that the exception object is not the return value of the method and the calling method can be interrupted too and so on and so on Typically, you'll throw a different class of exception for each different type of error. The information about the error is represented both inside the exception object and implicitly in the name of the exception class, so someone in the bigger context can figure out what to do with your exception.
9 Best Practices to Handle Exceptions in Java
Often, the only information is the type of exception, and nothing meaningful is stored within the exception object. By default, when an exception is thrown, the current method is interrupted, the calling method is interrupted too and so on till the main method. The executed code lines have been highlighted.
When no exception is thrown, the method flow executes the try statement and not the catch statement. As there is a thrown exception at line 5, the line 6 is not executed, but the exception is caught by the catch statement so the catch block is executed. The following code is also executed. Note that the catch statement takes an exception as parameter. There is a third case: The exception is thrown to the calling method.
Each catch block must take a parameter of a different throwable class. A thrown object may match several catch block but only the first catch block that matches the object will be executed. A catch-block will catch a thrown exception if and only if:.
9 Best Practices to Handle Exceptions in Java
This means that the catch block order is important. As a consequence, you can't put a catch block that catches all the exception which take a java. Exception as parameter before a catch block that catches a more specific exception as the second block could never be executed. At line 14, we use a multi-catch clause.
- Mistake 2: Catch unspecific exceptions!
- In Search of God - My Journey;
- 2. Prefer Specific Exceptions.
- Mistake 3: Log and throw an Exception.
- 7 Common Mistakes You Should Avoid When Handling Java Exceptions.
It is available since the JDK 7. This is a combination of several catch clauses and let's you handle exceptions in a single handler while also maintaining their types. So, instead of being boxed into a parent Exception super-class, they retain their individual types. You can also use the java. Throwable class here, since Throwable is the parent class for the application-specific Exception classes. However, this is discouraged in Java programming circles.
This is because Throwable happens to also be the parent class for the non-application specific Error classes which are not meant to be handled explicitly as they are catered for by the JVM itself. A finally block can be added after the catch blocks. A finally block is always executed, even when no exception is thrown, an exception is thrown and caught, or an exception is thrown and not caught.
It's a place to put code that should always be executed after an unsafe operation like a file close or a database disconnection. You can define a try block without catch block, however, in this case, it must be followed by a finally block. In the code section 6. Because methodA and methodB pass or throw exceptions, methodC must be prepared to handle them. This can be handled in two ways: The above example will cause a compilation error, as Java is very strict about exception handling. So the programmer is forced to handle any possible error condition at some point. A method can do two things with an exception: To work correctly, the original code can be modified in multiple ways.
For example, the following:. The AnotherException from methodB will be handled locally, while CustomException and SomeException will be thrown to the caller to handle it. Most of the developers are embarrassed when they have to choose between the two options.
9 Best Practices to Handle Exceptions in Java
This type of decision should not be taken at development time. If you are a development team, it should be discussed between all the developers in order to have a common exception handling policy. Add some exercises like the ones in Variables. From Wikibooks, open books for an open world. The unspecific throws clause hides all changes to the exceptions that a caller has to expect and handle. That might cause several unexpected errors that you need to find by a test case instead of a compiler error.
That tells the caller of your method which exceptional events need to be handled. It also allows you to update the throws clause when your method throws an additional exception. So your clients are aware of the change and even get an error if you change your throws clause. That is much easier to find and handle than an exception that only shows up when you run a particular test case.
Exception in the main method of your Java SE application. That provides several benefits. But keep in mind that the first catch block that handles the exception class or one of its superclasses will catch it. So, make sure to catch the most specific class first. Otherwise, your IDEs will show an error or warning message telling you about an unreachable code block. That is one of the most popular mistakes when handling Java exceptions.
It might seem logical to log the exception where it was thrown and then rethrow it to the caller who can implement a use case specific handling. But you should not do it for the following three reasons:. So, better only log the exception when you handle it. Like in the following code snippet. The doSomething method throws the exception.
And it then gets handled in the doEvenMore method which also writes a log message. Using exceptions to control the flow of your application is considered an anti-pattern for two main reasons:. So, better use proper conditions to break your loops or if-else-statements to decide which code blocks should be executed. You sometimes might want to wrap an exception in a different one. Maybe your team decided to use a custom business exception with error codes and a unified handling. When you instantiate a new exception, you should always set the caught exception as its cause.
Otherwise, you lose the message and stack trace that describe the exceptional event that caused your exception. The Exception class and all its subclasses provide several constructor methods which accept the original exception as a parameter and set it as the cause. When you generalize an exception, you catch a specific one, like a NumberFormatException , and throw an unspecific java.
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That is similar to but even worse than the first mistake I described in this post. It not only hides the information about the specific error case on your API, but it also makes it difficult to access. You need to catch the generic Exception class and then check the type of its cause. That removes all information about the exceptional event.
- 7 Common Mistakes You Should Avoid When Handling Java Exceptions.
- 1. Clean up Resources in a Finally Block or Use a Try-With-Resource Statement.
- Mistake 3: Log and throw an Exception.
The exceptions that you throw should always be as specific as possible.