Java Exception Handling - Exception Handling in Java
Java exception handling is managed via five keywords: try, catch, throw, throws, and finally. Exception Handling is a mechanism to handle runtime errors. In this Exception Handling in Java or Java Exceptions with checked, unchecked and errors with example and usage of try, catch, throw, throws and finally keywords. If an exception occurs within the try block that exception can be handled in catch block. Any exception that is thrown out of a method must be specified by a throws clause. Any code that absolutely must be executed after a try block must be put it in the finally block.
What is an Exception ?
An exception is an unwanted or unexpected event, which occurs during the execution of a program at run time, that disrupts the normal flow of the program execution. Java exception handling is managed by try, catch, throw, throws, and finally. If an exception occurs within the try block that exception can be handled in catch block. Any exception that is thrown out of a method must be specified by a throws clause. Any code that absolutely must be executed after a try block must be put it in the finally block.
- Error that occurs during runtime
- Exceptional event
- Cause normal program flow to be disrupted
Types of Exception in Java
- Checked Exception
- Unchecked Exception
The classes which directly inherit Throwable class except RuntimeException and Error are known as checked exceptions. Checked exceptions are checked at compile time. Checked Exceptions are IOException, SQLException etc.
The classes which inherit RuntimeException are known as unchecked exceptions. Unchecked exceptions are not checked at compile time, but they are checked at runtime. Unchecked Exceptions are ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc.
What are the keywords used in exception handling ?
There are five keywords used in Java exception.
List of five exception handling keywords are as follows.
Exception Handling keywords in Java
Java exception handling is managed via five keywords: try, catch, throw, throws, and finally. The brief description of Java Exception Handling keywords are as follows.
|try||The try keyword is used to specify a block where we should place exception code. The try block must be followed by either catch or finally. It means, we can't use try block alone.|
|catch||The catch block is used to handle the exception. It must be preceded by try block which means we can't use catch block alone. It can be followed by finally block later.|
|finally||The finally block is used to execute the important code of the program. It is executed whether an exception is handled or not.
A finally keyword is used to create a block of code that follows a try block. A finally block of code is always executed whether an exception has occurred or not.
|throw||The throw keyword is used to throw an exception.
throw keyword is used to throw an exception explicitly. Only object of Throwable class or its sub classes can be thrown.
Any method that is capable of causing exceptions must list all the exceptions possible during its execution, so that anyone calling that method gets a prior knowledge about which exceptions are to be handled. A method can do so by using the throws keyword.
|throws||The throws keyword is used to declare exceptions. It doesn't throw an exception. It specifies that there may occur an exception in the method. It is always used with method signature.
Throws keyword is used for exception handling without try & catch block. It specifies the exceptions that a method can throw to the caller and does not handle itself.
What is the difference between throw and throws ?
Following are the difference between throw and throws keywords .
|throw keyword is used to throw an exception explicitly.||throws keyword is used to declare an exception possible during its execution.|
|throw keyword is followed by an instance of Throwable class or one of its sub-classes.||throws keyword is followed by one or more Exception class names separated by commas.|
|throw keyword is declared inside a method body.||throws keyword is used with method signature (method declaration).|
|We cannot throw multiple exceptions using throw keyword.||We can declare multiple exceptions (separated by commas) using throws keyword.|
Java Exception Examples
- Divide by zero errors
- Accessing the elements of an array beyond its range
- Invalid input
- Hard disk crash
- Opening a non-existent file
- Heap memory exhausted
Java Exception Example
The default exception handler provided by java runtime will prints out the exception description, stack trace (Hierarchy of methods where the exception occurred) and Causes the program to terminate. The Exception message looks like the following message.
Exception in thread "main" java.lang.ArithmeticException: / by zero
at ExceptionExample1.main (ExceptionExample1.java:8)
What Happens When an Exception Occurs ?
- When an exception occurs within a method, the method creates an exception object and hands it off to the runtime system.
- The runtime system searches the call stack for a method that contains an exception handler
- When an appropriate handler is found, the runtime system passes the exception to the handler
- If the runtime system exhaustively searches all the methods on the call stack without finding an appropriate exception handler, the runtime system (and, consequently, the program) terminates and uses the default exception handler
Benefits of Java Exception Handling
- Separating Error-Handling code from regular business logic code
- Propagating errors up the call stack
- Grouping and differentiating error types
Separating Error Handling Code from Regular Code
Exceptions enable you to write the main flow of your code and to deal with the exceptional cases elsewhere. Note that exceptions don't spare you the effort of doing the work of detecting, reporting, and handling errors, but they do help you organize the work more effectively.
Catching Exceptions in Java with try catch
Java Exception Handling Example
In the following Java Exception Handling Example where we using a try catch statement to handle the exception.
Exception Handling in Java Example with try catch
Catching Exceptions in Java with multiple catch
Before Java 7, we used to catch multiple exceptions one by one as shown below. Multiple catches should be ordered from subclass to super class. If a catch block handles multiple exceptions in Java 7, you can separate them using a pipe (|) and in this case, exception parameter (ex) is final, so you can't change it. The byte code generated by this feature is smaller and reduce code redundancy. In Java 7, we can catch both these exceptions in a single catch block as follows.
Catching Exceptions in Java with multiple catch
Nested Exception Handling in Java
Catching Exceptions in Java with finally
If you catching exceptions in Java with finally, It is executed whether an exception is handled or not. The finally block is always executed. In the following example you can learn how to use catching exceptions in Java with finally
Catching Exceptions in java with finally
Throwing Exceptions with throw keyword
Java allows you to throw exceptions (generate exceptions). An exception you throw is an object.
Java Exception Example with throw keyword
Rules in Java Exception Handling
- A method is required to either catch or list all exceptions it might throw
— Except for Error or RuntimeException, or their subclasses
- If a method may cause an exception to occur but does not catch it, then it must say so using the throws keyword
— Applies to checked exceptions only
- Java compiler checks if the program either catches or lists the occurring checked exception
- If not, compiler error will occur
- Not subject to compile-time checking for exception handling
- Built-in unchecked exception classes Error, RuntimeException and Their subclasses
- Handling all these exceptions may make the program cluttered and may become a nuisance
Creating Your Own Exception Class
Steps to follow
- Create a class that extends the RuntimeException or the Exception class
- Customize the class i.e. Members and constructors may be added to the class
How To Use Your Own Exceptions
The following example shows how to use your own exceptions in java.
Java Exception Example with Your Own Exceptions
Java Exception Example with Your Own Exception with throw keyword
Exception in thread "main" error : conversion = '%'
You will get this error, when you put '%' at wrong place in your code like following method.
In the above error code we are swapped the order of the letter s with the sign % on the last one. So if you correct the error you should change s% to %s in your code like below code.