Java Exceptions & Exception Handling

Java exception handling is a Java application that helps to handle the error occurred in different phases. Whenever any error occurs an exception is thrown. Java exceptions are the errors that occur while executing a program and these occurs for different reason like when any invalid data entered as input, for a non-existing file is asked to access etc. Java exception is either generated by the JVM (Java Virtual Machine) or it is generated by the code resultant from a throw statement. Exception is an inbuilt class in the default package known as java.lang. The exception handling is a significant part of Java programming.

Types of Exception

There are two types of exception

1.       Checked Exception

2.       Unchecked Exception.

Checked exception
 
A checked exception is any subclass of Exception (or Exception itself), excluding the class RuntimeException and all its subclasses. These exceptions cannot be ignored at the time of compilation and the programmers need to deal with the prospect of the exception that will be thrown.

Examples of Checked Exception:

IOException

SQLException

Unchecked exception

Unchecked Exceptions mainly occur for the different programming errors such as accessing a null object (NullPointerException), accessing an element of an array that is out of bounds (ArrayBoundExceptions). Unchecked Exception is direct sub Class of RuntimeException.

Exception Hierarchy

The super class for exceptions is Throwable and it contains two subclasses Error and Exception. Every class in the hierarchy suffixes with Exception, but Error does not. All the exceptions are caused by problems within software  but Error is caused by hardware problems, The above of exceptions can be divided in two categories – unchecked exceptions and checked exceptions.

Throw

Throw is used to actually throw the exception. It is used to raise exception explicitly that means it is use when a user defined exception is raised.

Throws

Throws keyword, used to declare an exception. It gives the information regarding the exception to the programmer so the programmer can provide an exception handling code. They are not interchangeable.

Try block

A try statement is used to catch exceptions that might be thrown by an executed program. Whenever, a try block is in use a statement that might throw an exception and that prevent from program crashing if any exception occurs.

Catch Block

The catch block contains a series of legal Java statements. These statements are executed if and when the exception handler is invoked. The runtime system invokes the exception handler when the handler is the first one in the call stack whose type matches that of the exception thrown.

Finally

When a try block runs till the end, and there are no exception is

Thrown then the finally block will be executed after the try block but when an exception is thrown by the try block and it caught in one of the catch blocks the finally block will execute after the catch block.

Difference between Errors and Exception

Exceptions: An exception has two subclasses, checked and unchecked. The Exception is point out an error that may cause by the programmer.  An Exception handled at the application level

Errors: Errors are generated to indicate errors generated by the runtime environment. Errors don't have any subclasses while, tare always unchecked. Errors can be lethal in nature and revival from Error often is not possible. Usually indicate a system error or a problem with a low-level resource. Errors should be handled at the system level.

Example of Some Checked and Unchecked Exceptions

Checked IOException InterruptedException       NoSuchMethodException ClassNotFoundException CloneNotSupportedException FileNotFoundException ParseException InstantiationException

Unchecked NullPointerException StackOverflowError ArrayIndexOutOfBoundsException        IllegalArgumentException ExceptionInInitializerError IllegalStateException NumberFormatException ClassCastException NoClassDefFoundError