What is Exception in Java ?
An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions. When an error occurs within a method, the method creates an object and hands it off to the runtime system. The object, called an exception object, contains information about the error, including its type and the state of the program when the error occurred. Creating an exception object and handing it to the runtime system is called throwing an exception.
What happens when a method throws an exception ?
After a method throws an exception, the runtime system attempts to find an exception handler to handle it. The search begins with the method in which the error occurred and proceeds through the call stack in the reverse order in which the methods were called. When an appropriate handler is found, the runtime system passes the exception to the handler. If the runtime system is unable to find an appropriate exception handler,the runtime system terminates.
For example :
Suppose the main method calls a method named A . A calls B and B calls a method C . C method throws exception . The problem can be described with
the below diagram .
We can understand this in the steps given below :
An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions. When an error occurs within a method, the method creates an object and hands it off to the runtime system. The object, called an exception object, contains information about the error, including its type and the state of the program when the error occurred. Creating an exception object and handing it to the runtime system is called throwing an exception.
What happens when a method throws an exception ?
After a method throws an exception, the runtime system attempts to find an exception handler to handle it. The search begins with the method in which the error occurred and proceeds through the call stack in the reverse order in which the methods were called. When an appropriate handler is found, the runtime system passes the exception to the handler. If the runtime system is unable to find an appropriate exception handler,the runtime system terminates.
For example :
We can understand this in the steps given below :
- Exception occurs at method C() . The method creates an exception object and gives it to the runtime system.
- The runtime system starts looking for a matching exception handler . The search starts from method C() but because it does not provide any exception handler , the call propogates down the call stack.
- The runtime system looks for a handler in method B() . B also does not provide any handler so the call propagates down the call stack.
- A() provides a try/catch block that catches the type of exception that was thrown by C() so runtime system passes the exception to this handler.
What is OutOfMemoryError in Java?
OutOfMemoryError in Java is a subclass of java.lang.VirtualMachineError and it’s thrown by JVM when it ran out of heap memory. We can fix this error by providing more memory to run the java application through java options.
$>java MyProgram -Xms1024m -Xmx1024m -XX:PermSize=64M -XX:MaxPermSize=256m
Can we have an empty catch block?
We can have an empty catch block but it’s the example of worst programming. We should never have empty catch block because if the exception is caught by that block, we will have no information about the exception and it wil be a nightmare to debug it. There should be at least a logging statement to log the exception details in console or log files.
Can we write only try block without catch and finally blocks?
No, It shows compilation error. The try block must be followed by either catch or finally block. You can remove either catch block or finally block but not both.
What is Re-throwing an exception in java?
Exceptions raised in the try block are handled in the catch block. If it is unable to handle that exception, it can re-throw that exception using throw keyword. It is called re-throwing an exception.
1
2
3
4
5
6
7
8
9
10
11
| try{ String s = null; System.out.println(s.length()); //This statement throws NullPointerException}catch(NullPointerException ex){ System.out.println("NullPointerException is caught here"); throw ex; //Re |
No comments:
Post a Comment