The core advantage of exception handling is that normal flow of the application is maintained. Exception normally disrupts the normal flow of the application that is why we use exception handling.
Hierarchy of Exception classes
Types of Exception:
There are mainly two types of exceptions: checked and unchecked where error is considered as unchecked exception. The sun microsystem says there are three types of exceptions:
- Checked Exception
- Unchecked Exception
- Error
|
What is the difference between checked and unchecked exceptions ?
1)Checked Exception
The classes that extend Throwable class except RuntimeException and Error are known as checked exceptions e.g.IOException, SQLException etc. Checked exceptions are checked at compile-time.
|
2)Unchecked Exception
The classes that extend RuntimeException are known as unchecked exceptions e.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc. Unchecked exceptions are not checked at compile-time rather they are checked at runtime.
|
3)Error
Error is irrecoverable e.g. OutOfMemoryError, VirtualMachineError, AssertionError etc.
Use of try-catch block in Exception handling:
Five keywords used in Exception handling:
- try
- catch
- finally
- throw
- throws
|
try block
Enclose the code that might throw an exception in try block. It must be used within the method and must be followed by either catch or finally block.
catch block
Catch block is used to handle the Exception. It must be used after the try block.
|
The JVM firstly checks whether the exception is handled or not. If exception is not handled, JVM provides a default exception handler that performs the following tasks:
- Prints out exception description.
- Prints the stack trace (Hierarchy of methods where the exception occurred).
- Causes the program to terminate.
But if exception is handled by the application programmer, normal flow of the application is maintained i.e. rest of the code is executed.
Multiple catch block:
If you have to perform different tasks at the occrence of different Exceptions, use multple catch block.
Rule:At a time only one Exception is occured and at a time only one catch block is executed.
Rule:All catch blocks must be ordered from most specific to most general i.e. catch for ArithmeticException must come before catch for Exception .
Nested try block:
| try block within a try block is known as nested try block. |
Why use nested try block?
Sometimes a situation may arise where a part of a block may cause one error and the entire block itself may cause another error. In such cases, exception handlers have to be nested
finally block
The finally block is a block that is always executed. It is mainly used to perform some important tasks such as closing connection, stream etc.
Why use finally block?
- finally block can be used to put "cleanup" code such as closing a file,closing connection etc.
Rule: For each try block there can be zero or more catch blocks, but only one finally block.
Note: The finally block will not be executed if program exits(either by calling System.exit() or by causing a fatal error that causes the process to abort).
|
|
throw keyword
| The throw keyword is used to explictily throw an exception. |
We can throw either checked or uncheked exception. The throw keyword is mainly used to throw custom exception.
Exception propagation:
An exception is first thrown from the top of the stack and if it is not caught, it drops down the call stack to the previous method,If not caught there, the exception again drops down to the previous method, and so on until they are caught or until they reach the very bottom of the call stack.This is called exception propagation.
|
Rule: By default Unchecked Exceptions are forwarded in calling chain (propagated).
Rule: By default, Checked Exceptions are not forwarded in calling chain (propagated).
throws keyword
The throws keyword is used to declare an exception. It gives an information to the programmer that there may occur an exception so it is better for the programmer to provide the exception handling code so that normal flow can be maintained.
|
| Exception Handling is mainly used to handle the checked exceptions. If there occurs any unchecked exception such as NullPointerException, it is programmers fault that he is not performing check up before the code being used. |
Syntax of throws keyword:
- void method_name() throws exception_class_name{
- ...
- }
Which exception should we declare?
Ans) checked exception only, because:
- unchecked Exception: under your control so correct your code.
- error: beyond your control e.g. you are unable to do anything if there occurs VirtualMachineError or StackOverflowError.
Advantage of throws keyword:
Now Checked Exception can be propagated (forwarded in call stack).
Rule: If you are calling a method that declares an exception, you must either caught or declare the exception.
There are two cases:
- Case1:You caught the exception i.e. handle the exception using try/catch.
- Case2:You declare the exception i.e. specifying throws with the method.
|
Case1: You handle the exception
- In case you handle the exception, the code will be executed fine whether exception occurs during the program or not.
Case2: You declare the exception
- A)In case you declare the exception, if exception does not occur, the code will be executed fine.
- B)In case you declare the exception if exception occures, an exception will be thrown at runtime because throws does not handle the exception.
Difference between throw and throws:
| throw keyword | throws keyword |
|---|
| 1)throw is used to explicitly throw an exception. | throws is used to declare an exception. |
| 2)checked exception can not be propagated without throws. | checked exception can be propagated with throws. |
| 3)throw is followed by an instance. | throws is followed by class. |
| 4)throw is used within the method. | throws is used with the method signature. |
| 5)You cannot throw multiple exception | You can declare multiple exception e.g.
public void method()throws IOException,SQLException. |
|
Can we rethrow an exception?
Yes by throwing same exception in catch block.
ExceptionHandling with MethodOverriding
There are many rules if we talk about methodoverriding with exception handling. The Rules are as follows:
- If the superclass method does not declare an exception
- If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception but it can declare unchecked exception.
- If the superclass method declares an exception
- If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.
|
If the superclass method does not declare an exception
1) Rule: If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception.
2) Rule: If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception but can declare unchecked exception.
If the superclass method declares an exception
1) Rule: If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.
What is difference between Checked Exception and Unchecked Exception?
1)Checked Exception:
| The classes that extend Throwable class except RuntimeException and Error are known as checked exceptions e.g.IOException,SQLException etc. Checked exceptions are checked at compile-time. |
2)Unchecked Exception:
The classes that extend RuntimeException are known as unchecked exceptions e.g. ArithmeticException,NullPointerException etc. Unchecked exceptions are not checked at compile-time. more details...
What is the base class for Error and Exception?
Throwable.
Is it necessary that each try block must be followed by a catch block?
It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.
What is finally block?
|
Can finally block be used without catch?
- Yes, by try block. finally must be followed by either try or catch.more details...
|
|
Is there any case when finally will not be executed?
- finally block will not be executed if program exits(either by calling System.exit() or by causing a fatal error that causes the process to abort).more details...
Can subclass overriding method declare an exception if parent class method doesn't throw an exception ?
Yes but only unchecked exception not checked. more details...
What is exception propagation ?
Forwarding the exception object to the invoking method is known as exception propagation.more details...
|
|
|
|
|
|
|
|
|
|
|
