Exception and ErrorHandling in Java .ppt
- 2. ● This module is intended to give an idea about Exceptions and Errors. ● Covers different java constructs that are used to work with exceptions . ● Introduces user defined exceptions. Module Overview 2
- 3. Define Exceptions in java. Understand Exception Hierarchy and classes. Identifying exceptions in a program. Handle Exceptions properly and take necessary actions to work with exceptions. Understand different types of Exceptions. Identify the difference between Exceptions and Errors. Understand predefined Exceptions. Create user defined exceptions. Objectives 3
- 4. Exception:An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following: A user has entered invalid data. A file that needs to be opened cannot be found. A network connection has been lost in the middle of communications, or the JVM has run out of memory. Exception 4
- 5. Checked Exceptions: A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation. Runtime exceptions: A runtime exception is an exception that occurs that probably could have been avoided by the programmer. Runtime exceptions are ignored at the time of compliation. Errors: Errors are typically ignored in the code because programmer can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation. Types Of Exceptions 5
- 6. All exception classes are subtypes of the java.lang.Exception class. The exception class is a subclass of the Throwable class. Other than the exception class there is another subclass called Error which is derived from the Throwable class. Errors are conditions normally happen in case of severe failures, which are not handled by the java programs. Example : JVM is out of Memory. Exception Hierarchy 6
- 7. Diagramatic representation of Exception Hierarchy. Exception Hierarchy(Cont..) 7
- 8. Some important methods: public String getMessage() Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor. public Throwable getCause() Returns the cause of the exception as represented by a Throwable object. public void printStackTrace() Prints the result of toString() along with the stack trace to System.err, the error output stream. Throwable Class Methods
- 9. To work with exception java has provided following clauses. Try/Catch: A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following: Try{ //Protected code }catch(ExceptionName e1){ //Catch block } Working with Exceptions 9
- 10. Working with Exceptions(Cont..) 10 Catch(Specific): A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that occurred is listed in a catch block, the exception is passed to the catch block. A try block can be followed by multiple catch blocks.
- 11. Throws Keyword 11 Throws:If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature. You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword. Example: public void deposit(double amount) throws IOException { // Method implementation throw new IOException(); }
- 12. Throws: Amethod can declare that it throws more than one exception, in which case the exceptions are declared in a list separated by commas. import java.io.*; public class Bank{ public void withdraw(double amount) throws RemoteException, InsufficientFundsException { // Method implementation } //Remainder of class definition } Throws Keyword(Cont…) 12
- 13. The Finally Keyword 13 The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred. Using a finally block allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code. A finally block appears at the end of the catch blocks.
- 14. Syntax: try{ //Protected code }catch(ExceptionType1 e1){ //Catch block }catch(ExceptionType2 e2){ //Catch block }finally{ //The finally block always executes. } Finally(Cont..) 14
- 15. Important Points. A catch clause cannot exist without a try statement. It is not compulsory to have finally clauses when ever a try/catch block is present. The try block cannot be present without either catch clause or finally clause. Any code cannot be present in between the try, catch, finally blocks.
- 16. Exceptions can be created in Java. All exceptions must be a child of Throwable. If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class. If you want to write a runtime exception, you need to extend the RuntimeException class. User Defined Exceptions 16
- 17. Thank you
Editor's Notes
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- #3: Instructor Notes: By the end of this module, participants should be able to: <Obj1> <Obj 2>
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