Exceptions

1. 0 Exceptions Session 21

2. 0 Memory Upload • Creating Exceptions • Using exceptions to control object creation and validation

3. 0 Exceptions • To create exceptions we need to extend the Exception class • The exception class has one instance variable

4. 0 Checked Exceptions • A checked exception must: • Be caught • Be listed in the throws clause of the method where it may occur

5. 0 Unchecked Exceptions • An unchecked are: • Descendants of the RuntimeException

6. 0 Exception • public class MyException extends Exception{ • private String message; • public MyException( ){ • //empty constructor • } • public void setMessage(String newMsg){ • this.message = newMsg; • } • public String getMessage( ){ • return this.message; • } • } constructor set method get method

7. 0 The “Other” Class • The class with the main method is called the driver class • We create a class that can throw an exception

8. 0 The “Other” Class • public class ExcThr{ • public static void main(String [ ] arg) throws Exception{ • MyException me = new MyException( ); • throw me; • } //main method • } //class ends

9. 0 The “Other” Class • public class ExcThr{ • public static void main(String [ ] arg) throws Exception{ • MyException me = new MyException( ); • me.setMessage(“ICS111 Exception”); • throw me; • } //main method • } //class ends

10. 0 Exception Propagation • If an exception is not caught and handled where it occurs it will propagate back to the source. • In applications this will be the main method. Exception

11. 0 Running Several Classes • All classes must be in the same directory • When compiling a class that uses others Java will automatically find the other classes. • This applies to exceptions as well

12. 0 Time to Try it Out Creating and throwing exceptions. Exception messages

13. 0 Why do that? User, please do not create invalid objects • You can create exceptions to make your classes independent. • Think: • How can you prevent users from creating invalid objects?

14. 0 Where is the validation? • We can validate in the driver class. • This makes the Name class dependent in another class • So what can we do? Throw an exception within the Name class!!!!

15. 0 A simple class • public class Name{ • String name = “”; • public Name(String name){ • this.name = name; • } • public String getName( ){ • return this.name; • } constructor get method

16. 0 A simple class • public void setName(String newName){ • this.name = name; • } • public String toString( ){ • String s = “Name: ”+ this.name; • return s; • } • } //class name ends set method toString method

17. 0 Creating a Name Exception • Write down the rules for a valid name. • Name needs to be at least 3 characters long • Name cannot be blank

18. 0 Creating a Name Exception • So the exception will be thrown if: • Name is composed of only blanks even if it is longer than 2 characters • Name is less than 3 characters long. • Lets translate the rules into Java

19. 0 NameException • public class NameException extends Exception{ • private String message; • public NameException( ){ • //empty constructor • } • public void setMessage(String newMsg){ • this.message = newMsg; • } • public String getMessage( ){ • return this.message; • } • } constructor set method get method

20. 0 Name Class Modifications • //the constructor changes • public Name(String name) throws Exception{ • this.name = this.setName(name); • } The strategy? Centralized validation

21. 0 Name Class Modifications • public void setName(String newName ) throws Exeption{ • int len = 0; • newName = newName.trim( ); • len = newName.length( ); • if(len<3){ • NameException ne = new NameException( ); • ne.setMessage(“Invalid name length”); • throw ne; • } • this.name = newName; • }

22. 0 Name Class Modifications • public String getName( ){ • return this.name; • } • public String toString( ){ • String s = “Name: ”+ this.name; • return s; • } • } //class name ends Not in these methods

23. 0 The Driver Class • public class UsingNames{ • public static void main(String [ ] arg) throws Exception{ • Name n1 = new Name(“ABC”); • System.out.println(n1.toString( )); • Name n2 = new Name(“ W ”); • System.out.println(n1.toString( )); • } • } ABC

24. 0 Name Class Modifications • public void setName(String newName ) throws Exeption{ • int len = 0; • newName = newName.trim( ); • len = newName.length( ); • if(len<3){ • NameException ne = new NameException( ); • ne.setMessage(“Invalid name length”); • throw ne; • } • this.name = newName; • } ABC 3 ABC

25. 0 The Driver Class • public class UsingNames{ • public static void main(String [ ] arg) throws Exception{ • Name n1 = new Name(“ABC”); • System.out.println(n1.toString( )); • Name n2 = new Name(“ W ”); • System.out.println(n1.toString( )); • } • } W

26. 0 Name Class Modifications • public void setName(String newName ) throws Exeption{ • int len = 0; • newName = newName.trim( ); • len = newName.length( ); • if(len<3){ • NameException ne = new NameException( ); • ne.setMessage(“Invalid name length”); • throw ne; • } • this.name = newName; • } W W 1 Exception

27. 0 Exception Propagation UsingNames void main(String arg) Name String name Constructor Name(String name) String getMessage( ) void setMessage(String newMessage) NameException String message Constructor NameException( ) String getMessage( ) void setMessage(String newMessage) Exception Exception Class Created by JAVA

28. One Exception Several Messages • One Exception can display several messages as the program crashes • The message can be set on each of the different methods that may throw the exception

29. Validation in set methods • The methods that validate are usually the set methods. • The constructor validates by calling on the set methods • Each set method can add a different exception message in case of error.

30. Exceptions in Applets

31. 0 Time to Try it Out Exceptions and Objects

32. 0 Memory Defragmenter • Creating Exceptions • Setting exception messages • Validating objects using our own exceptions • E-mail any questions to blanca@hawaii.edu

33. 0 Task Manager • Answer the 5 webct questions • Read your e-mail • Visit WebCT, webct.hawaii.edu