Exceptions Session 21

Memory Upload Creating Exceptions Using exceptions to control object creation and validation

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

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

Unchecked Exceptions An unchecked are:  Descendants of the RuntimeException

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

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

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

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

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.

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

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

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

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!!!!

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

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

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

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

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

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

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; }

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

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

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

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

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 Exception 1

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

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

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.

Exceptions in Applets

Time to Try it Out Exceptions and Objects

Creating Exceptions Setting exception messages Validating objects using our own exceptions any questions to Memory Defragmenter

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