exceptions session 21. memory upload creating exceptions using exceptions to control object creation...

ExceptionsExceptions

Session 21

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• Creating Exceptions

• Using exceptions to control object creation and validation

ExceptionsExceptions

• To create exceptions we need to extend the Exception class

• The exception class has one instance variable

Checked ExceptionsChecked Exceptions

• A checked exception must: Be caught Be listed in the throws clause of the

method where it may occur

Unchecked ExceptionsUnchecked Exceptions

• An unchecked are: Descendants of the

RuntimeException

ExceptionException

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” ClassThe “Other” Class

• The class with the main method is called the driver class

• We create a class that can throw an exception

The “Other” ClassThe “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” ClassThe “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 PropagationException 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 ClassesRunning 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 OutTime to Try it Out

Creating and throwing

exceptions.Exception messages

Why do that?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?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 classA 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 classA 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 ExceptionCreating 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 ExceptionCreating 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

NameExceptionNameException

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 ModificationsName Class Modifications

//the constructor changes

public Name(String name) throws Exception{ this.name = this.setName(name);}

The strategy?Centralized validation

Name Class ModificationsName 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 ModificationsName 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 ClassThe 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 ModificationsName 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

The Driver ClassThe 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 ModificationsName 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 PropagationException Propagation

NameExceptionString messageConstructor NameException( )String getMessage( )void setMessage(String newMessage)

NameString nameConstructor Name(String name)String getMessage( )void setMessage(String newMessage)

UsingNamesvoid main(String arg)

Exception ClassCreated by JAVAException

One Exception Several MessagesOne 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 methodsValidation 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 AppletsExceptions in Applets

Time to Try it OutTime to Try it Out

Exceptions

and Objects

• Creating Exceptions

• Setting exception messages

• Validating objects using our own exceptions

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