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Exceptions Handling

Github Code repository

Exception Cases

                        
                            @startuml
                            actor User
                            User--> (file) #line:red;line.bold;text:red  : File not found
                            User--> (database) #line:red;line.bold;text:red  : Database unreachable
                            User--> (application) #line:red;line.bold;text:red  : Application error
                            User--> (network) #line:red;line.bold;text:red  : Network error
                            @enduml
                        
                      

Exception

Unanticipated events that interrupt the flow of the execution in a program. It would terminate the program abnormally.

Exception Handling

Providing an alternative way

Providing fiendly message to end user

Enough information to Debug the problem

Exception

• Compile Time Errors
• Logical Errors
• Runtime Errors

Compile Time Errors

• Cannot find symbol
• Incompatible Types
• Invalid Method Declaration

                        public static void main(String main[]){
                            System.out.println(j);
                        }
								
                    

Logical Errors

Commonly made by developers

                        //The output is -336
                        public static void main(String main[]){
                            int i=1000;
                            byte b=(byte)i;
                            System.out.println(b*14);
                            
                        }
								
                    

Runtime Errors

• Termination of program
• Unfriendly error message
• Improper shutdown of the resources

                        public class ShowRuntimeErrors {
                            public static void main(String[] args) {
                              System.out.println(1 / 0);
                            }
                          }
								
                    

What happens during exception?

• Name of the exception
• Description
• Location stacktrace
• JVM checks who is handling the exception
• Github code

Demo

Method Exception

NumberFormatException Challenge

ArithmeticException

                     public class Main {
                     public static void main(String[] args) {
                             System.out.println(1 / 0);
                         }
                         }
                           
                     

ArrayIndexOutOfBoundsException(LAB)

Exception Hierarchy

Why Exceptions?

• It is a way to signal errors on the application.
• Special object that indicates an error.
• Try/catch/finally are the keywords for handling exceptions
• Try=> normal code execution where everything is expected to go well.
• Catch=> if there is an error catch is called.
• Finally=> This is a cleanup code. This will run in any cases
• Exceptions are propagate to the call stack
• Exceptions object belongs throwable object hierarchy
• Exceptions can end your application
• Exception is a class

Try/Catch

                
                 try {
                 // code that may throw an exception
                 } catch (Exception e) {
                  // code to handle the exception
                 }

                  try {
                    System.out.println(1 / 0);
                 } catch (Exception e) {
                System.out.println("Error: " + e.getMessage());
                 }
                System.out.println("End of the program");   
                
                

Try catch finally syntax

Challenge

Exception Constructor

• Exceptions are objects like everything in java.
• It has constructors, methods just like other objects.
• Exception is a class
• Exception()/Exception(message)/Exception(cause)/Exception(message,cause)
Github Github code

Exception Handling

• Try (Wrap any code that could cause error)
• Catch ( block, print an error message)
• Finally ( block is executed regardless of whether an exception occurred or not)
• Throw (Error with custom message)
• Exception Handling code

Lab No Matching Exceptions Challenge

• NotMatchingException code
• Lab=> How do we correct this code?

Checked/Unchecked Exceptions

Lab Multiple Catch

Declaring, Throwing, and Catching Exceptions

Throwing Exception

LAB File I/O exception

Github code File IO

Throw Keyword

When exception is thrown it must of subclass

throw throwableInstance;

                        
                            ArithmeticException e = new ArithmeticException("you can't divide by zero");
                            throw e;

                            throw new ArithmeticException("you can't divide by zero");
                        
                    

Throw can be used

To throw a new instance of a built-in exception

Used inside a method or block of code to explicitly throw an exception.

Inside custom exception

Rules for throw

                        
                            ArithmeticException e=null ;
                            throw e;

                            ArithmeticException e = new ArithmeticException("you can't divide by zero");
                            throw e;
                            System.out.println("Why i am getting error");
                          
                            throw new Test();
                            class Test{}

                            class Test extends RuntimeException{}
                            throw new Test();
                        
                    

Throws

Github code File IO

Common Java Exceptions

                          String str= null;
                          int length = str.length(); //throws NullPointerException(Unchecked ex)

                          int[] arr = new int[5];
                          int x=array[9]; //throws ArrayIndexOutOfBoundsException(Unchecked ex)

                          int x=Integer.parseInt("abc"); //throws NumberFormatException(Unchecked ex)

                          Filereader fr = new FileReader("file.txt"); //throws FileNotFoundException(Checked ex)

                        
                    

Try With Resources

                        public static void main(String[] args) {
                            FileReader reader = null;
                            try {
                                reader = new FileReader("file.txt");
                                // Code that uses the reader
                            } catch (IOException e) {
                                // Exception handling
                            } finally {
                                if (reader != null) {
                                    try {
                                        reader.close();
                                    } catch (IOException e) {
                                        // Exception handling for close() method
                                    }
                                }
                            }
                    
                            try (FileReader reader1 = new FileReader("file.txt")) {
                                // Code that uses the resource
                            } catch (IOException e) {
                                // Exception handling
                            }
                            // The resource is automatically closed here
                    
                    
                        }
                        
                    
                   

Try With Multiple Resources

                    
                        public static void main(String[] args) {
                            try (
                                    FileReader reader = new FileReader("input.txt");
                                    FileWriter writer = new FileWriter("output.txt")
                            ) {
                                // reader = new FileReader("x.txt");
                                int character;
                                while ((character = reader.read()) != -1) {
                                    writer.write(character);
                                }
                                System.out.println("File copied successfully.");
                            } catch (IOException e) {
                                System.out.println("An error occurred while reading or writing the file.");
                                e.printStackTrace();
                            }
                        }
                        
                    
                   

Custom Exceptions

• Define custom exception classes if the predefined classes are not sufficient.
• Define custom exception classes by extending Exception or a subclass of Exception
• Use the exception classes in the API whenever possible
• Sample code on Github Custom Exceptions

Lab Custom Exception

Custom Exception

Errors vs Exception

Thank you