20. Exception.md

Exceptions

1. Error
   • Error objects describe internal errors, JVM execution errors or resource exhaustion.
   • Ex: StackOverflowError, OutOfMemoryError, NoClassDefFoundError, InternalError
2. Runtime Exception
   • Logic Error in the code needs fixing.
   • can also be used by devs to indicate a problem that needs correction ( IllegalArgumentException, IllegalStateException )
   • Ex: NullPointerException, ClassCastException, ArrayIndexOutOfBoundsException, NumberFormatException
3. Checked Exception
   • This type of exception needs to be handled by the devs.
   • This exception either needs to be handled with try/catch or add it to the function declaration
   • Ex: CloneNotSupportedException, FileNotFoundException, SQLException, AWTException

// handling Checked Exceptions
class Main(){
    
    // add throws exception to the method signature
   public static void doSomething() throws FileNotFoundException {
      throw new FileNotFoundException();
   }

   // surround with try/catch
   public static void doSomethingAgain(){
      try {
         throw new FileNotFoundException();
      } catch (FileNotFoundException e) {
         // handle error
      }
   }
    public static void main(String[] args){
    }
}

try/catch

• If the code throws exception, it needs to be handled using try/catch statement.
• The catch statement catches the type of exception declared in the catch block.

import java.io.IOException;

class Main{
    
public static void main(String[] args){
        try{
            // some code
        }catch(IOException ex) {
            // catch io exception
        }
}

}

• If there are multiple catch blocks the most specific exception needs to be at top and the generic at the last.

import java.io.IOException;

class Main{
    
public static void main(String[] args){
        try{
            // some code
        }catch(FileNotFoundException ex) {
            // catch FileNotFoundException
        }catch(IOException ex) {
           // catch IOException
        }
        
}

}

Finally Keyword

• Used in conjunction with try/catch
• finally block is guaranteed to run irrespective of whether the exception is thrown or not.
• It runs even if there is return or break statement.
• Only exception where finally block is not run is when System.exit(0) occurs before finally block.

try with resources

• Automatically closes the resources.

// syntax:

try(resource-specification){
    
}catch(Exception ex){
    
}