Java exception handling

            Throwable
               |
        ------------------
        |                |
      Error           Exception
                       |
           -------------------------
           |                       |
   Checked Exception       Unchecked Exception
  (compile-time)            (runtime exception)

Error VS Exception

Errors

• we CANNOT handle it (didn’t happen because of us)
• usually something we can’t even fix thru code
• examples
  • some problem with your hardware
  • out of memory error
  • stack overflow

Exception

• we CAN handle it
• Types
  • Runtime exception / Unchecked Exceptions
    • Not checked at compile time , happens during runtime.
    • Often due to programming errors like logic issues
    • Ex) NullPointerException
  • non-runtime exception / Checked exception
    • enforced at compile → must be caught with try-catch or declared with throws
    • The compiler forces you to handle them.

Syntax Errors (Separate)

• NOT Throwable objects (not part of the Error vs Exception hierarchy)
• They happen before the program runs — at compile time.
  • You must fix syntax errors or your program won’t compile

Exception handling

• We don’t want the code to stop running due to runtime errors ⇒ handle the exception
• Exceptions are actually a class

public static void main(String args[])
{
	int i = 4; //normal
	int j = 0; //normal
	int result;
	int nums = {2,3,4};
	
	try
	{
		result = i/j;
		System.out.println(nums[4]);
	} 
	// catch(Exception e)
	catch(ArithmeticException e)
	{
		System.out.println("ArithmeticException!");
	}
	catch(ArrayIndexOutOfBoundsException e)
	{
		System.out.println("ArrayIndexOutOfBoundsException!");
	}
	catch(Exception e)
	{
		System.out.println("Something went wrong.");
	}
	finally
	{
		System.out.println("BYE"); 
	}
	
}

• finally is NOT like else - it’s GUARANTEED to happen
  • to close files
    • every time u open a file it’s your responsibility to close it
  • release resources
  • clean up memory
  • log completion

if you want to throw the error by yourself

if (result==0)
{
	throw new ArithmeticException();
}

• since Exception is a class, you need to create an object of the exception

throws

Used in a method declaration to indicate that the method might throw exceptions.

public void divide(int a, int b) throws ArithmeticException {
    if (b == 0) {
        throw new ArithmeticException("Division by zero");
    }
    System.out.println(a / b);
}