Exceptions

Sequence of methods calling methods calling other methods….

sequenceDiagram
    A->>+B: message_call()
    B->>+C: message_call()
    C->>+D: message_call()
    D->>+E: message_call()
    E->>+F: message_call()
    F->>+G: message_call()
    G->>+H: message_call()
    H->>+I: message_call()
    I-->>-H: return
    H-->>-G: return
    G-->>-F: return
    F-->>-E: return
    E-->>-D: return
    D-->>-C: return
    C-->>-B: return
    B-->>-A: return

Something goes wrong

very deep down the call sequence…

sequenceDiagram
    A->>+B: message_call()
    B->>+C: message_call()
    C->>+D: message_call()
    D->>+E: message_call()
    E->>+F: message_call()
    F->>+G: message_call()
    G->>+H: message_call()
    H->>+I: message_call()
    I-->>-H: return 
    Note right of I: Something goes wrong!!
    H-->>-G: return
    G-->>-F: return
    F-->>-E: return
    E-->>-D: return
    D-->>-C: return
    C-->>-B: return
    B-->>-A: return

Where it could be fixed/handled

sequenceDiagram
    A->>+B: message_call()
    B->>+C: message_call()
    C->>+D: message_call()
    D->>+E: message_call()
    
    E->>+F: message_call()
    F->>+G: message_call()
    
    G->>+H: message_call()
    H->>+I: message_call()
    Note right of I: Something goes
WRONG!!
   
    I-->>-H: return 
    
    H-->>-G: return
    G-->>-F: return
    Note right of E: <- This method can
retry in a loop
    F-->>-E: return
    E-->>-D: return
    D-->>-C: return
    Note right of B: <- This method
can tell/ask user
    C-->>-B: return
    
    B-->>-A: return

But…. how to tell the methods / propagate the error??

Manual Error Propagation?

Requires adding error handling / Propagation logic to all methods in between where the error occurred until it can be handled:

sequenceDiagram
    A->>+B: message_call()
    B->>+C: message_call()
    C->>+D: message_call()
    D->>+E: message_call()
    
    E->>+F: message_call()
    F->>+G: message_call()
    
    G->>+H: message_call()
    H->>+I: message_call()
    Note right of I: Something goes
WRONG!!
   
    break error
        I-->H: propagate error
        end
    break error
        H-->G: propagate error
    end
    break error
        G-->>F: propagate error
    end
    break error
        F-->>E: propagate error
    end

     Note right of E: <<== This method can
retry in a loop
    I-->>-H: return 
   
    H-->>-G: return
    G-->>-F: return
    Note right of E: Retry did not help :-(
    break error
        E-->>D: propagate error
    end
    break error
        D-->>C: propagate error
    end
    break error
        C-->>B: propagate error
    end
    F-->>-E: return
    E-->>-D: return
    D-->>-C: return
   
    Note right of B:<<== This method
can tell/ask user
    C-->>-B: return
    
    B-->>-A: return

With Exceptions

if Exception A is thrown, control jumps to next enclosing catch A block up the call stack
all methods inbetween have to either
- do nothing if the Exception is Unchecked
- declare throws A in their method header if the Exception is Checked
see
- https://docs.oracle.com/javase/specs/jls/se24/html/jls-8.html#jls-MethodHeader
- “Here’s the bottom line guideline: If a client can reasonably be expected to recover from an exception, make it a checked exception. If a client cannot do anything to recover from the exception, make it an unchecked exception.” https://docs.oracle.com/javase/tutorial/essential/exceptions/runtime.html
- for example: almost every method can cause a NullPointerException - having to declare it in every method header would give no valuable information, just clutter up the code.
- you can catch Unchecked Exceptions in principle.
- any exception not

sequenceDiagram
    A->>+B: message_call()
    B->>+C: message_call()
    C->>+D: message_call()
    D->>+E: message_call()
    
    E->>+F: message_call()
    F->>+G: message_call()
    
    G->>+H: message_call()
    H->>+I: message_call()
    Note right of I: Something goes
WRONG!!
Throw Exception A
   Note right of E: <<== This method can
retry in a loop
    break Exception A
        I-->>E: control jumps to next catch A
    end
   Note right of E: <<== catch(Exception A){ retry in loop }
   Note right of E: Retry did not help - re-throw A
    
    Note right of B:<<== This method
can tell/ask user
    
    break Exception A
        E-->>B: control jumps to next catch A
    end
    Note right of B:<<== catch(Exception A)
{ tell/ask user }
    

     
    I-->>-H: return 
   
    H-->>-G: return
    G-->>-F: return
    
   
    F-->>-E: return
    E-->>-D: return
    D-->>-C: return
   
   C-->>-B: return
    
    B-->>-A: return