Inheritance and Lookup

4: doesNotUnderstand: aMessage

Goal

• A hook message is sent when the lookup fails
• Classes can customize the reaction to this failure

Message Sending

Sending a message is a two-step process:

1. look up the method matching the message
2. execute this method on the receiver

When No Method is Found

• if no method is found in the hierarchy, doesNotUnderstand: is sent to the receiver
• the initial message is passed as parameter

When No Method is Found

Step by Step...

node1 coucou: #stef

1. coucou: looked up in Node
2. not defined in Node -> continues in Object
3. not defined in Object -> send doesNotUnderstand: to node1
4. doesNotUnderstand: looked up in Node
5. not defined in Node -> continues in Object
6. doesNotUnderstand: is found and executed

doesNotUnderstand: is a Message

• doesNotUnderstand: is a message
• any class can implement the corresponding method
• used for automatic delegation, proxy, distribution

doesNotUnderstand: and the Debugger

When no class redefines doesNotUnderstand:

• • doesNotUnderstand: in Object is executed
  • this method raises a MessageNotUnderstood exception
  • when not handled, this exception opens a debugger

Catching MessageNotUnderstood Error

MessageNotUnderstood is just an exception, code can handle it:

Node >> sayHello
  self coucou: #stef

Node >> welcome
  [ self sayHello ]
  on: MessageNotUnderstood
  do: [ Transcript show: 'Something went wrong with a message' ]

What You Should Know

• The lookup sends doesNotUnderstand: when it can not find a method
• Each class can implement this method
• The default implementation of doesNotUnderstand: raises an exception that can be handled