Inheritance and Lookup

2: Lookup

Goal

• Understanding
  • message sending
  • method lookup
  • semantics of self

Inheritance

• Inheritance of state is static
• Inheritance of behavior is dynamic

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

Method Lookup

The lookup starts in the class of the receiver then:

• if the method is defined in the class, it is returned
• otherwise the search continues in the superclass

Some Lookup Cases

Sending the message color to aColoredRectangle

Some Lookup Cases

Sending the message area to aColoredRectangle

self Always Represents the Receiver

A new foo
> ...
B new foo
> ...

self Always Represents the Receiver

A new foo
> 10
B new foo
> 50

What is self/this?

Take 5 min and write the definition of self (this in Java).

• your definition should have two points:
  • what does self represent?
  • how is a method looked up when a message is sent to self?

self/this

• self represents the receiver of the message
• self in Pharo, this in Java
• The method lookup starts in the class of the receiver

self Always Represents the Receiver

A new bar
> ...
B new bar
> ...

self Always Represents the Receiver

A new bar
> 10
B new bar
> 50

self Always Represents the Receiver

    B new bar
    > 50

Evaluation of aB bar

1. aB's class is B
2. no method bar in B
3. look up in A - bar is found
4. method bar is executed
5. self refers to the receiver aB
6. foo is sent to self
7. look up foo in the aB's class: B
8. foo is found there and executed

self Always Represents the Receiver

    A new bar
    > ...
    B new bar
    > ...
    C new bar
    > ...

self Always Represents the Receiver

    A new bar
    > 10
    B new bar
    > 10
    C new bar
    > 50

What You Should Know

• self always represents the receiver
• Sending a message is a two-step process:
  1. Look up the method matching the message
  2. Execute this method on the receiver
• Method lookup maps a message to a method
• Method lookup starts in the class of the receiver
  • ...and goes up in the hierarchy