SE450: Overriding: Method Dispatch (aka Method Resolution) [13/35]

When an overridden method is invoked, we have to decide which implementation to run. There are two ways to do this: based on the static or dynamic type.

Recall

static refers to properties of the text of a program (synonyms are lexical, compile-time and early).

dynamic refers to properties of an execution of a program (synonyms are run-time and late).

declared type = compile-time type.
actual type = run-time type.

Both forms of method resolution proceed up the class hierarchy, using the superclass relation. In pseudo-code:

lookup(C,m)
  // return the address of code for method m
  if (C implements m) then
    return address of C.m
  else
    let B = the superclass of C
    return lookup(B,m)

Procedurally: if m is implemented in C then return the address of C.m else set C to be the superclass of C and repeat.

Static dispatch (early binding): start lookup with the declared type.

Dynamic dispatch (late binding): start lookup with the actual type.