a list = a Linear Recursive Structure (LRS or LRStruct)

a list = a Linear Recursive Structure(LRS or LRStruct) • What is a list? • the empty list is a list • a pair whose tail is a list is itself a list • This is a recursive definition! (1) is called the base case, and (2) is called the recursive case. • Note that traditional implementations do not follow this precise definition of what a list is: • many have no explicit representation of an empty list • none are recursive on the list; instead they recurse on a list node • This has implications for how the structure can support extension (see Visitor support, in later slides)

States • A list can therefore be in one of two states: • empty (corresponding to the base case) • non-empty (corresponding to the recursive case) • The state-based implementation we will study makes this distinction explicit in the representation

Empty vs. NonEmpty state(look Ma, no NullPointerException!) • An LRS object delegates all calls to its LRS State object – which can respond to all messages. • Empty and NonEmpty states respond differently. • There is never a null pointer in the structure! (Think about the implications of this for a while.)

What is basic (invariant) list functionality? • insert new item at front • remove item from front • set/get first item (head) • set/get rest (tail) • plus (in Java) methods inherited from Object (toString, equals, etc)

How are these methods defined? public LRStruct<E> insertFront(E item) { return _state.insertFront(this, item); } public E removeFront() { return _state.removeFront(this); } public LRStruct<E> setDatum(E item) { return _state.setDatum(this, item); } public E getDatum() { return _state.getDatum(this); } public LRStruct<E> setRest(LRStruct<E> rest) { return _state.setRest(this, rest); } public LRStruct<E> getRest() { return _state.getRest(this); }

LRStruct delegates to State! • All of these methods delegate to the state of the LRStruct. • States polymorphically determine what happens.

a An empty LRS • LRS a = new LRS(); LRS Empty _state

a Inserting an item into an empty LRS • a.insertFront(“fred”); LRS Empty _state

Empty a Inserting an item into an empty LRS • a.insertFront(“fred”); LRS _state

Empty a _state Inserting an item into an empty LRS • a.insertFront(“fred”); LRS _dat=“fred” _tail NonEmpty

Empty a _state _state Inserting an item into an empty LRS • a.insertFront(“fred”); LRS _dat=“fred” _tail NonEmpty LRS

a list = a Linear Recursive Structure (LRS or LRStruct)