No homework this week Stage 2 starts next week

Code review Team with N members is assigned N submissions to review Discuss submissions within team Everyone on team writes up one review Write-ups are done on an individual basis Due on Monday Stage 2 begins on Monday

Today Software design issues –In context of list definition Recursion –In context of list definition

Lists We’ve seen an array-based list implementation, the ArrayList. Advantage of an array-based implementation: –fast access to a specific index –typically less space usage than other options Disadvantage of an array-based implementation: –can be expensive to insert items –resizing is an expensive operation

Amortization Resizing of array-based data structures involves a tradeoff: –many insertions (as in our Bag) are very efficient since no allocation of space is required (an array is allocated as a big block of memory) –some insertions (as in our Bag) are very expensive since resizing must take place

Linked List A linked list is a list implementation which spreads out the cost of space allocation evenly to all insertions. Each insertion involves allocation of space

Advantages/Disadvantages Advantages –predictable cost of insertion –efficient insertion at any point in structure Disadvantages –extra space required to store links –inefficient indexing

Comparison of storage A pair has a first and a second element When pairs are used to construct lists, the first is called the “head” or the “car” the second is called the “tail”, “rest” or “cdr” the pair is called a “cons cell” or simply a “cons”.

Linked list implementations Textbook discusses a typical linked list implementation java.util.LinkedList is another typical implementation These implementations have procedural, not object-oriented, roots.

Traditional implementations provide a large number of methods – making it difficult to reuse if less functionality is desired; provide no means, beyond inheritance, to extend functionality to suit a specific situation; and adding functionality requires knowledge of internal structure of the list.

So… These implementations are inflexible, and do not exhibit good OO design. In lecture we will discuss a state-based list implementation which can be easily extended with new functionality.

Variant/Invariant decomposition Design principle which leads to cohesive and decoupled components That which is invariant is put into one component Variant properties/behaviors are factored out into separate components Compare this to what we saw with e.g. Observer pattern: –decoupling of event generation (invariant) from event handling (variant)

a list = a Linear Recursive Structure (LRS or LRStruct) What is a list? 1.the empty list is a list 2.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