Recitation

1
Recitation 8

• Godfrey Tan

2
Administrivia

• Please be succinct in your answers!
• PS5 due next Tuesday
• PS6 due Tuesday after Spring Break
• Form Project groups!
• 3-4 people
• Email me by next Tuesday
• Project to be posted right after Spring Break

3
PS3/4 Spec

• Never include implementation detail in the spec
  for public interfaces
• adds Edges to the HashMap
• Be specific.
• // removes edge connecting Nodes src and dst
• void removeEdge(Node src, Node dst)

4
PS3/4 Rep Exposure

• Class Graph
• HashMap nodes // keynode, valueedge list
• List getEdges(Node n)
• return (List) nodes.get(n) // die die
• List getEdges(Node n)
• return
• Collections.unmodifiableList((List)
  nodes.get(n))

5
PS3/4 Object Models

• Problem OM
• NO implementation details indicate WHAT abstract
  data are maintained
• Sets reflect the abstract data (say Nodes and
  Edges)
• Draw by examining specifications
• Code OM
• Refinement of POM
• Indicate important implementation details about
  HOW you maintain data (like Hashtable, ArrayList,
  )
• Draw by examining source code

6
Notions of Equivalence

• Behaviorally Equivalent
• If there is no sequence of (mutative) operations
  that can distinguish them
• Observationally Equivalent (similar)
• If no difference can be observed through observer
  operations

7
Behaviorally Equivalent equals

• Mutable types
• Can only be equals to itself
• StringBuffer s new StringBuffer(thing)
• StringBuffer t new StringBuffer(thing)
• s.equals(t) ?
• Immutable types
• If all the fields in both objects are equals
• String a thing String b thing
• a.equals(b) ?

8
Observationally Equivalent similar

• Mutable types
• If all the fields in both objects are similar
• StringBuffer s new StringBuffer(thing)
• StringBuffer t new StringBuffer(thing)
• s.similar(t) ?
• Immutable types
• If all the fields in both objects are similar
• String a thing String b thing
• a.similar(b) ?

9
Implementation

• public class MyObject extends Object
• MyObject(boolean mutable)
• ismutable mutable
• boolean equals(Object other)
• if(ismutable)
• // ?
• else
• // ?

10
Implementation

• public class MyObject extends Object
• MyObject(boolean mutable)
• ismutable mutable
• boolean equals(Object other)
• if(ismutable)
• return super.equals(other)
• else
• return similar(other)

11
Implmentation

• public class MyObject
• boolean similar(Object other)
• // check whether each field is
• // similar by value and return the
• // conjunction () of the results.

12
Implementation

• public class MyObject extends Object
• MyObject(boolean mutable)
• ismutable mutable
• int hashCode()
• if(ismutable)
• // ?
• else
• // ?

13
Implementation

• public class MyObject extends Object
• MyObject(boolean mutable)
• ismutable mutable
• int hashCode()
• if(ismutable)
• return super.hashCode()
• else
• // sum of each fields hashcodes

14
Remember!

• a.equals(b) ?
• a.hashCode() b.hashCode()
• This must be satisfied even when all the fields
  are primitive types

15
Weakest Precondition

• For each statement S with pre-assertion P
• Assume post-assertion Q is true
• (Substitute S in Q) wp(S,Q)
• Check P ? wp(S,Q)
• If false, we have trouble!
• Example
• Pre-assertion P y 0
• Code S x y 1
• Post-assertion Q x 1

16
Proof

• Example
• Pre-assertion P y 0
• Code S x y 1
• Post-assertion Q x 1
• Assume x 1
• Substitute x y 1
• y 1 1
• Check y 0 ? y 1 1
• TRUE

17
Loops P while b S Q

• Unrolled-loop analysis hard
• Solution find a loop invariant I s.t.
• Base Case P ? I
• For each iteration where b is true
• after b and S executed, I still holds
• When b is false, loop terminates
• I still true and Q is true

18
Example

• // remove all elements in the list equal
• // to the first element
• int i 1
• while(i
• if( list.get(i).equals(list.get(0)) )
• list.remove(i)
• else
• i
• What is the loop invariant?

19
Example

• // remove all elements in the list equal
• // to the first element
• int i 1
• while(i
• if( list.get(i).equals(list.get(0)) )
• list.remove(i)
• else
• i
• What is the loop invariant?
• for all 0 list 0

20
PS5

• Build a back-end for a Foliotracker application
  and a test application
• For PS6, you will have to build a front-end GUI

21
Components

• Portfolio
• Identified by name
• Contains stock positions
• Stock Position
• Identified by ticker symbol
• Contains of shares
• Price at which stock is bought (optional)

22
Requirements

• Must support multiple portfolios
• Data must be persistent
• Flexible so that a front-end application can use
  your API easily
• Get price for a stock
• polling vs callback vs on-demand
• Modifying Stock Positions (buy/sell)