CS61B L06 Control Structures (1)Garcia / Yelick Fall 2003 © UCB  Dan Garcia ( Kathy Yelick  ( inst.eecs.berkeley.edu/~cs61b/ 1 Handout: notes Computer Science 61B Data Structures and Advanced Programming Lecture 9 – Arrays and Invariants

CS61B L06 Control Structures (2)Garcia / Yelick Fall 2003 © UCB Where is Dan? Hurricane Isabel No office hours this week for Dan

CS61B L06 Control Structures (3)Garcia / Yelick Fall 2003 © UCB Vectors Review Vectors are objects that look like: The boxes can only hold references to objects –Not 1, 2.3, or other basic types. –Can mix objects: 1 Vector may contain Accounts, Motes, Vectors… Elements are numbered from 0 to size-1 Vectors are a mutable type –The objects can be replaced by other objects –Vectors can grow and shrink In Java library: import java.util.*;

CS61B L06 Control Structures (4)Garcia / Yelick Fall 2003 © UCB Object: The Universal Parent Java is an object-oriented language –One type inherits from another –You can use the subtype (child type) anywhere the supertype (parent type) is expected –Given a Vector v of >=2 elements: »Object obj = v.get(1); is legal »obj.intValue() is a compile-time error »(Integer obj).intValue() compiles, but is a runtime error if obj is not really an Integer »obj.toString() is always legal … more about this in a couple of weeks Object Account Vector array Integer obj ?

CS61B L06 Control Structures (5)Garcia / Yelick Fall 2003 © UCB Arrays are Similar Arrays are objects that look like: The boxes can hold references to –Objects or primitives, e.g., 1, 2.3 –But type of all primitive elements must be the same The boxes are numbered from 0 to size-1 Arrays are a mutable type –The elements can be replaced by other elements –But, they cannot grow and shrink Arrays are built into the Java language, not in the library, so no need to “import”

CS61B L06 Control Structures (6)Garcia / Yelick Fall 2003 © UCB Creating Arrays Declaration uses [] and element type: int [] intArr; can contain int values Account [] acctArr; can contain “Account” refs, or nulls Object [] objArr; can contain any Object ref, or nulls To create array object, use new intArr = new int[3]; acctArr = new Account[2]; objArr = new Object[1]; intArr acctArr01 objArr

CS61B L06 Control Structures (7)Garcia / Yelick Fall 2003 © UCB Using Arrays One can get/set elements using [] syntax: intArr[0] = 0; // don’t rely on defaults for now intArr[1] = 2; intArr[2] = 4; int y = intArr[1]; As with Vectors, need to have the object before using it: int [] a2; a2[0] = 2; // error – no array allocated yet Shorthand for initialization (only with decl): int [] a3 = {2, 4, 6}; a2 = {1, 2, 3}; // not allowed

CS61B L06 Control Structures (8)Garcia / Yelick Fall 2003 © UCB Types and Arrays Arrays element types must match declaration: intArr[1] = new Account(100); // compiler error intArr[1] = new Integer(1); // compiler error But you can have an array of Objects Object [] objArr = {new Account(100), new Integer(1)}; objArr[0] = new Integer(2); objArr01 myBalance x

CS61B L06 Control Structures (9)Garcia / Yelick Fall 2003 © UCB Multidimensional Arrays A multidimensional array in Java is an array of arrays: int [][] x = new int [3][2]; int [][] y = new int[2][]; y[0] = new int[2]; y[1] = new int[1] y[1][0] = 3; x y 1 0 3

CS61B L06 Control Structures (10)Garcia / Yelick Fall 2003 © UCB “Growing” and “Shrinking” Arrays Once an array is created, its size does not change However, we can copy smaller ones to larger ones int [] myArr = {3, 6, 5}; //... use myArr, but what if it’s now too small intArr tmpArr201 x myArr int [] tmpArr = new int [2*myArr.length + 1]; for (int k = 0; k < myArr.length; k++) { tmpArr[k] = myArr[k]; } // rest of tmpArr uninitialized (default 0) myArr = tmpArr; See also System.arrayCopy

CS61B L06 Control Structures (11)Garcia / Yelick Fall 2003 © UCB An Aside: Understanding main Recall the signature of the main method: public static void main (String [] args) This says main takes an array of Strings E.g., public static void main (String [] args) { System.out.println(args[0]); int i = Integer.parseInt(args[1]); System.out.println(“args[1] = “ + i); } As with reading from System.in, more error checking should be used.

CS61B L06 Control Structures (12)Garcia / Yelick Fall 2003 © UCB Administrivia Reading assignments –For Wednesday : Liskov 10 and –For Friday: ADW Chapter 8 CSUA Help session for advanced Unix postponed. HW #3 due 11:59pm –Correction in hw3.html (api was OK). »See hw3/Errata.txt (similarly for proj1) Quiz in 1 week1: Monday, –In-class, open book/notes, last2 slides contains topics Project #1 handed out on Friday, due ~2 wks

CS61B L06 Control Structures (13)Garcia / Yelick Fall 2003 © UCB Designing Data Abstractions Major questions to answer: –How does the abstract object behave to the outside world? (the specification) –How do we represent the data internally in our implementation? (the implementation) –What properties do we enforce on the state of our internal representation? (representation invariant) –How does a concrete value map to an abstract one? (abstraction function) Project 1: design a Landlord class –Specification: given »Do not add public methods that your tester tests –Implementation: Use a Vector of plots »Options: store “rented” or “vacant”, sorted?

CS61B L06 Control Structures (14)Garcia / Yelick Fall 2003 © UCB Representation Invariant Representation invariant: a property of the concrete (implementation) object that holds on entry/exit of all methods in the class –What values may private instance variables have? –Methods assume the invariant holds on entry –Methods that change the instance variables must ensure the invariant still holds when they exit Why use a representation invariant? –Allows us to reason about the correctness of each method in isolation –May make some method faster or easier to write –Self-discipline: you make up and enforce the rules

CS61B L06 Control Structures (15)Garcia / Yelick Fall 2003 © UCB Representation Invariant - Fraction Recall: Fractions are immutable (in the abstract), and toString() shows in reduced form Possible invariants for the Fraction class: –weak invariant: myDenom != 0 »toString() can’t assume anything, always must reduce –weak invariant with benevolent side-effects »Same as above, except toString() replaces instance variables with reduced form after calling gcd() »“Benevolent” because it does a good thing Future calls to gcd will be very fast The side effect is not user-visible –strong invariant: always store in reduced form »myDenom != 0 AND gcd(myNumer, myDenom) == 1

CS61B L06 Control Structures (16)Garcia / Yelick Fall 2003 © UCB Abstraction Function Expresses the “meaning” of the values in the object’s instance variables as a whole –Tells us how interpret the concrete state of the instance variables to recover the abstract object they represent This Fraction object represents the abstract rational number ½ : So does this one: Abstraction functions are often many-to-one –Several concrete states may represent the same abstract object The abstraction function for Fraction is division: –AF(c) = c.myNumer / c.myDenom Do 3 different implementation have different AFs? 1 myNumer 2 myDenom 4 myNumer 8 myDenom 1212

CS61B L06 Control Structures (17)Garcia / Yelick Fall 2003 © UCB Representation Invariant – repOk() Often a good idea to include a “self-test” method that checks whether the invariant holds: true if the object representation * is in a legal state satisfying the invariant, * otherwise return false */ public boolean repOk() { return ((myDenominator != 0) && (gcd(myNumer, myDenom) == 1)); } When to call repOk? –When invariant may be broken: at the end of all constructors and methods that change the object (mutators) as a self-diagnostic –Also on entry to methods that rely on the invariant Since check may be costly, may turn it off in “released” version of code using: assert repOk(); // more in next lecture

CS61B L06 Control Structures (18)Garcia / Yelick Fall 2003 © UCB Representation Exposure A well-designed data abstraction can guarantee its representation invariants –Variables are usually private to ensure invariants User may still be able to violate the invariants –Representation exposure: giving the user of an abstract data type access to the internals, allowing them to violate representation invariants If all fields are private, how does this happen? –Taking mutable objects from user and putting them into the internal state –Returning mutable parts of the state

CS61B L06 Control Structures (19)Garcia / Yelick Fall 2003 © UCB PRS Question The following code for proj1 has as an invariant: Plots in myPlot (which store the beginning/size) do not overlap How many “lines” need to be changed to ensure the user cannot break the invariant? 1: none 2: 2 3: 2&3 4: 2&4 5: 3&4 6: 2-4 7: 2-5 1)public class Landlord { 2) public Vector myPlots; 3) Landlord (Vector initP) { myPlots = initP; } 4) Vector getPlots () { return myPlots; } 5) Plot plotAt(int i) { return myPlots.get(i); } 6) }

CS61B L06 Control Structures (20)Garcia / Yelick Fall 2003 © UCB Data Abstraction Summary Data Abstraction is a powerful programming tool –Provide an abstract data object and a set of useful operations to the outside world –Hide the implementation details Representation Invariant –Rules about how the internal variables are managed –Include a repOk() method that checks whether any of the rules have been violated at run time Abstraction Function –Describes the mapping from your concrete representation using instance variables to the abstract object your class represents Benevolent side effects –Modifications of internal state that are not externally visible

CS61B L06 Control Structures (21)Garcia / Yelick Fall 2003 © UCB Announcements - Quiz Open book/open notes Topics: –Basic Java 1.Writing and using classes and methods 2.Conditionals, loops, and recursion 3.Parameter passing 4.Primitive values vs. objects (and references to them) 5.public and private 6.static and non-static 7.Exceptions 8.Vectors/Enumerations Note: repOK and loop invariants will not be on this modifies”) –Testing

CS61B L06 Control Structures (22)Garcia / Yelick Fall 2003 © UCB Announcements - Quiz Review session –Tuesday or Wednesday evening: watch newsgroup for details Exam questions often use examples you have seen. –We will include the code, but you will have time problems if you’re seeing it for the first time. Examples from labs and homeworks (through lab 4 and homework 3): –Account –Fraction –StringFormat (hw) –StringCheck (lab) –IslamicDate –Vector and Enumeration (lab) Solutions for labs and homeworks online