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Review for the AP CS Exam

Object Oriented Program Design– Program Design

• Read and Understand class specifications and relationships among the classes(has a and is a relationships)

• Decompose a problem into classes, define relationships and responsibilities of those classes

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– Top down approach• Make a list of all of the things that a program must

accomplish• Each of those things becomes its own module

(perhaps a method or perhaps a class with several methods)

– Bottom up approach• Starts with specific, detailed way of achieving

something(ie recursive area fill or interacting with a piece of hardware)

• Builds program up around that detail

Program design

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Class Design• Design and implement a class• Design an interface (remember that an interface

has only methods, no data)• Extend a class using inheritance (Design question)• Remember that any class that implements an

interface must implement all methods in the interface

• Use extends and implements appropriately• declare constructors and methods with

– meaningful names– appropriate parameters– appropriate return types (remember constructors do not

have return types)

Class design

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Class Design• appropriate declaration of methods as private or

public• all data should be private

Class design

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1. Program ImplementationA. Classes that fill common needs should be

built so they can be reused by other programs. OO design is an important part of program implementation

B. Implementation Techniques1. Encapsulation and information hiding

2. Procedural abstraction

Program Implementation

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C. Programming constructs1. Primitives vs references

1. Remember that references must be associated with an object

2. Beware of aliasing:when two references point to the same object

Program Implementation

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Here is an explanation from borland’s web site;

D. Round-Off Problems : 1. One of the problems with floating point numbers is round-

off. Round off errors occur when attempting to represent certain numbers in any number base. For example, 1/3 is not exactly representable in base ten, while 1/10th is easily representable. But since we're dealing with computers, we are specifically in base two numbers. As opposed to base ten, 1/10th is not exactly representable in base two. For example, the fractional portions of base two are: 1/2 1/4 1/8 1/16 1/32 1/64 1/128 1/256 1/512 The numbers 1/2, 1/4, 1/8, all powers of two, are exactly representable in a computer. But since 1/10 lies between 1/8 and 1/16, it is not exactly representable using binary notation. So internally the computer has to decide which fractional binary portions to add together to sum close to 1/10. For example: 1/2 1/4 1/8 1/16 1/32 1/64 1/128 1/256 1/512 0 0 0 1 1 1 0 0 0 this adds up to: 0.1093 which is close to 0.1000 but could easily be rounded to .11 so the computer internal algorithm must try to find another combination of binary fractions which come closer to 0.1000 When it's internal algorithm is satisfied, it will have a number which is CLOSE to 1/10th but not EXACT. This inexactness is known as ROUND-OFF error.

Floating Point Number Probs

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E. Constant declarations1. final double WIDTH = 1.1619;

2. Use these whenever you are using a numeric value that you type several places in a program

F. Variable declarations5. Scope of variables

1. Shadowing problem: Most local variable gets precedence

Constant Declarations

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G. Method declarations1. A method’s signature consists of name,

parameter types and order of parameters2. Overloaded methods have the same name but

different parameters3. Constructors are often overloaded4. What is a static method?a method that does not

require an instance of the class to be created5. What is a private method?accessible only to class

member methods

Method declarations

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H. Interfaces1. A collection of methods that a class must

implement2. An interface you should know

a)interface java.lang.Comparable

int compareTo(Object other)// return value < 0 if this is less than other// return value = 0 if this is equal to other// return value > 0 if this is greater than other

Interface Declarations

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Example

Double x = new Double(1.618);

Double y = new Double(3.14159);

//Write a statement that compares x to y and prints out “phi is less than pi”

if (x.compareTo(y) < 0)

Interface Declarations

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I. Math ops

1. +,-,/,*,%

2. two types of division

3. type casting (int) and (double)

4. Students are expected to understand "truncation towards 0" behavior as well as the fact that positive floating-point numbers can be rounded to the nearest integer as (int)(x + 0.5), negative numbers as (int)(x - 0.5)

5. Math.abs(double x), Math.sqrt(double x), Math.pow(double base, double exp)

Math Operators

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J. Control techniques1. Methods2. break; //breaks out of nearest ________3. conditional (if, if else, switch case optional)4. iteration (for and while only required)5. recursion

a. Powerful technique when a method invokes itself

b. Must have some sort of stopping or base case

c. Remember to use McCarthy method or something similar to trace

d. many method calls decrease efficiency

Control

For each loop used heavily

The for each loop will probably be used heavily in the exam– ArrayList<String> songs = some list– Write a for each loop to print the songs

– int[] nums = some array of integers– Write a for each loop to find the average as a

double

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Program Analysis– Testing/Debugging

• Test Libraries and Classes in isolation• Identify boundary cases

– Understand error handling• ArithmeticException

– Divide by zero• IndexOutOfBoundsException• ClassCastException

– Attempt to cast to subclass of which it is not an instance

• NullPointerException– Needed an object, got the shaft!!!

• IllegalArgumentException

Program Analysis

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Algo Analysis– Big Oh Notation– Asymptotic growth limit– O(n) linear– O(n2) quadratic (two dim array, nested for)– O(logn) divide and conquer(assume base 2, this is

comp sci man!!)– O(1) constant time– O(nlogn) better sorting algo’s including logn= find

where this element belongs and n to move each element into position

Analysis of Algorithms

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Analysis of AlgorithmsData Search Insert

(once the place is found)

Rem Get first

Array Unordered

Array Ordered

ArrayList

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int : 32 bit signed integer double: 64 bit signed floating point boolean: true or false

Data Structures

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Classes and Class Hierarchies

Remember chess piece example abstract base class with abstract method

isValidMove() Children must redefine this method Example of dynamic binding or late

binding Calls to isValidMove are _____________

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Classes and Class Hierarchies

“Cast down the hierarchy” Ok to cast from Object to Integer assuming class

is an Integer Ok to cast from piece to pawn if you know the

piece is a pawn Children inherit all methods and classes (but

cannot play with ______________) Also, static methods and vars are one per class

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One/Two dimensional arrays

Once created a certain size, fixed use .length, the member var Use [ ] to access elements starting at 0 …length-1 0 and <length works for boundaries Use array[0].length for column dimensions Use r and c for loop control vars for ease of

reading Must create individual objects if array of

references

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ArrayList

int size() boolean add(Object x) Object get(int index) Object set(int index, Object x)

// replaces the element at index with x // returns the element formerly at the specified position

void add(int index, Object x) // inserts x at position index, sliding elements // at position index and higher to the right // (adds 1 to their indices) and adjusts size

Object remove(int index) // removes element from position index, sliding elements // at position index + 1 and higher to the left // (subtracts 1 from their indices) and adjusts size

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Stacks

______________ data structures Good when you need to untangle some

type of operations that have to be suspended and returned to

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Stack interface

public interface Stack { // postcondition: returns true if stack is empty,

false otherwise boolean isEmpty(); // precondition: stack is [e1, e2, ...,

en] with n >= 0 // postcondition: stack is [e1, e2, ..., en, x]

void push(Object x); // precondition: stack is [e1, e2, ..., en] with n >= 1 // postcondition: stack is [e1, e2, ..., e(n-1)]; returns en // throws an unchecked exception if the stack is empty

Object pop(); // precondition: stack is [e1, e2, ..., en] with n >= 1 // postcondition: returns en // throws an unchecked exception if the stack is empty

Object peekTop(); }

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GridWorld GridWorld is an object oriented universe where

Actors can be called on to act() each step public Color getColor() { } public void setColor(Color newColor) { } public int getDirection() { } public void setDirection(int newDirection) { } public Grid getGrid() { } public Location getLocation() { } public void putSelfInGrid(Grid gr, Location loc) { } public void removeSelfFromGrid() { } public void moveTo(Location newLocation) { } public void act() { } public String toString() { }

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GridWorld Location represents a row, col (y,x) in the

grid For convenience, Location also has

constants set up to handle the various directions

use getRow() and getCol() to get the y and x locations resepectively

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GridWorld public static final int LEFT = -90; public static final int RIGHT = 90; public static final int HALF_LEFT = -45; public static final int HALF_RIGHT = 45; public static final int FULL_CIRCLE = 360; public static final int HALF_CIRCLE = 180; public static final int AHEAD = 0; public static final int NORTH = 0; public static final int NORTHEAST = 45; public static final int EAST = 90; public static final int SOUTHEAST = 135; public static final int SOUTH = 180; public static final int SOUTHWEST = 225; public static final int WEST = 270; public static final int NORTHWEST = 315;

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GridWorld A BoundedGrid implements the Grid Interface It uses a ____________________ called

______________to store the elements private Object[][] occupantArray; // the array storing the

grid elements getNumRows() and getNumCols() will handle the

dimensions

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GridWorld A BoundedGrid implements the Grid Interface It uses a ____________________ called ______________to store

the elements private Object[][] occupantArray; // the array storing the grid

elements getNumRows() and getNumCols() will handle the dimensions isValid(Location loc) is an important method to know public boolean isValid(Location loc) { return 0 <= loc.getRow() && loc.getRow() < getNumRows() && 0 <= loc.getCol() && loc.getCol() < getNumCols(); }

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GridWorld public E get(Location loc) { if (!isValid(loc)) throw new IllegalArgumentException("Location " + loc + " is not valid"); return (E) occupantArray[loc.getRow()][loc.getCol()]; //

unavoidable warning } public E put(Location loc, E obj) {} public E remove(Location loc) {}

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GridWorld Critters! public void act() { } public ArrayList getActors() {} public void processActors(ArrayList actors) { } public ArrayList getMoveLocations() { } public Location selectMoveLocation(ArrayList locs) {} public void makeMove(Location loc) { }

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Strings class java.lang.String implements java.lang.Comparable int compareTo(Object other)

// specified by java.lang.Comparable boolean equals(Object other) int length() String substring(int from, int to)

// returns the substring beginning at from // and ending at to-1

String substring(int from)// returns substring(from, length())

int indexOf(String s)// returns the index of the first occurrence of s; // returns -1 if not found

Keep in mind substring first you want, first you don’t want. Also, there is a substring with only one argument, starts at that

char and goes to end substring probably a better idea that charAt, since chars not in

subset

Math.random()

All of the random() calls are made using Math.random()

This gives you a number between _______ and ________Example, use Math.random() to generate a # for choosing and element from an ArrayList called locations

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Sorting Quadratic Sorts O(n2)

– Selection• Choose the maximal or minimal element by a loop and then swap

that with the current place you are moving through• Minimizes swaps

– Insertion• Pick an element, shift everyone out of the way to make room for

it (Church pew shuffle)– Bubble

• Nested for loops allow one element to “bubble” to the front or back each time

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Sorting nlogn Sorts

– Merge• Start breaking up your list into smaller lists and merge them

together• Requires temporary storage area of size n for merging into

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SortingSort Best Average WorstSelection

Insertion

Merge

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Thats all folks!!!!• Neo journeyed to the machine city and faced off with Smith• Frodo took the ring to the Mountain of Doom• On May 3, you will accomplish your mission, the APCS exam• It is your purpose!