1. Barbara Ericson ericson@cc.gatech.edu
AP CS Exam Overview
Barbara Ericson
Georgia Institute of Technology

2. Georgia Institute of Technology
Learning Goals
Understand the AP CS Exam
What are the details of the exam?
How is the exam graded?
What is on the exam?
Tips for taking the exam
Resources to use in studying for the exam
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Exam Details
Two Sections (3 hour exam)
40 Multiple choice questions (1 hour 30 min)
5 answers to each question (a – e)
Only one right answer
At least 5 questions on the case study
A Topics: oo, Java basics, cs terms, testing, design decisions
AB Topics: data structures, big-O, and more design
Free-response questions (1 hour 30 min)
4 questions (2-3 parts)
A Topics: arrays, strings, classes, interfaces, sorting, searching, Java library classes, design
AB Topics: 2-d arrays, data structures
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How the Exam is Graded
Total score is
score = multipleChoiceWeight * (# correctAnswers – 0.25 * #wrongAnswers) + freeResponseWeight * freeResponseScore
Equal weight to multiple choice and free response sections
Each free response question is worth 9 points (partial credit can be given)
Free response questions are graded according to a grading standard (rubric) by high school and college teachers
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5. Final Grade Calculation (1999)
Max score 80
(1.00 * MC * FR) AB
Max Score 100
(1.25 * MC * FR)
Composite
Score
AP Grade
60-80 5
70-100
45-59 4
60-69
33-44 3
41-59
25-32 2
31-40
0-24 1
0-30
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Exam Hints
Read the question before reading the code
Do the easiest questions first
If you read the question and have ruled out a couple of answers guess the answer
Don’t write tricky, non-standard code
If a question has parts answer the parts you can
Don’t cross out what you have if you don’t add anything new
Write neatly and indent properly
Follow Java conventions for names and use good names
Don’t worry about imports or comments
Don’t forget the return statement
Check and follow the pre and post conditions
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7. Primitive Variables vs Object Variables
Primitive variables are like sliders
Allocates space for the variable value
The value can be between a min and max based on the amount of space
Object variables are like a ticket to an event
Help you find your seat
ref
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Object Variables
Remember that declaring an object variable declares a reference to an object of that type
It doesn’t create the object
Example
String s; // declares a reference
s.length(); // throws a null pointer exception
s = “Hello”; // now it references a string
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9. Inheritance versus Association
“is a” versus “has a”
Use inheritance when the child is really “a kind of” the parent
A SlowFish “is a kind of” Fish
A Truck “is a kind of” Vehicle
Don’t use inheritance when the child can’t be substituted for the parent
A Wall isn’t a kind of Fish
A Vehicle isn’t a kind of Person
Use “has a” when one object has an object of another class associated with it
A Vehicle “has a” person associated with it (the owner)
A course session “has a” course associated with it
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Inheritance Test
Which is correct?
A high school is a kind of school?
An exam is a kind of question?
A marching band is a kind of school?
A dictionary is a kind of book?
A cat is a kind of animal?
You must be able to substitute the child for the parent
If I need a book will a dictionary do?
If I need a school will a marching band do?
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Inheritance
You can call inherited public methods directly
You can access inherited public fields directly
You can access inherited private methods indirectly
Through public methods
You can access inherited private fields indirectly
Through public accessors and modifiers
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Inheritance Example
public class ContactInfo {
private String name;
private String phoneNumber;
public ContactInfo(String theName, String thePhoneNumber)
this.name = theName;
this.phoneNumber = thePhoneNumber; }
public String getName() { return name; }
public String getPhoneNumber() { return phoneNumber; }
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13. Inheritance Example - continued
public class ExtendedContactInfo extends ContactInfo {
private String nickname;
public ExtendedContactInfo (String nickname,
String name,
String phoneNumber)
// missing code }
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14. Inheritance Example - continued
What can go in place of // missing code?
super(theName,thePhoneNumber); this.nickname = theNickname;
this.name = theName; this.phoneNumber = thePhoneNumber; this.nickname = theNickname;
this.nickname = theNickname; super(theName, thePhoneNumber);
this.nickname = theNickname; this.name = theName; this.phoneNumber = thePhoneNumber;
This is question 39.
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Inheritance
The child class extends the parent class
public class ChildClass extends ParentClass
No extends means extends Object
Objects of the child class inherit all the fields and methods of the parent class
But can’t directly access private fields or methods
Use public accessors and modifiers
Can invoke parent constructor to initialize
Use super(paramList) as first line in constructor
If none found, one will be provided (no-arg)
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Interfaces
An interface is a way two classes can communicate without worrying about what class each one is.
Just need to know the methods
Let’s you plug in different classes
As long as they implement the interface
Interfaces can only have abstract methods and constants in them
Declare an interface with
public interface Name
A class can implement more than one interface
public class Name extends Parent implements Interface1, Interface2, …
One interface can inherit from another
Actionable inherits from Drawable in the revised case study
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17. Explaining Interfaces
First point out what happens when you hardcode a class
Like in MBCS with Fish
Then show how interfaces let you add new classes
Like Wall in the revised MBCS
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Polymorphism
Ability to call a method based on the type of the object the method
In Java usually refers to inheritance-based
Method is based on parent class
All objects have a reference to their class
All objects know what class they are
All methods are invoked by checking with the class of the object first
No matter what it is declared as
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Polymorphism Example
See ShapePanel
A shape panel has a list of shapes
ShapeCanvas has a list of shapes
When we ask a shape to draw
It first checks with it’s class to see if it has a draw method and if so will execute that
So the correct shape is drawn
If Oval draw an oval
If Rectangle draw a rectangle
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20. Polymorphism Questions
Expect questions on what is the output when a method is called on a child object that is declared as a parent object
Remember that objects know what class they are
And all methods are resolved starting with the object’s class then going up the inheritance tree
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Static
Static really means exists on the object that defines the class
A class named Class
Objects all have a reference to their class
getClass() will return it
Static variables are shared by all objects of a class
Static methods can be called using
ClassName.method()
Static methods can’t access object fields
No access to a current object (no this)
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Run Time Exceptions
NullPointerException, indicating an attempt to reference an object using an object variable that is null
int[] gradeArray;
System.out.println(gradeArray[0]);
ArrayIndexOutOfBoundsException, indicating an attempt to access an element that is not within an array’s range of indexes
gradeArray = new int[10];
System.out.println(gradeArray[10]);
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Run Time Exceptions
ArithmeticException, such as division by zero for integers
System.out.println(9/0);
ClassCastException, which occurs when an attempt is made to cast a variable to a class that it does not match
List nameList = new ArrayList();
nameList.add(new Integer(5));
nameList.add(“Susan”);
String name = (String) nameList.get(0);
Students in AB should know how to throw exceptions like the IllegalArgumentException.
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24. Throwing Exceptions - AB
IllegalArgumentException, indicating an argument to a method that is not legal for that method
public void setUser(String user) {
if (user.trim().equals(""))
{ throw new
IllegalArgumentException("missing user name"); }
else { this.user = user; } }
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Summary
The A Exam covers
Basics: variables, loops, conditionals
Debugging: Runtime exceptions, types of errors, techniques
OO concepts: classes, objects, encapsulation, information hiding, interfaces, inheritance
CS concepts: sorting, searching, algorithms, numbers in different bases, one-dimensional arrays, pre- and post- conditions, assertions
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26. Georgia Institute of Technology
Summary
The AB exam covers
Data Structures
Two-dimensional arrays, linked lists, stacks, queues, trees, heaps, priority queues, sets, maps
Algorithms
Big-Oh notation, worst-case and average-case time and space analysis
Searching using hashing
Sorting using quicksort and heapsort.
Invariants
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