AP Exam Review – Allan Didier

Rough Scoring Guide: AP Score : % correct
- 5 : 80%
- 4 : 60%
- 3 : 46%
- 2 : 36%
AP CS A exam content
Java Quick Reference Sheet
- This sheet will be provided to you during the exam.
Keywords and Concepts
- Public vs Private
  - public: visible outside the class (and to static methods)
  - private: not visible outside the class (or to static methods).
- Void vs non-void
  - void: does something but does not return anything
    - Ex: void setName(String newName);
  - non-void: returns a variable of the type declared in the method
    - Ex: String getName();
- Declare
  - stating a variable and the type of variable
  - Ex: int x; Account a;
- Instantiate
  - setting aside memory space for an object, i.e. running the constructor with the word “new”
  - Ex: Account a = new Account();
- Initialize
  - setting the initial value of an variable
  - Ex: int x = 9;
  - Object variables are often initialized in the constructor.
- Composition
  - Composed of (“has a”).
  - Ex: An array of integers. The array is composed of integers.
- Inheritance
  - Parent / child class relationships
  - Extends
- Polymorphism
  - The ability of objects to change types and be related by inheritance
  - Casting a parent class object as a child class.
- Overloading
  - Methods with same name but different parameters in the same class.
  - Ex: changeYear (int x); changeYear (String y)
- Overriding
  - Methods with the same name and parameters, but in parent/child classes. Child class methods override the parent class methods.
  - Ex: parent and child toString() methods
- Final
  - Once set, the variable value cannot be changed.
- Static
  - Variable: the variable has one value for all objects of that type.
  - Method: the method can be called without having to create an object of the class
  - Example
    - Math library methods (random, abs, sqrt, pow) are static. You can call them directly.
      - Math.pow(x,y);
      - Math.abs(x);
    - Scanner and Random library methods are not static. You have to create a Scanner or Random object and then call the method from that object.
      - Scanner scan = new Scanner();
      - scan.nextLine();
      - Scanner.nextLine() won’t work.
- Super
  - access the parent class method
- This
  - Refers to the class variable, not the local variable.
Binary Numbers
- Bits and bytes. Know basically what they are.
- You should not have to do any binary to decimal conversions.
Primitive Variables
- All lower case first letter. Upper case signifies an object.
- int: 4 bytes:
  - .MAX_ and .MIN_ Value
  - -2,147,483,648 to 2,147,483,647
- long: 8 bytes
- float (decimals): 4 bytes
- double: 8 bytes
- What happens when you go over the limit of an int or float?
  - int x = 2147483647 + 1; will not thrown an error.
  - x == -21474863648
  - I’ll explain this in class.
- boolean: true or false
- char: single quotes. Double quotes are Strings.
Math class
- .abs(): Absolute value
- .pow(x,y): x to the y power
- .random(): random double from 0 to 1
Strings
- Methods and Manipulation
  - Must know for the exam
    - .substring
    - .indexOf
    - .equals
    - .compareTo
  - Not on the Java Subset sheet, but you should know.
    - .concat
    - .contains
    - .ignoreCase and toLowerUpperCase
- Strings are immutable
  - You cannot change the value of a String once it is set. You can reassign the String variable to a new value, but you cannot change the value.
  - Won’t work
    - String name = “John”;
    - name.concat (“ Doe);
    - name + ” Doe”;
  - Will work
    - String name = “John”;
    - name = name.concat (“ Doe);
    - name = name + ” Doe”;
    - name += ” Doe”;
Operators
- Increment
  - By one: ++ (i++ or ++i)
  - By amount: +=
  - Multiply by amount *=
- Decrement
  - By one – – (i- – or – -i)
  - By amount -=
  - Divide by amount: /=
- Modulus, remainder, %
  - 16 % 3 == 1
Order of Operations
- (12 % 3 *3 + 2 * 3 / 8) * 4 = ?
Boolean logic
- Or ||
- And &&
- Equivalency ==
  - The single “=” means set equal to. They will dock points in the FRQ if you mix these up as it changes the logic.
- DeMorgan’s Law: The distributive property of not !
  - ! (a && b) == !a || !b
  - ! (a || b) == !a && !b
  - !(x > 7) == (x <= 7)
  - Example:
    !( (x > y) && (z == w) ) is equivalent to ( (x <= y) || (z != w) )
Random numbers
- Math Library
  - Math. random()
  - The only one needed for the AP Exam
  - Double from 0 – 1.
  - Multiply by a number to change the range.
  - Add a number to change the lower bound.
- Random library
  - Random rand = new Random();
  - int or float within a range
  - Won’t show up on MCQ, but you can use it on FRQ.
Case or Switch Statements
- String month;
- Switch (month){
  - case “January”:
    - do this;
    - break;
  - case “February”:}
Loops
- While
- Do while
- For
- ForEach
  - String[] weekday = [“Mon”, “Tues”, “Wed”];
  - for (day in weekday){
    do this;}
Variable scope
- Local vs. global (class) variables
- this. references the class variable
- Variables only exist within the area (scope) in which they are defined. Example:
  - int x = 9;
  - void doThis (int x) { x ++; }
  - doThis (x);
  - System.out.print (x);
  - x == 9 because the print statement is outside the method. The x in the doThis method == 10, but this never actually changes the x outside the method.
Primitive vs object variable changing
- x=y, change y, print x
  - Primitives and Strings don’t change x (even though Strings are objects, they are immutable)
  - Objects, arrays, and ArrayLists will change x
Error Messages and Exceptions
- ArithmeticException
  - division by zero
- NullPointerException
  - Trying to use methods in an object that has not been initialized or instantiated.
  - Ex: Account b; b.withrdraw (amount);
- ArrayIndexOutOfBoundsException
  - Trying to access a location in an array that does not exist.
- IndexOutOfBoundsException
  - Trying to access a location in an arrayList or other object that doesn’t exist.
- IllegalArgumentException
  - Passing in the wrong variable type into a method.
  - Ex. void setValue(int x){}; setValue(“zero”);
- ConcurrentModificationException
  - Trying to modify an object when it is not allowed.
  - Ex. Removing elements from an ArrayList while using a ForEach loop.
- NaN
  - Not a Number
  - Square root of negative number, etc.
Other Classes
- Wrapper Class
  - ArrayLists cannot use primitive variables.
  - The Wrapper classes are the object version of primitives. They use a capital letter.
  - Integer, Float, Double, Boolean, Character, etc
  - Strings don’t have a Wrapper class because they are objects, not primitive variables.
- Object Class
  - The parent of all classes.
  - This may show up in the MCQs.
- List Class
  - The parent of ArrayLists. (Technically an Interface that inherits the Collections class.)
  - Again, this may show up in the MCQs.
Searching and Sorting
- You will not have to do searching or sorting for the FRQ. These will only show up in the MCQ.
- The main questions directly regarding these will be efficiency questions.
  - Linear search is best for searching unsorted lists.
  - Binary search is best for searching sorted lists.
  - Selection sort is good for quickly finding the greatest/least items in an unsorted list.
  - Insertion sort is good at adding items into an already sorted list.
- Merge sort is a recursive sorting algorithm that you should know about, but not know how to code. It is more efficient than Selection or Insertion sort, but much more complicated. I’ll explain it in class.
Free Response Question Tips
- Focus on the logic not the semantics.
- You only need to know and use the Java code covered in this class.
- You can use Java code not covered in this class, as long as it works. You don’t get any extra credit for it, though.
- You don’t get docked for inefficient code, only code that doesn’t work.
- You must use Java code, not Python, C++, or any other language. You may get docked if you use code from other languages or just writing logic.
- If you can’t write the && symbol, try your best and write the word “and” above your symbol or comment your code so they can understand your logic.