Allan Didier

Allan Didier

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Allan Didier > Mr. Didier’s Classes > AP Computer Science > AP Computer Science A Exam Info > AP Exam Review

AP Computer Science

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AP Exam Review

  • 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
  • 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 > 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
  • Loops
    • While
    • Do while
    • For
    • ForEach
  • 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. 
      • 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. 
  • 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.