AP CSA (Java) Key Ideas

Unit 1: Using Objects and Methods

IDE (Integrated development environment) provides tools to write, compile, and run code
compiler: converts human code to binary & detects errors
errors: syntax (language used incorrectly), logic, run-time (eg. Null Pointer), exception (not detected by compiler and interrupts execution, eg. Arithmetic Exception from division by 0 or IndexOutOfBoundsException)
escape: \” and \\ and \n
variable names: letters, digits, and underscores
- no: beginning with digit, including spaces or reserved words
- final goes directly before variable type (eg. public static final double WEIGHT = 49.9;)
casting: (double) (12 / 5) = 2.0 and (double 25 / 4 = 16.25
integer overflow wraps around (logic error)
rounding: (int) (x + 0.5) for positive; (int) (x – 0.5) for negative
procedural abstraction: tell method job without code

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class methods and attributes are static (associated with class)
- static methods call by ClassName.methodName();
- bad practice to call static methods on objects (will run theoretically, but marked wrong on exam)
when in same class, can just call name
public attributes can directly call with period after class or object, private need getter/setters

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example of using split: String[] fruits = sentence.split(“,”);
- ^ does not remove whitespace
not on exam, but: .toUpperCase() & .toLowerCase()
cannot call method on null object
inheritance: public class Dog extends Animal {}
- has attributes & methods from main class
- subclasses may have own methods (override main)

algorithms incorporate sequencing, selection (if), and repetition (for, while)
when there is && or || in if statement, order of the conditions matter
- checks first one and then second
- first one must check not null so that second condition does not run (would result in NullPointerException)
De Morgan’s Law
- !(a && b) = !a || !b
- !(a || b) = !a && !b
== compares reference IDs or primitive values
- must use .equals() for Strings and other reference objects
- many classes define their own .equals() methods
roundoff error may result in “Mathematical Inconsistency”
default boolean = false, int = 0
“Off by one error” results in value one less or more than target (common in loops, check operators carefully)
- OutOfBoundsException when accessing nonexistent index

abstraction: attributes and method signatures, simple presentation through diagrams or natural language
data abstraction: simple understanding of data attributes
procedural abstraction: understanding function of methods
encapsulation; private attributes and public methods for safety
maximize system reliability (running expectedly without failure) by testing on various conditions and more users
publish open source code to avoid intellectual property concerns
default values for arrays: 0 and 0.0 and false and null

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when primitive types or Strings passed in as parameter, methods cannot change it
setting object variable to other object points it to same reference so they are the same thing (changes to one changes both)
- works for arrays and objects too but not Strings or ints
FRQ #2 classes: does not matter if variables are initialized in the method or in class declaration

algorithmic bias: systematic prejudice manifested from skewed data training or flawed design
array.length; and ArrayList.size()
values in array are default in not set
can traverse arrays with for, while, or enhanced for-loops
- enhanced: for (String s : alphabet) {}
- ^ statements using s inside enhanced loop cannot modify it
create file object and use Scanner on it:
- File file = new File (“students.text”);
- Scanner scan = new Scanner (file);
- while (scan.hasNextLine){
  - String line = scan.nextLine();
  - System.out.println(line);}
- scan.close() after done with file
hasNext() checks item (separated by whitespace, while hasNextLine() checks entire line
scan.hasNextLine() does not advance the scanner
import java.util.io and then heading public method (parameter) throws IOException{} so that program stops if file is invalid

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array and ArrayList are mutable, meaning can change when passed into method parameter
int / Integer and double / Double
- autoboxing: automatically to wrapper class
- unboxing: automatically to primitive type
- wrapper class objects can be used in ArrayList
- happens automatically in the background, usually won’t cause errors unless:
  - unboxing null object
  - trying to compare value of wrapper objects with == instead of .equals()
parsing (on reference sheet) converts String to an int or double
int[][] array = new int[4][5]
ArrayList<Type> al = new ArrayList<Type>();
- or just <>();

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linear search: checks each element to see if matches target
binary search: has min and max and checks if mid is the target, otherwise changes bounds
- mid = (min + max) / 2 so left has more
- binary more efficiency than linear, unless array length is very small
selection sort: look for smallest, then swap
- inefficient because needs to go through every
insertion sort: move element forwards while is less
merge sort: recursively divide array in half, then combine to a sorted version
- left array will have more if odd number of elements
- go through logic of merge sort (tracing merge sort steps will likely be on MCQs)

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complexity O(n): how many executions of computing need to run program
- eg. complexity of O(4n)
recursion: need base case
- if infinite calls because never reaches base case, throws CallStackOverflowException
- keep track of commands, especially if they come after the recursive call