Unit 6 Arrays

The unit I taught

Unit 7 ArrayList

Can change size
A class with many methods
Not designed to store primitives, they store object references
Slightly slower than arrays

7.1 Introduction to ArrayList

Primitive Data Types:
- Boolean
- Char
- Double
- Int
Wrapper Class Data Types
- Boolean
- Character
- Double
- Integer

import java.util.ArrayList; //you must import the java.util package

// Instantiating: ArrayList<DataType> variableName = new ArrayList<>(n);
//DataType must be nonprimitive data type

public class introArrayList {
    public static void main (String[] args) {
        ArrayList<Integer> e1 = new ArrayList<Integer>(); //empty
        ArrayList<String> e2 = new ArrayList<String>(5); //5 elements
        ArrayList<Dogs> e3 = new ArrayList<Dogs>(); //you can store whatever objects you want
    }
}

7.2 ArrayList Methods

size();
- Returns the number of elements in the list
add(obj);
- Adds elements at the end
remove(index);
- Removes element at a specific index
set(index, object);
- Replaces element at index with new object
get(index);
- Returns element at index

Unit 8 2D Arrays

Objectives

Representing collections of related primitive and object reference data using two dimensional array objects

2D Array Vocab

Arrays: a data structure used to implement a collection (list) of primitive or object reference data
Element: a single value in the array
Index: the position of the element in the array (it starts from 0)
Array length: the number of elements in the array (public and final)

// Initializing a sample Array
public class Test {

    public static void main(String[] args) {
 
       int[][] arr = {
          { 1, 2, 3 },
          { 4, 5, 6 },
          { 7, 8, 9 }
       };
 
       System.out.println("arr[0][0] = " + arr[0][0]);
       System.out.println("arr[1][2] = " + arr[1][2]);
       System.out.println("arr[2][1] = " + arr[2][1]);
       
    }
 
 }
 Test.main(null);

arr[0][0] = 1
arr[1][2] = 6
arr[2][1] = 8

Nested Loops

Can be used to traverse a 2D array

// Example of a nested loop for traversing an array
public class Test {

    public static void main(String[] args) {
 
      String[][] arr = {
         { "a", "f", "g", "l" },
         { "b", "e", "h", "k" },
         { "c", "d", "i", "j" }
      };
 
      for (int row = 0; row < 3; row++) {
         for (int col = 0; col < 4; col++) {
            System.out.print(arr[row][col] + " ");
         }
        System.out.println(" ");
      }
       
    }
 
 }
 Test.main(null);

a f g l  
b e h k  
c d i j

Unit 9 Inheritance

Notes of Key Concepts

Each class can extend from it's base class
When a class extends from its base class, it inherits the methods and other attributes
Protected: an access modifier so that the attribute isn't affected by access modifier that isn't affected by outside modifiers
If the attributes of the subclass are exactly the same as the attributes of the superclass, then we can just use the constructor of the superclass.
An additional attribute that's added to the subclass class does not have to be added to the superclass if it does not apply to it
Overriding methods allows the subclass to provide specific implementation of a method
- Has to be already provided by the super-class
If a method in a subclass has the same name, same parameters or signature, and same return type as a method in its super-class, then the method in the subclass will override the method in the super-class.
Uses of the super keyword
- Use constructors in the superclass
- Use the methods in the superclass in the child class
You can use a inheritance hierarchy to organize references
Polymorphism: You can do one action many ways
- Runtime Polymorphism: method overriding
- Compile Time Polymorphism: also using method overrloading
Object class is the superclass of all other classes in Java
toString Method: Prints out the attributes of an object and converts a string object to string
equals Method: Compares two strings and returns a boolean value of true if equal

Unit 10 Recursion

Learning Objectives

Learn about how to write simple recursive methods and determine the purpose or output of a recursive method by tracing
Determine the result of recursive method calls
Primarily in the multiple choice section; students were asked to determine the result or describe the behavior of a recursive method
5 to 7.5% exam weighting

10.1 Recursion

A recursive method is a method that calls itself - a subproblem that calls itself repeatedly
To parts to the method:
- a base case
- recursive call
After multiple calls, the base case is reached where recursion is stopped and a value is returned
- Should be written first to avoid infinite recursion
Recursions are similar to loops - recursions can be written as loops
Iteration vs. Recursion:
- Iteration is used when we execute a set of instructions repeatedly (through a loop) until the condition becomes false
- Recursion is used when the solution to a bigger problem can be expressed in terms of smaller problems
- Main Difference: recursion uses function calls vs. iteration uses for and while loops

10.2 Binary Search

Binary search algorithm
- Data must be in sorted order
- Keeps halving array until value is found
- More efficient than linear search (O(log2n) vs O(n))

Selection Sort

Linear Recursion:
- A function that only makes a single call to itself each time the function runs (as opposed to one that would call itself multiple times during its execution)
Selection Sort:
- The algorithm works by repeatedly finding the minimum element (considering ascending order) from the unsorted part and putting it at the end of the sorted part

Merge Sort

Can be used to sort ArrayList structures
Uses a Divide and Conquer algorithm
It divides the input array into two halves, calls itself for the two halves, and then merges the two sorted halves
merge() function is used for merging two halves