CollegeBoard Notes

Unit 1 Primitive Types

  • Primitive types categorized into two types of data types
    • Primitive data types - includes byte, short, int, long, float, double, boolean and char
    • Non-primitive data types - such as String, Arrays and Classes
  • There are eight primitive types
    • A Byte (1 byte) and stores whole numbers from -128 to 127
    • A short (2 bytes) and stores whole numbers from -32,768 to 32,767
    • An int (4 bytes) and stores whole numbers from -2,147,483,648 to 2,147,483,647
    • A long (8 bytes) and stores whole numbers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
    • A float (4 bytes) and stores fractional numbers which is great when storing 6 to 7 decimal digits
    • A double (8 bytes) and stores fractional numbers which is great when storing 15 decimal digits
    • A boolean (1 bit) and stores only 2 values, true or false
    • A char (2 bytes) and stores a single character/letter or ASCII values

Unit 2 Using Objects

  • Java is object-oriented programming language (OOP)
    • Everything in Java is associated with classes and objects, along with its attributes and methods. In real life, a car is an object and has attributes, like weight and color, and methods, like drive and brake.
    • A Class is like an object constructor, or a "blueprint" for creating objects.
  • To create classes, use the keyword "class" (usually main class is created)
  • In Java, an object is created from a class. We have already created the class named Main, so now we can use this to create objects.
  • You can also create multiple objects for one class

Unit 3 Boolean Expressions and if Statements

  • Boolean is a primitive data type that can only store true or false values
  • Boolean can be declared with the keyword "boolean"
import java.util.Scanner;
public class booleanExample //define class using class keyword
{
    public static void main(String[] args) //java main method
{
    boolean isCorrect = true; //declare the boolean variables using boolean keyword
    boolean isWrong = false; 
    if (isCorrect)
    {
        System.out.println("You are right.");
    }
    else
    {
        System.out.println("You are wrong.");
    }
}
}
booleanExample.main(null);

You are right.

This can also be used to compare values and numbers. For example, using a comparison operator, like the > operator, we can find out if an expression is true. This can be easily used in real life situations like the following.

int myAge = 16; // stating the two integers with their values
int votingAge = 18;
System.out.println(myAge > votingAge); // returns false, because 16 is less than 18

false

Unit 4 Iteration

  • An Iterator is an object that loops through collection
  • To use an Iterator in Java, you must import it from the java.util package.
  • To get an Iterator, the iterator() method can be used
  • There are three types of iterations that can be used
    • For Loops
    • While Loops
    • Recursions
  • You can also add or remove numbers using an Iterator
import java.util.ArrayList;
import java.util.Iterator;

public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> numbers = new ArrayList<Integer>();
    numbers.add(12);
    numbers.add(8);
    numbers.add(2);
    numbers.add(23);
    Iterator<Integer> it = numbers.iterator();
    while(it.hasNext()) {
      Integer i = it.next();
      if(i < 10) {
        it.remove();
      }
    }
    System.out.println(numbers);
  }
}

Unit 5 Writing Class

  • There are a couple of steps to writing a class
    • make sure to use the class keyword to create a class in the start
    • To create an object of Main, specify the class name, followed by the object name, and use the keyword new
      • Constructors are used to create new objects
  • Instance variables are used to store any information regarding objects created in a class
  • Methods are used to show programs a new command they must run
    • specific methods are used to perform specific tasks in a class

Unit 6 Array

  • Used to store multiple values in a single variable
  • To declare an array, define the variable type with square brackets and insert values formatted in a comma-separated list, using curly braces
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
  • Access array elements by referring to the index number in the array. The counting for the elements starts at 0. 
System.out.println(cars[0]);
  • You can also change the value of a specific element using the index number
  • To find out how many elements an array has, use the length property
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
System.out.println(cars.length);

Unit 7 Arraylist

  • The ArrayList class is a resizable array, which can be found in the java.util package
  • The size of an Array cannot be modified so if you want to add or remove elements to/from an array, you have to create a new one whereas for Arraylists, elements can be added and removed whenever you want
    • The syntax for each type is also different
  • The ArrayList class has many useful methods. For example, to add elements to the ArrayList, use the add() method
import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> cars = new ArrayList<String>();
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    cars.add("Mazda");
    System.out.println(cars);
  }
}
  • To access an element in the ArrayList, use the get() method and refer to the index number
cars.get(0);

Unit 8 2D Array

  • A 2D Array is the simplest form of a multidimensional array
  • The syntax for declaration is 
data_type[][] array_name = new data_type[x][y];
        int[][] arr = new int[10][20];
  • The syntax for initialization is
array_name[row_index][column_index] = value;
        arr[0][0] = 1;
  • Elements in two-dimensional arrays are commonly referred by x[i][j] where ‘i’ is the row number and ‘j’ is the column number
  • To output all the elements of a 2D array, use nested for loops
    • For this 2 for loops are needed, one to traverse the rows and another to traverse the columns

Unit 9 Inheritance

  • Inheritance is essentially what attributes/methods the child class inherits from the parent class
    • This can be used with the fibonacci sequence mini-lab earlier in the trimester
    • Is useful when you want to inherit the same attributed and methods from another class
      • "this" is used when referring to the current object in the method
      • "super" is used to call any superclass method like the parent class

Unit 10 Recursion

  • Recursion is used to make a function call itself and breaks complicated problems down into simple problems
  • Use recursion to add all of the numbers up to 10
public class Main {
    public static void main(String[] args) {
      int result = sum(10);
      System.out.println(result);
    }
    public static int sum(int k) {
      if (k > 0) {
        return k + sum(k - 1);
      } else {
        return 0;
      }
    }
  }

When the sum() function is called, it adds parameter k to the sum of all numbers smaller than k and returns the result. When k becomes 0, the function just returns 0. Since the function does not call itself when k is 0, the program stops there and returns the result.