javaNotes.md

Java

• an object orientated language
• uses brace brackets

Comments

• single-line start with double slash "//"
• multi-line start with slash asterisk "/*"

Variables

• declare type then variable name = value

int myNumber = 525;

Operators

• relational operators don't need the same two operands

Control Flow

• if statement

  • syntax

  if (condition) {
    // code goes here
  } else if {
    // more code goes here
  } else {
    // even more code
  }

  • shortened for binary cases (ternary conditional statement)

  (condition) ? executedIfTrue : executedIfFalse;

• switch statement

  • syntax

   /*
   break statement exits the case statement
   otherwise it will continue to check whether it matches anything else
   */
  int myNumber = 5;
  
  switch (myNumber) {
  
    case 1: System.out.println("Case 1");
    break;
  
    case 2: System.out.println("Case 2");
    break;
  
    default: System.out.println("Default case");
    break;  
  }

• for loops

  • syntax

  for (int counter = 0; counter < 5; counter++) {
  
    System.out.println("Counter: " + counter);
  
  }
  
  // or (for each loop)
  
  for (int number : arrayOfNumbers) {
  
    System.out.println("The number is " + number);
  
  }

Classes

• keyword class
• each class needs a class constructor
• instance variables
  • details associated with the objects (from the classes)
• methods
  • built-in main method
• void means that it will not return a value
• if we want to return a value, specify the value type (int, char etc.)
• inheritance
  • use the "extends" keyword

class Dog extends Animal{

  // an instance variable
  int age;

  // class constructor
  public Dog(int dogsAge) {

    age = dogsAge;
  }

  // a method
  public void bark() {

    System.out.println("Woof");
  }

  // built-in main method
  public static void main(String[] args) {

    // creating an instance of the class
    Dog myDog = new Dog(3);
    myDog.bark();
  }
}

Data structures

• .size() to find size of structure
• arrays
  • keyword "ArrayList"

  ArrayList<Integer> quizGrades = new ArrayList<Integer>();

  • use .add(value) to add values to the array
    • to insert use .add(index, value)
  • use .get(index) to access the values
• dictionaries
  • keyword "HashMap"
  • consists of keys and values

  HashMap<String, Integer> myDict = new HashMap<String, Integer>();

  • use .put(key, value) to add key value pairs to the dictionary
  • use .get(key) to access the value associate with the key
  • use .keySet() to access the keyset of a dictionary