OOP Concepts


Updated: 03 September 2023


  • A blueprint for an object
  • Fields
  • Parameters
  • Methods


  • What classes have in common
  • Abstracted features
  • Override or extend methods
  • Avoid duplicate
  • Anything in superclass will be reflected in all subclasses


  • Create objects
  • Allow them to interact


Allows us to validate or perform input information for a field and use Setters and Getters

Instance vs Local variables

  • Instance - Defined inside of a class
  • Local - Defined in a method


  • Allow the subclass to redifine methods in the parents
  • Refer to subclasses by their superclass type
  • Can treat the objects as if they’re of the superclass type
  • Cannot access methods that only exist in the subclass

Abstract classes

  • Cannot be an object in itself
  • May contain state and/or implementation
  • Subclasses cannot inherit protected fields


  • Does not contain an implementation
  • Only abstract methods
  • Allows classes of different inheritance trees can still communicate
  • Avoid interfaces that just force redefinition by subclasses



public class Animal {

	private String name;
	private double height;
	private int weight;
	private String favFood;
	private double speed;
	private String sound;

	public void setName(String newName){ name = newName; }
	public String getName(){ return name; }

	public void setHeight(double newHeight){ height = newHeight; }
	public double getHeight(){ return height; }

	public void setWeight(int newWeight){
		if (newWeight > 0){
			weight = newWeight;
		} else {
			System.out.println("Weight must be bigger than 0");
	public double getWeight(){ return weight; }

	public void setFavFood(String newFavFood){ favFood = newFavFood; }
	public String getFavFood(){ return favFood; }

	public void setSpeed(double newSpeed){ speed = newSpeed; }
	public double getSpeed(){ return speed; }

	public void setSound(String newSound){ sound = newSound; }
	public String getSound(){ return sound; }

	// A private method can only be accessed by other public methods
	// that are in the same class

	private void bePrivate(){
		System.out.println("I'm a private method");

	public static void main(String[] args){

		Animal dog = new Animal();






public class Dog extends Animal{

	public void digHole(){

		System.out.println("Dug a hole");


	public void changeVar(int randNum){

		randNum = 12;

		System.out.println("randNum in method value: " + randNum);


	/* This private method can only be accessed through using other
	 * methods in the class */

	private void bePrivate(){
		System.out.println("In a private method");

	public void accessPrivate(){

	// The constructor initializes all objects

	public Dog(){

		// Executes the parents constructor
		// Every class has a constructor whether you make it or not


		// Sets bark for all Dog objects by default





public class Cat extends Animal{

	// The constructor initializes all objects

	public Cat(){

		// Executes the parents constructor
		// Every class has a constructor whether you make it or not


		// Sets bark for all Dog objects by default



	// If you want to make sure a method isn't overridden mark it as Final

	final void attack(){
		// Do stuff that can never change

	// A field marked with final can't be changed

	public static final double FAVNUMBER = 3.14;

	// A class labeled as final can't be extended



public class WorkWithAnimals{

	int justANum = 10;

	public static void main(String[] args){

		Dog fido = new Dog();




		// Everything is pass by value
		// The original is not effected by changes in methods

		int randNum = 10;

		System.out.println("randNum after method call: " + randNum);

		// Objects are passed by reference to the original object
		// Changes in methods do effect the object


		System.out.println("Dog name after method call: " + fido.getName());

		System.out.println("Animal Sound: " + fido.getSound());

		// Create a Dog and Cat object with the super class
		// but the Dog and Cat reference type

		Animal doggy = new Dog();
		Animal kitty = new Cat();

		System.out.println("Doggy says: " + doggy.getSound());
		System.out.println("Kitty says: " + kitty.getSound() + "\n");

		// Now you can make arrays of Animals and everything just works

		Animal[] animals = new Animal[4];
		animals[0] = doggy;
		animals[1] = kitty;

		System.out.println("Doggy says: " +animals[0].getSound());
		System.out.println("Kitty says: " +animals[1].getSound() + "\n");

		// Sends Animal objects for processing in a method


		// Polymorphism allows you to write methods that don't need to
		// change if new subclasses are created.

		// You can't reference methods, or fields that aren't in Animal
		// if you do, you'll have to cast to the required object

		((Dog) doggy).digHole();

		// You can't use non-static variables or methods in a static function

		// System.out.println(justANum);

		// sayHello();

		// You can't call a private method even if you define it in
		// the subclass

		// fido.bePrivate();

		// You can execute a private method by using another public
		// method in the class


		// Creating a Giraffe from an abstract class

		Giraffe giraffe = new Giraffe();




	// Any methods that are in a class and not tied to an object must
	// be labeled static. Every object created by this class will
	// share just one static method

	public static void changeObjectName(Dog fido){



	// Receives Animal objects and makes them speak

	public static void speakAnimal(Animal randAnimal){

		System.out.println("Animal says: " + randAnimal.getSound());


	// This is a non-static method used to demonstrate that you can't
	// call a non-static method inside a static method

	public void sayHello(){





//If you don't want the user to create objects from
//a class mark it as abstract.
//Subclasses can still extend it

abstract public class Creature{

	// protected fields are like private fields except
	// subclasses can inherit them

	protected String name;
	protected double height;
	protected int weight;
	protected String favFood;
	protected double speed;
	protected String sound;

	// There are no abstract fields in Java, but
	// there are abstract methods. Every method
	// marked abstract must be overridden
	// Not all methods must be abstract and you
	// can also use static methods

	public abstract void setName(String newName);
	public abstract String getName();

	public abstract void setHeight(double newheight);
	public abstract double getHeight();

	public abstract void setWeight(double newWeight);
	public abstract double getWeight();

	public abstract void setFavFood(String newFood);
	public abstract String getFavFood();

	public abstract void setSpeed(double newSpeed);
	public abstract double getSpeed();

	public abstract void setSound(String newSound);
	public abstract String getSound();



public class Giraffe extends Creature{

	private String name;

	public void setName(String newName) {
		name = newName;


	public String getName() {
		// TODO Auto-generated method stub
		return name;

	public void setWeight(double newWeight) {
		// TODO Auto-generated method stub


	public double getWeight() {
		// TODO Auto-generated method stub
		return 0;