- 9.1 Superclasses and Subclasses
- 9.2 Writing Constructors for Subclasses
- 9.3 Overriding Methods
- 9.4 Super Keyword
- Why would we want to do this?
- Here is a Complete Example of the Inheritance Hierarchy from above:
- Learning Objectives
- What is Polymorphism?
- Static and Dynamic types
- Compile-time vs Runtime methods
- Hacks
9.1 Superclasses and Subclasses
A hierarchy of classes is created, extending attributes into subclasses of information (ie. Automobile –> Trucks and Sedans –> Ford, BMW, Nissan, Toyota).
Pre-Requisites
Before we start you need to remember what classes and methods are. Scopes of variables are also important to this section because you need to know which classes can access which variables before extending a variable across classes.
Scope of Variables:
Variables can be declared as private or public.
Popcorn Hack: What are scope do private and public variables allow?
Answer: Private variables can only be accessed within the class while public variables can be used globally.
Private | Public |
---|---|
only directly accessible within the class they are declared | accessible anywhere inside and outside their class |
These are different types of variables. Only INSTANCE
and STATIC
variables can be declared as private or public.
Variable Type | Definition | Scope within Class | Scope to Subclasses |
---|---|---|---|
Local Variables | variables within methods | cannot be accessed outside method | cannot be accessed outside class |
Instance Variables | variables within a class but not inside a method | can be accessed in entire class | can be accessed in subclasses |
Static Variables | variables that belong to a class, not instance | can be accessed in entire class | can be accessed in subclasses |
Parameter Variables | variables specific in passing values to the method | cannot be accessed outside method | do not affect inheritance |
Now we can go into class hierarchies.
Class Hierarchy
Definitions
- Superclasses - a class that contains all the common attributes and behaviors that could be shared among other classes (a blueprint for subclasses)
- Subclasses - extends the behaviors to is specified by a superclass; can also have additional specific attributes
- “Is-A” Relationship - the relationship when a subclass extends a superclass (ie. Automobile –> Sedan; a Sedan “is-a” automobile)
extends
Keyword
extends the subclass from the superclass
class Automobile {
public String brand; // public instance var
private String model; // private instance var
public Automobile(String brand, String model) {
this.brand = brand;
this.model = model;
}
public void start() {
System.out.println("Car is starting");
}
}
class Truck extends Automobile {
public int cargoCapacity; // subclass specific var
// instance that is specific to the Truck subclass, with vars from Automobile class
public Truck(String brand, String model, int cargoCapacity) {
super(brand, model); // inherited vars
this.cargoCapacity = cargoCapacity;
}
// specific method to Truck
public void loadCargo() {
System.out.println("Loading cargo into the truck");
}
}
class Sedan extends Automobile {
public boolean isLuxury; // subclass specific var
// instance that is specific to the Sedan subclass, with vars from Automobile class
public Sedan(String brand, String model, boolean isLuxury) {
super(brand, model); // inherited vars
this.isLuxury = isLuxury;
}
// specific method to Sedan
public void accelerate() {
System.out.println("Sedan is accelerating");
}
}
public class Main {
public static void main(String[] args) {
Automobile car = new Automobile("Toyota", "Camry");
Truck truck = new Truck("Ford", "F-150", 1000);
Sedan sedan = new Sedan("BMW", "328i", true);
// automobile methods and variables
System.out.println(car.brand); // Accessing public variable
// System.out.println(car.model); // compilation error because of the private var
car.start();
// truck methods and variables
System.out.println(truck.brand); // inherited public var
System.out.println(truck.cargoCapacity); // public var specific to truck
truck.loadCargo();
// sedan methods and variables
System.out.println(sedan.brand); // inherited public var
System.out.println(sedan.isLuxury); // public var specific to sedan
sedan.accelerate();
}
}
Main.main(null);
Toyota
Car is starting
Ford
1000
Loading cargo into the truck
BMW
true
Sedan is accelerating
This example shows how the Automobile
class is extended twice, with the Truck
and Sedan
subclasses.
Popcorn Hack: If I were to declare a variable
color
that isprivate
in the class Automobile, would I be able to extend and directly access that variable to the subclassTruck
orSedan
?
A: No, you would need to encapsulate the private variable as shown above, through a method in the superclass and then inherit that var with super()
, which will be explained later.
9.2 Writing Constructors for Subclasses
Learning Objectives
- Constructors are not inherited
- When a subclass’s constructor doesn’t explicitly call a superclass’s constructor using
super
, Java inserts a call to the superclass’s no-argument constructor. - The actual parameters passed in the call to the superclass constructor provide values that the constructor can use to initialize the object’s instance variables.
- Regardless of whether the superclass constructor is called implicitly or explicitly, the process of calling superclass constructors continues until the Object constructor is called. At this point, all of the constructors within the hierarchy execute beginning with the Object constructor.
Important note: Constructors are NOT inherited by the subclass. See this in action below.
// TO BE INCLUDED EARLIER IN THE LESSON. IT IS NECESSARY FOR THE FUNCTIONALITY OF THIS SECTION.
class Vehicle {
public int year;
public String manufacturer;
public Vehicle(int year, String manufacturer) { // constructor for parent class
this.year = year;
this.manufacturer = manufacturer;
}
public Vehicle() {
this.year = 2000;
this.manufacturer = "Unknown";
}
// method to be used later
public void drive() {
System.out.println("The driver is driving the car.");
}
}
class Car extends Vehicle {
public String model;
public Car(String model) {
this.model = model;
}
}
public class VehicleDemonstration {
public static void main(String[] args) {
Car myCar = new Car("Altima");
System.out.println("Year: " + myCar.year);
System.out.println("Manufacturer: " + myCar.manufacturer);
System.out.println("Model: " + myCar.model);
}
}
VehicleDemonstration.main(null);
Year: 2000
Manufacturer: Unknown
Model: Altima
As you can see, the output uses the no-argument construction info from the base Vehicle
constructor.
The super
keyword can be used to change parent constructor values.
class NewCar extends Vehicle {
public String model;
public NewCar(int year, String manufacturer, String model) {
super(year, manufacturer); // see the use of super here
// what happens if you use no arguments with super()? see reminders below
this.model = model;
}
}
public class VehicleDemonstration2 {
public static void main(String[] args) {
NewCar myCar = new NewCar(2016, "Nissan", "Altima");
System.out.println("Year: " + myCar.year);
System.out.println("Manufacturer: " + myCar.manufacturer);
System.out.println("Model: " + myCar.model);
}
}
VehicleDemonstration2.main(null);
Year: 2016
Manufacturer: Nissan
Model: Altima
Key Reminders:
- If you do call
super()
in your constructor, it has to be the first line of the constructor. - You cannot assign values to parent attributes/variables without using
super()
. - If you call
super()
with no arguments, it will use the no-argument parent constructor. This also happens automatically if you don’t include anysuper()
call.
9.3 Overriding Methods
Learning Objectives
- Method overriding occurs when a public method in a subclass has the same method signature as a public method in the superclass.
- Any method that is called must be defined within its own class or its superclass.
- A subclass is usually designed to have modified (overwritten) or additional methods or instance variables.
- A subclass will inherit all public methods from the superclass; these methods remain public in the subclass.
There are three options for methods to be used by subclasses:
- Methods inherited from the parent class
- Unique methods written for the subclass
- Override parent methods to modify its implementation
The first two should make sense. Let’s see an example of overriding parent methods below.
public class NuroCar extends Vehicle {
private String deliveryItem;
// unique constructor
public NuroCar(int year, String manufacturer, String deliveryItem) {
super(year, manufacturer); // another use of super
this.deliveryItem = deliveryItem;
}
// HERE is the overridden function
public void drive() {
System.out.println("This car is driving itself!");
}
}
public class VehicleDemonstration3 {
public static void main(String[] args) {
NuroCar pizzaCar = new NuroCar(2023, "Nuro", "Pizza");
// here's the call to the overridden function
pizzaCar.drive();
}
}
VehicleDemonstration3.main(null);
This car is driving itself!
This can be very helpful if you want a certain parent method to function slightly differently for a certain subclass.
Popcorn Hack
A parent class Animal
is often used to show how subclasses can differ from their parent classes. An Animal
parent class is provided in the cell below. Create a subclass of a certain species that overrides a parent method and uses `super` to call to the parent's constructor while adding its own unique attributes.
Hint:
// parent class
public class Animal {
public String species;
public int milesPerHour;
// no argument constructor
public Animal() {
this.species = "Unknown";
this.milesPerHour = 10;
}
// constructor with arguments
public Animal(String species, int milesPerHour) {
this.species = species;
this.milesPerHour = milesPerHour;
}
// parent method
public void move() {
System.out.println("The " + this.species.toLowerCase() + " runs at " + this.milesPerHour + " miles per hour.");
}
}
// your subclass goes here
public class Cheetah extends Animal {
private int age;
public Cheetah(String species, int milesPerHour, int age) {
super(species, milesPerHour);
this.age = age;
}
@Override
public void move() {
System.out.println("The " + this.species.toLowerCase() + " is " + this.age + " years old and runs at " + this.milesPerHour + " miles per hour.");
}
}
public class Main {
public static void main(String[] args) {
Cheetah anim = new Cheetah("Cheetah", 75, 6);
anim.move();
}
}
Main.main(null);
The cheetah is 6 years old and runs at 75 miles per hour.
9.4 Super Keyword
Using the super keyword to call a superclass’s method.
public class Performer { //superclass
public void practice(){
System.out.println("Honing my craft!");
}
public void perform(){
System.out.println("Performing for an audience!");
}
}
public class Dancer extends Performer { //subclass
public void perform(){
System.out.println("Dancing on the stage!");
}
}
public class BalletDancer extends Dancer { //subclass
public void jete(){
System.out.println("Leaping...");
}
public void pirouette(){
System.out.println("Spinning...");
}
public void perform(){
jete();
pirouette();
}
public static void main(String[] args){
BalletDancer derrick = new BalletDancer();
derrick.practice();
derrick.perform();
}
}
System.out.println("BalletDancer class: ");
BalletDancer.main(null);
BalletDancer class:
Honing my craft!
Leaping...
Spinning...
public class Performer { //superclass of Dancer class
public void practice(){
System.out.println("Honing my craft!");
}
public void perform(){
System.out.println("Performing for an audience!");
}
}
public class Dancer extends Performer { //superclass of BalletDancer class
public void perform(){
System.out.println("Dancing on the stage!");
}
}
public class BalletDancer extends Dancer {
public void jete(){
System.out.println("Leaping...");
}
public void pirouette(){
System.out.println("Spinning...");
}
public void perform(){
// got rid of perform() because it had stack overflow error
jete();
pirouette();
}
public static void main(String[] args){
BalletDancer derrick = new BalletDancer();
derrick.practice();
derrick.perform();
}
}
System.out.println("BalletDancer class: ");
BalletDancer.main(null);
BalletDancer class:
Honing my craft!
Leaping...
Spinning...
Note:
Super keyword can be placed in any order as it prints out chronologically.
public void perform(){
super.perform();
jete();
pirouette();
}
"Honing my craft!
Dancing on the stage!
Leaping...
Spinning..."
However, if we were to write:
public void perform(){
jete();
pirouette();
super.perform();
}
"Honing my craft!
Leaping...
Spinning...
Dancing on the stage!"
What if we use 2 super keywords?
public class Performer { //superclass
public void practice(){
System.out.println("Honing my craft!");
}
public void perform(){
System.out.println("Performing for an audience!");
}
}
public class Dancer extends Performer { //subclass
public void perform(){
super.perform();
System.out.println("Dancing on the stage!");
}
}
public class BalletDancer extends Dancer { //subclass
public void jete(){
System.out.println("Leaping...");
}
public void pirouette(){
System.out.println("Spinning...");
}
public void perform(){
super.perform();
jete();
pirouette();
}
public static void main(String[] args){
BalletDancer derrick = new BalletDancer();
derrick.practice();
derrick.perform();
}
}
System.out.println("BalletDancer class: ");
BalletDancer.main(null);
BalletDancer class:
Honing my craft!
Performing for an audience!
Dancing on the stage!
Leaping...
Spinning...
Popcorn Hack
Create a subclass and a superclass by calling the methods from the superclass from the subclass using the keyword super.
// place code here
public class Movie {
public void watch() {
System.out.println("Watching a movie!");
}
}
public class ActionMovie extends Movie{
public void watch() {
super.watch();
System.out.println("My heart is beating like crazy!");
}
public static void main(String[] args) {
ActionMovie movie = new ActionMovie();
movie.watch();
}
}
ActionMovie.main(null);
Watching a movie!
My heart is beating like crazy!
9.5 Creating References Using Inheritance Hierarchies
Inheritance Hierarchies is the idea that:
- If we have a public superclass (A) that then has a public subclass (B) and that public subclass then has another public subclass (C), then the subclass C is also a subclass of the superclass A and so subclass C is extending superclass A
Here is an example of this:
// Class Declarations:
// Superclass (A)
public class Vehicles{}
// Subclass (B)
public class Car extends Vehicles{}
// Subclass (C)
public class Sedan extends Car{}
// Subclass (D)
public class Truck extends Vehicles{}
Here is a diagram displaying this relationship:
-
This diagram shows the the vehicle superclass which then has two subclasses: Car and Truck, however, Car has another subclass called Sedan
-
Based on this diagram, we know that Truck has a “is-a” relationship with Vehicle, Car also has a “is-a” relationship with Vehicle, and since Car has an “is-a” relationship with Vehicle, so does Sedan
-
So Sedan also has a “is-a” relationship with Vehicle
If we assume that all of the class contain a constructor with no arguments, this then allows for us to declare a superclass reference variable to hold a subclass object:
Vehicles v1 = new Truck();
Vehicles v2 = new Car();
// instead of doing
Car v3 = new Sedan();
// We have the option to do this instead
Vehicles v4 = new Sedan();
However, it is important to remember that you can’t declare a Subclass variable and put in a Superclass object
// don't do this
Sedan v5 = new Vehicle();
Popcorn Hack: Using a superclass and subclasses of your choice, assuming that the classes contain a constructor with no arguments, create Class Declarations and Superclass References
// show here
public class Instrument {
public String brand;
public double cost;
public Instrument() {
this.brand = "unknown";
this.cost = 0.0;
}
public Instrument(String brand, double cost) {
this.brand = brand;
this.cost = cost;
}
public void play() {
System.out.println("The " + brand + " costs " + cost + " dollars");
}
}
public class Guitar extends Instrument {
private String material;
public Guitar(String brand, double cost, String material) {
super(brand, cost);
this.material = material;
}
@Override
public void play() {
super.play();
System.out.println("The " + brand + " is made of " + material);
}
}
public class BassGuitar extends Guitar {
private int weight;
private String color;
public BassGuitar(String brand, double cost, String material, int weight, String color) {
super(brand, cost, material);
this.weight = weight;
this.color = color;
}
@Override
public void play() {
super.play();
System.out.println("The " + brand + " weighs " + weight + " pounds");
System.out.println("The " + brand + " has a " + color + " color");
}
}
public class Main {
public static void main(String[] args) {
Instrument inst1 = new Guitar("Yamaha", 449.99, "Spruce Wood");
Instrument inst2 = new BassGuitar("Fender", 1179.99, "Alder Wood", 9, "Blue");
System.out.println("Guitar Info:");
inst1.play();
System.out.println();
System.out.println("Bass Guitar Info");
inst2.play();
}
}
Main.main(null)
Guitar Info:
The Yamaha costs 449.99 dollars
The Yamaha is made of Spruce Wood
Bass Guitar Info
The Fender costs 1179.99 dollars
The Fender is made of Alder Wood
The Fender weighs 9 pounds
The Fender has a Blue color
Why would we want to do this?
-
When doing object oriented programming, we are able to create a superclass with common attributes and then have subclasses with more specific traits
-
By doing this, we get rid of code redundancy and it also makes the easier to reuse common aspects without needing to write them out every time while still presenting the option to override any attribute from the superclass to better represent the subclass
-
All the subclasses adhere to one superclass so this makes updates to code much easier and allows you to take advantage of polymorphism to unify the code and allow for more flexible and manageable code (more about this in 9.6)
Here is a Complete Example of the Inheritance Hierarchy from above:
class Vehicle {
private String brand; // private instance variable
private int year; // private instance variable
public Vehicle(String brand, int year) {
this.brand = brand;
this.year = year;
}
public void displayInfo() { // common variables for each vehicle
System.out.println("Brand: " + brand);
System.out.println("Year: " + year);
}
}
class Truck extends Vehicle {
// feature that only a truck can have
private double maxLoadCapacity;
public Truck(String brand, int year, double maxLoadCapacity) {
super(brand, year); // Inherits the variables from superclass
this.maxLoadCapacity = maxLoadCapacity;
}
@Override // Override allows for displayInfo to now also show the trait specific to the Truck while still keeping the main variables
public void displayInfo() {
super.displayInfo(); // Reuse the displayInfo method from the superclass
System.out.println("Max Load Capacity: " + maxLoadCapacity + " tons");
}
}
class Car extends Vehicle {
// something that cars have
private int numberOfDoors;
public Car(String brand, int year, int numberOfDoors) {
super(brand, year);
this.numberOfDoors = numberOfDoors;
}
@Override // Override allows for displayInfo to now also show the trait specific to the Car while still keeping the main variables
public void displayInfo() {
super.displayInfo(); // Reuse the displayInfo method from the superclass
System.out.println("Number of Doors: " + numberOfDoors);
}
}
class Sedan extends Car {
// Sedan is luxury so trait specific to sedan
private boolean leatherSeats;
public Sedan(String brand, int year, int numberOfDoors, boolean leatherSeats) {
super(brand, year, numberOfDoors);
this.leatherSeats = leatherSeats;
}
@Override // Override allows for displayInfo to now also show the trait specific to the Sedan while still keeping the main variables
public void displayInfo() {
super.displayInfo(); // Reuse the displayInfo method from the superclass
System.out.println("Leather Seats: " + leatherSeats);
}
}
public class Main {
public static void main(String[] args) {
Vehicle v1 = new Truck("Ford", 2023, 10.5);
Vehicle v2 = new Car("Toyota", 2023, 4);
Vehicle v3 = new Sedan("Honda", 2023, 4, true);
System.out.println("Truck Information:");
v1.displayInfo();
System.out.println("\nCar Information:");
v2.displayInfo();
System.out.println("\nSedan Information:");
v3.displayInfo();
}
}
Main.main(null)
Truck Information:
Brand: Ford
Year: 2023
Max Load Capacity: 10.5 tons
Car Information:
Brand: Toyota
Year: 2023
Number of Doors: 4
Sedan Information:
Brand: Honda
Year: 2023
Number of Doors: 4
Leather Seats: true
Popcorn Hack: In your own words describe the importance of Inheritance Hierarchies
Answer: Inheritance Hierarchies are important because they enable the reuse of common attributes or behaviors and also gets rid of code redundancy. They also allow the updating of code to be much easier.
9.6 Polymorphism
Learning Objectives
MOD-3.D: Call methods in an inheritance relationship.
MOD-3.D.1: Utilize the Object class through inheritance.
MOD-3.D.2: At compile time, methods in or inherited by the declared type determine the correctness of a non-static method call.
MOD-3.D.3: At run-time, the method in the actual object type is executed for a non-static method call.
What is Polymorphism?
Polymorphism is a greek word meaning many-formed (poly=many, morph=form). In the context of programming, Polymorphic behavior is defined as being able to reference objects of different types at different points during compilation.
One example of Java polymorphism, is our ability to store references to objects instantiated from any class AND its subclasses in a variable. Another example is for methods, where we override the behavior of a method in a subclass such that it differs from the parent class. We successfully perform polymorphism when we override non-static methods and execute them from the correct, corresponding class at runtime.
Here is a simple example of polymorphism:
// This is our Parent class
class Shape {
public void draw() {
System.out.println("Drawing a shape");
}
}
// Subclass 1
class Circle extends Shape {
@Override
public void draw() {
System.out.println("Drawing a circle");
}
}
// Subclass 2
class Rectangle extends Shape {
@Override
public void draw() {
System.out.println("Drawing a rectangle");
}
}
public class Main {
public static void main(String[] args) {
// Here we have reference variables circle and rectangle defined with static type "Shape"
Shape circle = new Circle(); // Circle has dynamic type "Circle"
Shape rectangle = new Rectangle(); // Rectangle has dynamic type "Rectangle"
// Calling the draw() method
circle.draw(); // Output: Drawing a circle
rectangle.draw(); // Output: Drawing a rectangle
}
}
Main.main(null);
Drawing a circle
Drawing a rectangle
Even though the circle and rectangle have identical data-types, the compiler knows to call the correct method at run-time. We have successfully performed polymorphism
Popcorn hack
- Create any example of polymorphism, perferably with a class from your project, with corresponding methods and attributes as well.
// Program your code here
class Question {
private String questionText;
public Question(String questionText) {
this.questionText = questionText;
}
public void display() {
System.out.println(questionText);
}
public boolean checkAnswer(String userAnswer) {
return false;
}
}
class MultipleChoiceQuestion extends Question {
private String[] choices;
private int correctChoiceIndex;
public MultipleChoiceQuestion(String questionText, String[] choices, int correctChoiceIndex) {
super(questionText);
this.choices = choices;
this.correctChoiceIndex = correctChoiceIndex;
}
@Override
public void display() {
super.display();
for (int i = 0; i < choices.length; i++) {
System.out.println((i + 1) + ". " + choices[i]);
}
}
@Override
public boolean checkAnswer(String userAnswer) {
int userChoice = Integer.parseInt(userAnswer);
return userChoice == (correctChoiceIndex + 1);
}
}
class TrueFalseQuestion extends Question {
private boolean correctAnswer;
public TrueFalseQuestion(String questionText, boolean correctAnswer) {
super(questionText);
this.correctAnswer = correctAnswer;
}
@Override
public boolean checkAnswer(String userAnswer) {
return userAnswer.equalsIgnoreCase(Boolean.toString(correctAnswer));
}
}
public class Main {
public static void main(String[] args) {
Question[] questions = new Question[3];
questions[0] = new MultipleChoiceQuestion("What is the capital of France?",
new String[]{"Paris", "Berlin", "Madrid", "Rome"}, 0);
questions[1] = new TrueFalseQuestion("Is the Earth round?", true);
questions[2] = new MultipleChoiceQuestion("Which planet is known as the Red Planet?",
new String[]{"Mars", "Venus", "Jupiter", "Saturn"}, 0);
for (Question question : questions) {
question.display();
}
}
}
Main.main(null);
What is the capital of France?
1. Paris
2. Berlin
3. Madrid
4. Rome
Is the Earth round?
Which planet is known as the Red Planet?
1. Mars
2. Venus
3. Jupiter
4. Saturn
Static and Dynamic types
Typically in Java, a reference variable storing an object instantiated from a child class can only store other objects of that class. If we try to store an object of another class, we’d get an error i.e.
Integer myNumber = new Integer(6); // Instantiates an Integer object
System.out.println(myNumber); // Would print Integer type 6 normally
myNumber = new Double(6.1); // Compile error occurs, incompatible types
System.out.println(myNumber); // This line is not reached
But what if we want want a reference variable to store objects of other classes? We get an error
Popcorn Hack
- Given these lines of code, identify the static and dynamic variables in each scenario
Animal myPet = new Dog();
Object1 thing = new Object2();
Vehicle myRide = new Helicopter();
Answers
Static Variables: Animal
, Object1
, Vehicle
Dynamic Variables: Dog
, Object2
, Helicopter
Compile-time vs Runtime methods
Sometimes, when we perform polymorphism, we have to be careful to distinguish between our compile time and run-time methods. When we instantiate a dynamic variable that is of a different type from our static variable, our program at run time will attempt to search for the methods and attributes in the superclass, NOT the child class that we created. Although the compiled program will still output the correct outputs of the method in the child class, it will not be able to find a method that exists in the child class but not in the parent class.
To resolve this issue, we need use Down-casting.
- Downcasting: Essentially telling the compiler to refer to a variable’s subclass (the programmer specifies this), instead of the static type. We are telling the compiler to search our dynamic type for the methods and attributes that we want to access.
Below is an example of compile versus run-time methods and downcasting that I used in our group’s project.
import java.util.HashMap;
// This is our parent class for all child classes that make up a graph
public class GraphObject {
private int value;
// constructor
public GraphObject(int value) {
this.value = value;
}
// getter method
public int getValue() {
return this.value;
}
// setter method
public void setValue(int value) {
this.value = value;
}
// Method overloading to print object to terminal
@Override
public String toString() {
return Integer.toString(this.value);
}
}
// This is our child class. Represents a vertex or a node in a graph
public class GraphNode extends GraphObject {
private HashMap<Integer,Integer> edges; // Hashmap of destination vertices and weights of paths <Target:Weight>
// Constructor of child class
public GraphNode(int value, HashMap<Integer,Integer> edges){
super(value); // super keyword to run constructor of parent class, which sets the value of the node
this.edges = edges; // adds any needed edges
}
public HashMap<Integer,Integer> getEdges() {
return this.edges;
}
public void addEdge(int targetId, int weight) {
this.edges.put(targetId, weight);
}
public void removeEdge(int targetId) {
this.edges.remove(targetId);
}
// Overloading the toString method of the parent class, using super to get the output of the parent class
@Override
public String toString() {
return "GraphNode[value=" + super.toString() + ",edges=" + this.edges + "]";
}
}
HashMap<Integer, Integer> node1Connections = new HashMap<Integer, Integer>();
node1Connections.put(2, 7);
node1Connections.put(3, 2);
node1Connections.put(4, 10);
GraphObject node1 = new GraphNode(1, node1Connections);
System.out.println(node1.toString());
((GraphNode)node1).addEdge(5,6); // Here, we downcast node1, which is a GraphObject, to what it references, which is GraphNode
// node1.addEdge(5,6); // This wouldn't work as the compiler attempts to find addEdge() in GraphObject, which it can't, throwing an error
System.out.println(node1.toString());
GraphNode[value=1,edges={2=7, 3=2, 4=10}]
GraphNode[value=1,edges={2=7, 3=2, 4=10, 5=6}]
Popcorn Hacks
- Define each of the following terms in your own words
- Downcasting - Casting an object reference from a superclass to a subclass.
- Static Type - A variable that cannot change during the program’s execution.
- Dynamic Type - A variable that can be changed during the program’s execution.
- Polymorphism - Allows objects of different classes to be treated as objects of common superclass.
- Compile-time method - A method that is determined and bound at compile time.
- Run-time method - Method that is determined and bound at runtime.
9.7 Object Superclass
-
The Object class is the superclass of all other classes as well as data types and is a part of the java.lang package
-
If a parent class doesn’t specify by using the
extends
keyword, the class will inherit from the Object -
String toString()
andboolean equals(object other)
are the most frequently used and subclasses of Object override the equals and toString methods by using class-specific implementations
Normally when you use the toString
method without overriding it, it returns a hash code value as seen in the example below:
class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
}
public class ToStringNoOverride {
public static void main(String[] args) {
Person person = new Person("Alice", 25);
// When you print this out, it will return the hash code since its using the default method for toString
System.out.println(person);
}
}
ToStringNoOverride.main(null)
REPL.$JShell$63E$Person@2376aa49
However, we can fix this by creating a new method by overriding the normal toString
method and building our own representation of the object
class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public String toString() { // We override the toString method here
return "Person Name: " + name + "\nPerson Age: " + age; // We then add our own custom string representation
}
}
public class ToStringOverride {
public static void main(String[] args) {
Person person = new Person("Alice", 25);
// When you print the person object, it calls the overridden toString method we made
System.out.println(person);
}
}
ToStringOverride.main(null)
Person Name: Alice
Person Age: 25
Now, we want to compare 2 people by using boolean equals(Object other)
and determine if they are the same
class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public String toString() {
return "Person Name: " + name + "\nPerson Age: " + age;
}
}
public class EqualsNoOverride {
public static void main(String[] args) {
Person person1 = new Person("Alice", 25);
Person person2 = new Person("Bryce", 25);
System.out.println("Person 1 Age equals Person 2 Age: " + person1.equals(person2)); // use the equals method to compare the two people
}
}
EqualsNoOverride.main(null)
Person 1 Age equals Person 2 Age: false
False is returned when you use the boolean equals(Object other)
because the parameters is of Type Object superclass is unable to detect them since the Object class saves them under two different memory addresses and to fix this, we have to make an override method for this.
To do this, we use instanceof
and casting
class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
@Override // We override the equals method
public boolean equals(Object other) {
if (!(other instanceof Person)) // Using instanceof, we are making sure that the we compare objects of the same class
return false;
Person that = (Person) other; // Here we perform casting to make sure that other is converted to Person
return this.age == that.age;
}
@Override
public String toString() {
return "Person Name: " + name + "\nPerson Age: " + age;
}
}
public class EqualsOverride {
public static void main(String[] args) {
Person person1 = new Person("Alice", 25);
Person person2 = new Person("Bryce", 25);
System.out.println("Person 1 Age equals Person 2 Age: " + person1.equals(person2));
}
}
EqualsOverride.main(null)
Person 1 Age equals Person 2 Age: true
Hacks
Create a superclass with at least 2 subclasses based on your own topic.
- Create a DrawIO diagram for your structure and label them with superclasses and subclasses
- Create a superclass on your own topic
- Create at least two subclasses
- Each class must create at least two methods, one private and public variable, and examples of local, static, instance, and parameter variables
- Override at least one method
- Use the
super
keyword at least once