Java 15_ interfaces, abstract classes, abstract methods etc

Java doesn't allow you to inherit from more than one class.
when you want to add additional functionality?
You create an interface. Interfaces are empty classes. They provide all of the methods you must use, but none of the code.

Define an interface. They normally

public interface Drivable {
    
    // You can put fields in an interface, but understand 
    // that their values are final and can't be changed
    double PI = 3.14159265;
    
    // All methods in an interface must be implemented
    // They are public and abstract by default
    // An abstract method must be defined by any class 
    // that uses the interface
    int getWheels();
    
    // You can't define a method as final and abstract
    // final means the method can't be changed and 
    // abstract means it must be changed
    void setWheels(int numWheels);
    
    double getSpeed();
    
    void setSpeed(double speed);
    
}

Create an abstract

If you want to create a class in which every method
doesn't have to be implemented use abstract classes.

// Create an abstract class with the abstract keyword
public abstract class Crashable{
    
    boolean carDrivable = true;
    
    public void youCrashed(){
        this.carDrivable = false;
    }
    
    public abstract void setCarStrength(int carStrength);
    
    public abstract int getCarStrength();
    
}

A class part of an abstract class

/* You define that you want a class to use an interface
 * with the implements keyword. This class must create
 * a method for each method defined in Drivable
 * You can implement more than 1 interface like this
 * public class Vehicle implements Drivable, Crashable
 */
// You make a class part of an abstract class by using 
//the extends keyword
public class Vehicle extends Crashable implements Drivable {
    
    int numOfWheels = 2;
    double theSpeed = 0;
    
    int carStrength = 0;

All methods must be as visible as those in the interface. If they are public in the interface they must be public in the subclass

    public int getWheels(){
        return this.numOfWheels;
    }
    
    public void setWheels(int numWheels){
        this.numOfWheels = numWheels;
    }
    
    public double getSpeed(){
        return this.theSpeed;
    }
    
    public void setSpeed(double speed){
        this.theSpeed = speed;
    }
    
    public Vehicle(int wheels, double speed){
        this.numOfWheels = wheels;
        this.theSpeed = speed;
    }
    
    public void setCarStrength(int carStrength){
        this.carStrength = carStrength;
    }
    
    public int getCarStrength(){
        return this.carStrength;
    }
    
}