I have a superclass called DecrementCarousel which has a method that returns an object called CarouselRun. CarouselRun has its own methods which I need to override in HalvingCarousel, but I don't know how. DecrementCarousel:

public class DecrementingCarousel {
    static int [] arr ;
    static int capacity;
    int counter = 0;
    boolean alreadyExecuted = false;
    boolean alreadyRun = false;

    public DecrementingCarousel(int capacity) {
        DecrementingCarousel.capacity = capacity;
        arr = new int[capacity];

    }

    public boolean addElement(int element){
        if (alreadyExecuted) return false;
        if (counter < capacity && element > 0) {
            arr[counter] = element;
            counter  ;
            return true;
        }
        return false;
    }

    public CarouselRun run(){
        alreadyExecuted = true;
        if (alreadyRun) return null;
        alreadyRun = true;
        return new CarouselRun();

    }


    }

Here are methods in CarouselRun:

public class CarouselRun {
    int position = 0;

    public int next() {
        int count = 0;
        while (count < arr.length && arr[position %= arr.length] <= 0) {
            position  ;
            count  ;
        }
        if (count == arr.length) return -1;
        return arr[position  ]--;
    }

    public boolean isFinished() {
        for (int var: arr) {
            if (var > 0) return false;
        }
        return true;
    }

}

How to override these CarouselRun methods in a subclass called HalvingCarousel? According to the task HalvingCarousel can only extend DecrementCarousel

CodePudding user response：

Inheritance

This can be done like any other simple inheritance. You just need to declare the class HavingCarousel as a child of CarouselRun using the extends keyword and the use annotation @Override to declare that you want to override the methods of the parent class.

Parent class:

public class CarouselRun {
    int position = 0;

    public int next() {
        System.out.println("Carousel run implementation of next()");
        return 1;
    }

    public boolean isFinished() {
        System.out.println("Carousel run implementation of isFinished()");
        return true;
    }

}

Child class:

public class HalvingCarousel extends CarouselRun{
    @Override
    public int next() {
        System.out.println("HavingCarousel implementation of next()");
        return 2;
    }

    @Override
    public boolean isFinished() {
        System.out.println("HavingCarousel implementation of isFinished()");
        return true
    }
}

Interfaces

Looking at your code it could also be worth exploring interfaces instead of inheritance. Intefaces are generally more flexible and a class can implement many interfaces while it can only inherit from one parent class so interfaces should be used if there are no strong reasons opposing that like for example a huge portion of shared code which as far as I can see you will not be having.

Interface for a carousel

public interface ICarousel {
    public int next();
    public boolean isFinished();
}

CarouselRun implements the interface:

public class CarouselRun implements ICarousel {
    int position = 0;

    @Override
    public int next() {
        System.out.println("Carousel run implementation of next()");
        return 1;
    }

    @Override
    public boolean isFinished() {
        System.out.println("Carousel run implementation of isFinished()");
        return true;
    }
}

HalvingCarousel implements the interface:

public class HalvingCarousel implements ICarousel{
    @Override
    public int next() {
        System.out.println("HavingCarousel implementation of next()");
        return 2;
    }

    @Override
    public boolean isFinished() {
        System.out.println("HavingCarousel implementation of isFinished()");
        return true;
    }
}

Further Reading

From reading your code I believe that you still need to read more on basic Java concepts like interfaces, classes, inheritance, static vs instance attributes and modifiers. For example I can't see why you would declare your array arr as static but then initialize it in the constructor and then apparently you are trying to access the array arr in a totally unrelated instance of class CarouselRun. This will not work and it does not really make sense, but as these are basic concepts I cannot explain all of this here.

CodePudding user response：

To override behaviour of CarouselRun you need a child class, overriding its' methods. Then you need to override DecrementingCarousel.run() in order to return the subclass of CarouselRun.

public class AnotherCarouselRun extends CarouselRun {

    @Override
    public int next() {
        //override behaviour as needed
        return 0;
    }

    @Override
    public boolean isFinished() {
        //override behaviour as needed
        return false;
    }
}

Override methods in AnotherCarouselRun to suit your needs.

Then override behaviour of DecrementingCatousel.run()

public class HalvingCarousel extends DecrementingCarousel {

    public HalvingCarousel(int capacity) {
        super(capacity);
    }

    @Override
    public CarouselRun run() {
        //do other stuff
        return new AnotherCarouselRun();
    }
}

AnotherCarouselRun is a CarouselRun, so you are abiding to the contract of run(). Another option is to use anonymous class:

public class HalvingCarousel extends DecrementingCarousel {

    public HalvingCarousel(int capacity) {
        super(capacity);
    }

    @Override
    public CarouselRun run() {
        //do other stuff
        return new CarouselRun() {

            //this is anonymous class

            @Override
            public int next() {
                //override behaviour as needed
                return 0;
            }

            @Override
            public boolean isFinished() {
                //override behaviour as needed
                return false;
            }
        };
    }
}

This is virtually the same as first option, but you are subclassing CarouselRun as anonymous class. They are mostly used when you need the class only once.