CodeGym/Help with Java Tasks/How do i write the intersects method?

Level 26

Port-Harcourt

7/29/19
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12comments

How do i write the intersects method?

Question about the task

Under discussion

But there's more.
The BaseObject class needs more methods.
For now, these will be empty draw(Canvas canvas) and move() methods.
Subclasses will have to override them and implement the necessary functionality (you can leave them empty for now—we just need the code to compile).
Declare these methods, but make them abstract.
Also, make the BaseObject class itself abstract.

And we'll need to determine whether the ball hit a brick or the paddle.
This is how we'll do that:
In this same class, we'll create a special method: boolean intersects(BaseObject o)
It will determine whether objects intersect. If they intersect, return true; if not, false.

Because we're treating objects as circles for convenience, I propose the following collision-detection algorithm: If the center of the circle of one object lies within the circle of another, then we'll conclude that they've collided.

Or to put it more simply:
distance_between_objects <= max(radius_of_the_first_object, radius_of_the_second_object)

Requirements:

The BaseObject class must be abstract.
The BaseObject class must declare an abstract draw() method with one Canvas parameter.
An abstract move() method without parameters must be declared in the BaseObject class.
The BaseObject class must implement an intersects() method with one BaseObject parameter. It must return true if the current object intersects the argument.

package com.codegym.task.task24.task2413; public class Ball extends BaseObject { public Ball(double x, double y, double radius) { super(x, y, radius); } @Override void draw(Canvas canvas) { } @Override void move() { } }

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- FniJnn -

Level 33 , Paris, France

15 January 2020, 22:08

I don't think it's important if i give you that, it's just some basics mathematics. return Math.sqrt((Math.max(o.getX(), this.getX()) - Math.min(o.getX(), this.getX())) * (Math.max(o.getX(), this.getX()) - Math.min(o.getX(), this.getX())) + (Math.max(o.getY(), this.getY()) - Math.min(o.getY(), this.getY())) * (Math.max(o.getY(), this.getY()) - Math.min(o.getY(), this.getY()))) <= Math.max(o.getRadius(), this.getRadius());

Johannes

Level 27 , Centurion, Pretoria, South-Africa

3 May 2020, 09:23

basic maths.......?

Henrique

Level 41 , São Paulo, Brazil

28 July 2020, 18:08

Hi, can you explain this, please?

- FniJnn -

Level 33 , Paris, France

10 August 2020, 16:37

Hi, it's just the application of the Pythagorean theorem. I know it's very badly written, i was beggining Java aswell ^^

hidden #10459091

Level 28

6 August 2019, 18:51

Firstly - you don't need to declare and caluculate radius of those objects because they are both declared and set in the "radius" field (radius is required by the constructors). Secondly - I don't think that you distance formula (distance between two points) is correct. It should be rather:

double distance = Math.sqrt(Math.pow((this.getX() - o.getX()), 2.0) + Math.pow((this.getY() - o.getY()), 2.0));

Lastly your if statement is incorrect. As they stated in the task it should be something like this

if (distance <= Math.max(this.getRadius(), o.getRadius())){
            return true;
        }
        return false;

Hope it helps!

null

Level 29 , Poznan, Poland

10 September 2019, 12:17

if (distance <= Math.max(this.getRadius(), o.getRadius())){
    return true;
}
 return false;

You can simplify it to:

return distance <= Math.max(this.getRadius(), o.getRadius());

hidden #10459091

Level 28

11 September 2019, 09:42

That is absolutely true. Thanks for pointing that out!

Henrique

Level 41 , São Paulo, Brazil

28 July 2020, 18:08

Can someone explain this, please?

Halnik111

Level 29 , Nitra, Slovakia

31 July 2020, 20:22

every object has its center (x with y) and every object has some radius they cover (thats that radius variable) and what u need to do is calculate distance between 2 of those objects centers (x and y). then all u need to do is compare objects radius with distance (for example if radius is set to 3 and other to 4 and distance between two objects is <= radius(3) || radius(4)) returns true as these objects made collision. (someone needs to approve this. this is how i understand it so it doesnt mean its 100% right)

Henrique

Level 41 , São Paulo, Brazil

3 August 2020, 13:51

Thanks, Halnik111. Yeah, that's what I understood of this task too (but I'm not 100% certain as well). However, my biggest problem with this task is the code:

double distance = Math.sqrt(Math.pow((this.getX() - o.getX()), 2.0) + Math.pow((this.getY() - o.getY()), 2.0))

I didn't get this part, and some people are saying that this is basic math, but for me this is not basic at all, and I'm having a hard time trying to understand this code (the math and the logic of the code). Do you have any idea?

Onur Bal

Level 27 , Istanbul, Turkey

18 October 2020, 12:05

Let me give an example: Object 1: x = 2 y = 4 Object 2: x = 3 y = 6 So on a plane they would look like this (y axis) 6 . O2 5 . / 4 . O1 3 . 2 . 1 . 0 . . . . . . 0 1 2 3 4 5 (x axis) Here the "/" represents the distance between the two objects. Now how would we go about finding the length of this straight line between the two? First of all, a distance can either be 0, or longer, right? It can't be a negative number. But, if you were to subtract Object 1's values from that of Object 2's (x diff: 2 - 3 = -1, y diff: 4 - 6 = -2), we get two negative numbers, which obviously can't be the distance between two objects. The solution to this is simple: the square of any number is positive. -1 ** 2, or Math.pow(-1, 2) becomes a 1, and -2 ** 2, or Math.pow(-2, 2) becomes a 4. Hence the reason behind this part of the code:

Math.pow(this.getX() - o.getX()), 2.0)

and

Math.pow(this.getY() - o.getY()), 2.0);

We are simply calculating the squares of the differences. After we get the values from these calculations, we add the two differences together. With the sample coordinates above, it would become "1 + 4 = 5". Now, because we squared the numbers, we must take their square root to get our actual solution.

Math.sqrt(Math.pow((this.getX() - o.getX()), 2.0) + Math.pow((this.getY() - o.getY()), 2.0))

which in our example is equal to

Math.sqrt(5)

I hope that clears it all up.

Misiu

Level 41 , Gdansk, Poland

18 October 2020, 12:39

Wow, what an explanation. Admire :)