Friday, June 3, 2016

Super basic object stuff

I am not going to rework this right now, I think it gets the main points across as is.

From a recent Reddit post:

What you are asking about is quite basic to object oriented programming. Let's start with the first point I'd like to make: PersonA and PersonB are exactly the same thing, except the data is different. However, they cannot be simple methods that return integer values, that will never fly. The values you're setting up are lost as soon as the methods end.
Instead, you want to create objects of type Person. You do this by creating a class called Person and setting up variables to represent the values you want. At its absolute most basic (and this isn't correct, it's just showing the idea) it would look like this:
public class Person {
    public int x;
    public int y;
    public int range;
}
This is more akin to what we used to call a struct in c. Ordinarily you'd have a lot more stuff in there, but adding that right now could inhibit understanding so I want to take this slowly.
With the above class you could do this in the class you've posted:
public static void main(String [] args) {
    Person a = new Person();
    a.x = 200;
    a.y = 100;
    a.range = 160;

    Person b = new Person();       
    b.x = 100;
    b.y = 400;
    b.range = 170;
After that, you could use 'a.x' or 'b.range' the same way you were originally trying to use 'xA' or 'rangeB'.
Remember when I said that I was just showing the basics? Well, objects are a lot more powerful than just bags of variables. Let's use just another couple of features.
First, let's make a 'constructor' for Person so you can slim down your code:
public class Person {
    ... //What you already have
    public Person(int newX, int newY, int newRange) {
        x = newX;
        y = newY;
        range = newRange;
    }
}
Now your main can do this:
Person a = new Person(200, 100, 160);
Isn't that nicer?
Next, let's add 'accessor' methods to Person. Accessors are also called 'getters' and there are good reasons to use them. Trust me on this, I'm on the train and don't have time to fully explain:
public class Person {
    ... //What you already have

    public int getX() {
        return x;
    }

    //Do the same thing for 'y' and 'range'
}
At this point, you could still do 'a.x = 5' or 'range = b.range'. We can prevent that by marking the variables as 'private'
public class Person {
    private int x;
    private int y;
    private int range;
    ... //what you otherwise already have
}
But wait! Now when I try to write a.x=5 I get some horrible error and everything is broken and the world is ending!
No, you just need 'setters', also known as modifiers to do the job for you:
public class Person {
    ... //Everything you already have
    public void setX(int newX) {
        x = newX;
    }
    // Follow the same pattern for y and range
}
Now the equivalent to 'int range = a.range' is 'int range = a.getRange()', and the equivalent of 'a.range = 50' is 'a.setRange(50)'.
There's a LOT more to OOP, we've really just scratched the surface. But just encapsulating your data in this way will go a long way towards making your programs more readable, maintainable and robust.
I have no doubt that questions are coming up in your mind. I tried to make this clear and left out a few niceties in the interest of getting it written quickly and not overloading you, but it's a lot to absorb. Please ask questions when you need clarification.