Processing button class - class

Question

The exercise is as follows:
Rewrite programming exercise 3 from lecture 6 by
creating a class called Button to replace the arrays.
a) Create the class and define class variables that
hold information about position, dimensions and
color. In addition a class variable should be made,
which contains the title of the particular button.
Use the constructor to set the initial values of the
class variables.
So basically, I have to convert a previous exercise I have done into a class.
This is how I made the previous exercise in case you need it: http://pastebin.com/RqM6hj6K
So I tried to convert it into class, but apparently it gives me an error and I cannot see how to fix it.
My teached also said that I don't have to keep it as an array, and could instead make many variables instead of the data in the array.
The language is processing and gives error code NullPointerException
class Button
{
int[] nums;
Button(int n1, int n2, int n3, int n4)
{
nums[0] = n1;
nums[1] = n2;
nums[2] = n3;
nums[3] = n4;
}
void display()
{
fill(255, 0, 0);
rect(nums[0], nums[1], nums[2], nums[3]);
}
};
void setup()
{
size(800, 800);
Button butt = new Button(75, 250, 200, 200);
butt.display();
}

Answer 1

You're only declared nums, but not initialized it.
This results in a NullPointerException: in the constructor you're accessing nums[0], but nums doesn't have a length yet. Try this:
class Button
{
//remember to initialize/allocate the array
int[] nums = new int[4];
Button(int n1, int n2, int n3, int n4)
{
nums[0] = n1;
nums[1] = n2;
nums[2] = n3;
nums[3] = n4;
}
void display()
{
fill(255, 0, 0);
rect(nums[0], nums[1], nums[2], nums[3]);
}
};
void setup()
{
size(800, 800);
Button butt = new Button(75, 250, 200, 200);
butt.display();
}
In the future, always make sure the variables you try to access properties of(arrays/objects) are initialized/allocated first(otherwise you'll get the NullPointerException again and it's no fun)
As #v.k. so nicely points out, it's better to have readable code and remove some of the redundancy.
Before the x,y,width and height of your button were stored in an array. That is all the array could do: store data and that's it! Your class however can not only store the same data as individual easy to read properties, but can also do more: functions! (e.g. display())
So, the more readable version:
class Button
{
//remember to initialize/allocate the array
int x,y,width,height;
Button(int x,int y,int width,int height)
{
this.x = x;
this.y = y;
this.width = width;
this.height = height;
}
void display()
{
fill(255, 0, 0);
rect(x,y,width,height);//why don't we use this.here or everywhere ?
}
};
void setup()
{
size(800, 800);
Button butt = new Button(75, 250, 200, 200);
butt.display();
}
Yeah, it's sorta easier to read, but what's the deal with this you may ask ?
Well, it's a keyword that allows you to gain access to the object's instance (which ever that may be in the future when you choose to instantiate) and therefore it's properties (classes version of variables) and methods (classes version of functions). There's quite a lot of neat things to learn in terms of OOP in Java, but you can take one step at a time with a nice and visual approach in Processing.
If you haven't already, check out Daniel Shiffman's Objects tutorial
Best of luck learning OOP in Processing!