Question

I’m currently trying to draw shapes with 2D Arrays. In my class there is a global array defined with public char canvas[][];

Up until now, I have only declared arrays with char canvas[][] = new char[height][width];

If this Array has already been declared, and I’m not supposed to amend the code I’ve been given, how do I call an instance of that array so that I can use it?

thanks.

(edit)

class DrawingSystem {

    public char canvas[][];

       public static void makeNewCanvas(int tmpWidth, int tmpHeight) {

        canvas[][] = new char[tmpHeight][tmpWidth];

        for (int row=0; row<tmpHeight; row++) {
            for (int col=0; col<tmpWidth; col++) {
                canvas[row][col] = ' ';
            }
        }       
    }

Answer 1

You have an incompatibility between static methods and instance variables.

Think about it this way: an instance variable is associated with a specific instance of a class; a static variable is associated with the class itself. You call static methods via the class:

ClassI.callStaticMethod();

Whereas you call an instance method via an instance of the class:

public ClassI classObj = new ClassI();
classObj.callInstanceMethod();

In the code you posted, there’s an instance variable (“canvas”) being set in a static method (main is associated with the Class, not an instance).

Therefore, you’ll need to create instance methods to modify/update your “canvas”, and create an instance of the class within the static function. This object (an “instance”) can be used to update the instance variable.

Here’s an example:

public class Foo {
    public char canvas[][];

    public static void main(String[] args) {
        Foo fooObj = new Foo(); //creates an instance of this class
        fooObj.createCanvas(2, 2);
        fooObj.modifyCanvas(0, 0, 'c');
    }

    public void createCanvas(int x, int y) {
        canvas = new char[x][y];
    }
    public void modifyCanvas(int x, int y, char c) {
        canvas[x][y] = c;
    }
}

This obviously isn’t a one-to-one correlation to your assignment, but I’m sure you’ll be able to adapt it to what you’re doing 🙂