Why does this get Method return a null value?

Question

I can not figure out why the return value for method getButton is null.

I am trying to create a maze game. The setUp method, in the MazeFrame, class draws the maze. Each step within the maze is a button which, in this program, is a Step object which extends JButton.

I need the getButton method to work, so I can call it in other classes and refer to a particular step/button by its index, but this method is bent on always returning null, instead of the correct button I am referring to.

The only time it does not return null is when I call it with a = 6, b = 6, which is the step.length - 1. Then it returns the correct button.

I tried creating an ArrayList where I stored each Step/Button as soon as they were created. Then I had the getButton method run through this array list and return the step within it that had the same index values (a and b, i and j, row and column) as the index values passed into getButton.

The return value is still null.

public class MazeFrame extends JFrame {
    Container contentPane;
    private int buttonCount = 7;
    private Step[][] steps;
    private ArrayList<Step> locations = new ArrayList<>();

    public static void main(String[] args) {
        FirstMaze thisOne = new FirstMaze();
        thisOne.SetUp();
        thisOne.pack();
        thisOne.setVisible(true);
        thisOne.setSize(new Dimension(400, 400));
    }

    public void SetUp() {
        //sets up container and sets layout grid
        contentPane = this.getContentPane();
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        contentPane.setLayout(new GridLayout(buttonCount, buttonCount));

        //run through the grid
        for (int i = 0; i < buttonCount; i++) {
            for (int j = 0; j < buttonCount; j++) {
                steps = new Step[buttonCount][buttonCount];

                //create and add buttons to the container
                //for the start button
                if (i == 0 && j == 0) {
                    contentPane.add(steps[i][j] = new Step(i, j, "start", this));
                }
                //for the goal button
                else if (i == buttonCount - 1 && j == buttonCount - 1) {
                    contentPane.add(steps[i][j] = new Step(i, j, "goal", this));
                }
                //every other button
                else {
                    contentPane.add(steps[i][j] = new Step(i, j, "intermediary", this));
                }
                steps[i][j].setTransitions(steps[i][j].getTransitions(i, j));
                getButton(i, j); //call to getButton
            }
        }
    }

    public Step getButton(int a, int b) {
        System.out.println("is the step: a,b null? " + (steps[a][b] == null)); //Print statement is a test
        return steps[a][b];
    }
}

The print statement in getButton is a test, and it confirms that the return value is null.

Answer 1

Your for loop to initialize the array either needs to be placed in a constructor or enclosed in curly brackets to use it as an initialization block.

public class MyClass {
    // wrap the loop in curly brackets
    {
        for (...)
    }
    ...
}

If your code is within a static class, you need to mark your initialization block as static, too.

public static class MyClass {
    // wrap the loop in curly brackets
    static {
        for (...)
    }
    ...
}

Personally, I recommend putting this code in a constructor method.

Answer 2

Here:

    for (int i = 0; i < buttonCount; i++) {
        for (int j = 0; j < buttonCount; j++) {
            steps = new Step[buttonCount][buttonCount];
            ... 

In each iteration of your loop, you set steps to a new empty array and then set one element in it.

The steps = line should not be inside your loops. Move it to before:

    steps = new Step[buttonCount][buttonCount];
    for (int i = 0; i < buttonCount; i++) {
        for (int j = 0; j < buttonCount; j++) {
            ...