Question

I’ve been searching a lot for this problem and I can’t find a solution. I’m trying to build a mini-game and I have a method for creating platforms. I have a class with every platform parameters, and I made a class array so i can have multiple platforms at the same time.

Problem: When I try to call the method for constructing the platform by sending the parameters I want, it gives me a NullPointerException. The method was working before, but with everything static and so i couldnt have multiple instances of that class, and now I removed the static fields from the platform class and it gives me the NullPointerException every time I call the method.

I copied the part of the code that gives me the error, the error goes the following way:

public static void main(String[] args) {
        Game ex = new Game();
        new Thread(ex).start();
    }

In Game class:

public Load_Stage load = new Load_Stage();
public Game() {
        -other variables initializatin-
        Initialize_Items();
        load.Stage_1(); // <--- problem this way

In Load_Stage class:

public class Load_Stage {
    public Platforms plat = new Platforms();

    public void Stage_1(){    
        Stage_Builder.Build_Platform(200, 500, 300, plat.platform1);
        Stage_Builder.Build_Platform(100, 200, 100, plat.platform1);
    }

}

And inside the Stage_Builder class:

public class Stage_Builder {

    public static final int max_platforms = 10;
    public static Platform_1[] p1 = new Platform_1[max_platforms];
    public static boolean[] platform_on = new boolean[max_platforms];    

    public Stage_Builder() {
        for (int c = 0; c < platform_on.length; c++) {
            platform_on[c] = false;
        }
    }
    public static void Build_Platform(int x, int y, int width, ImageIcon[] type) { // BUILDS A PLATFORM

        for (int b = 0; b < max_platforms; b++) {
            if (platform_on[b] == false) { 
                p1[b].Construct(x, y, width, type); // <-- NullPointerException here
                platform_on[b] = true; 
                break;
            }
        }
    }
}

Thanks beforehand.

EDIT: Here’s the Platform_1 class (sorry for forgetting about it):

public class Platform_1 {

    private int platform_begin_width = 30;
    private int platform_middle_width = 20;
    public int blocks_number = 0;
    public ImageIcon[] platform_floors = new ImageIcon[500];
    private int current_width = 0;
    public int [] platform_x = new int [500];
    public int platform_y = 0;
    public int platform_width = 0;

    public void Construct(int x, int y, int width, ImageIcon [] type) {        
        platform_width = width;
        platform_y = y;
        for (int c = 0; current_width <= platform_width; c++) { 
            if (c == 0) {
                platform_x[c] = x;
                platform_floors[c] = type[0];
                current_width += platform_begin_width;
            } else if ((current_width + platform_middle_width) > platform_width) {
                platform_floors[c] = type[2];
                blocks_number = c + 1;
                platform_x[c] = current_width + x;
                current_width += platform_middle_width;
            } else {
                platform_floors[c] = type[1];
                platform_x[c] = current_width + x;
                current_width += platform_middle_width;
            }
        }        
    }
}

And the Platforms class:

public class Platforms {

    public ImageIcon[] platform1 = {new ImageIcon("Resources/Sprites/Stage_Objects/Platform1/begin.png"),
        new ImageIcon("Resources/Sprites/Stage_Objects/Platform1/middle.png"),
        new ImageIcon("Resources/Sprites/Stage_Objects/Platform1/end.png")};
}

Answer 1

The problem and solution are both obvious.

public static Platform_1[] p1 = new Platform_1[max_platforms];

After this line of code executes, p1 is an array of references of type Platform_1 that are all null.

Executing this line of code tells you so right away:

            p1[b].Construct(x, y, width, type); // <-- NullPointerException here

The solution is to intialize the p1 array to point to non-null instances of Platform_1.

Something like this would work:

for (int i = 0; < p1.length; ++i) {
    p1[i] = new Platform1();
}