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I've got a class which features some ArrayList of BufferedImage, but I've got a serious problem. There is 2 possibilities :

-The ArrayList are all static, and so their getting methods are : this works fine, as the app is launching and the animation is running perfectly. But I can't have differents animations since there are statics.

-The ArrayList (and their getters) are not static : I get a NullPointerException when getDown() is called, which point at the precise moment where this one is called.

Before that, I used simple arrays and I believed that use ArrayLists would solve the problem, but there is no difference.

I don't understand why it's doing that, could you please help me on this matter ?

public class AnimUnit { private static final int width = 32, height = 32, nbframe = 4; private ArrayList<BufferedImage> down; private ArrayList<BufferedImage> up; private ArrayList<BufferedImage> right; private ArrayList<BufferedImage> left; private ArrayList<BufferedImage> idle; public AnimUnit(SpriteSheet sheet) { this.down = new ArrayList<BufferedImage>(); this.up = new ArrayList<BufferedImage>(); this.left = new ArrayList<BufferedImage>(); this.right = new ArrayList<BufferedImage>(); this.idle = new ArrayList<BufferedImage>(); for(int i = 0; i < nbframe; i++) down.add(sheet.crop((width*2)+2, (height*i)+i, width, height)); for(int i = 0; i < nbframe; i++) up.add(sheet.crop((width*3)+3, (height*i)+i, width, height)); for(int i = 0; i < nbframe; i++) left.add(sheet.crop((width)+1, (height*i)+i, width, height)); for(int i = 0; i < nbframe; i++) right.add(sheet.crop((width*4)+4, (height*i)+i, width, height)); for(int i = 1; i < nbframe; i++) idle.add(sheet.crop(0, (height*i)+i, width, height)); } public static int getWidth() { return width; } public static int getHeight() { return height; } public ArrayList<BufferedImage> getDown() { return down; } public ArrayList<BufferedImage> getUp() { return up; } public ArrayList<BufferedImage> getRight() { return right; } public ArrayList<BufferedImage> getLeft() { return left; } public ArrayList<BufferedImage> getIdle() { return idle; }
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    Could you share the client code calling methods of AnimUnit? In general, static methods/members should be avoided in object-oriented programming. You should be working with the object instances instead. Commented Jul 28, 2017 at 22:25
  • I recommend to use Singleton pattern instead of static methods for "random" class method access. Commented Jul 28, 2017 at 23:01
  • I removed the "statics" from the code since it seems people misunderstand my point. I would want my code to run properly with instances, but its keeps giving me NullPointerException if they are not statics, and this is odd to me. I don't find any way to solve the problem... Commented Jul 28, 2017 at 23:51
  • Many consider singletons an anti-pattern Commented Jul 28, 2017 at 23:53

3 Answers 3

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Currently, the properties maintained in your class are all static. You're using a constructor to assign values to them, which could be misleading to user of your class since your class has no non-static properties. If your constructor isn't called, then they're not initialized (and will throw null pointer exceptions when accessed), but a constructed object of nothing but static methods is kind of useless.

Remove the word "static" from all your properties and methods, and I think it will work like you'd want and expect.

AnimUnit animUnitA=new animUnit(spriteSheetA); AnimUnit animUnitB=new animUnit(spriteSheetB); ArrayList<BufferedImage> downA=animUnitA.getDown(); ArrayList<BufferedImage> downB=animUnitB.getDown();
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2 Comments

I think you misunderstood my post. I know that I can't use this class properly with statics ListArray, but I get NullPointerExceptions when I remove them (of course, I remove the "statics" before the attributes AND the getters)
You've edited the code in your post, so you should verify whether your circumstances have changed. When your attributes were static, you could access them without initializing them. Now that they're non-static, you can't attempt to access them unless you've constructed your object (which initializes them).
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Remove the word "static" from all your properties and methods, and whenever you initialize something to null initialize it to "" instead.

Example:

Instead of:

String xyz = null;

Try:

String xyz = "";
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It won't change anything since I never initialize something to "null" or ""
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Ok I just made some tests and fianlly found out that the NullPointerException was about an instance of the AnimUnit class itself, not one the ArrayLists. Thanks to all anyway, even if the problem wasn't about the arrays at all

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2 Comments

So delete your futile question.
Sorry, but even is this question is this futile, I cannot delete it because "many people invested time and effort in answering it".

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