Calling methods with different parameters in Java

Question

I am in the process of programming a simple program that deals with object recognition. What causes problem is that I am considering and making a gui, but the problem here is that it won't let me call a method. I've looked around the site, but got no luck. Tried different suggestions from others, still nothing. I just don't know how I should call a method with different parameters.

Here's what I've tried..

public class Main {

    private static ImageProcessor ip;
    public static void main(String[] args) {
        Main_ m = new Main_();
        m.main(ip);
    }
}

Here's the desired method that I am trying to call..

public class Main_ implements PlugInFilter{

......

    public static String launch(ImageProcessor ip){
            ip  = FiltreGaussien_.apply(ip, 3);
            ImageProcessor result = Otsu_.apply(ip);
            Canny_ cannyFilter = new Canny_(result);
            result = cannyFilter.apply(5);
    
            Hough_ houghFilter = new Hough_(result);
            List<Line> lines = houghFilter.apply();
    
    
            try{
                Card card = new Card(lines);
                ip = card.extractCorner(ip);
                ImagePlus imp = new ImagePlus("...", ip);
                new ImageWindow(imp);
            }
            catch(RuntimeException e){
    
            }
    
            ip = Otsu_.apply(ip);
            TemplateMatching_ matcher = new TemplateMatching_();
            return matcher.launch(ip);
        }
}

And my console gives me this..

Exception in thread "main" java.lang.NullPointerException
    at ij.process.ByteProcessor.<init>(ByteProcessor.java:96)
    at main.mean.FiltreGaussien_.apply(FiltreGaussien_.java:69)
    at main.card_detection.Main_.launch(Main_.java:30)
    at main.card_detection.Main_.main(Main_.java:22)
    at main.Main.main(Main.java:11)

Process finished with exit code 1

What am I doing wrong?

Your problem is that FiltreGaussien_.apply is incorrectly initializing a ByteProcessor - this is what the stack trace tells you. Since we have neither the code nor the documentation for FiltreGaussian, we cannot say what is wrong with it/ — user15187356, Apr 15 '21 at 00:42
Some variable is not initialized. Check your code in line 69 of FilterGaussian, as the stack trace says. May be related to how you swallow (`catch` and ignore) the exception in `launch`. Use the debugger in your IDE to step through the code to see what variable is `null`. Or [edit] your question and trim your code down to a [mcve]. — Robert, Apr 15 '21 at 00:46
Does this answer your question? [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — Charlie Armstrong, Apr 15 '21 at 01:03
@CharlieArmstrong. For me it did after I got the answer by anar1501. — Henrik.A, Apr 15 '21 at 01:23

score 1 · Accepted Answer · answered Apr 15 '21 at 00:47

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable. It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't. But you can - and your IDE will help you here. Run your program in the debugger and when it fails, the IDE will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, it will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

Thanks for your time and your good explanation, now I understand the problem. Time to look at it with the debugger :-) — Henrik.A, Apr 15 '21 at 01:21

Calling methods with different parameters in Java

1 Answers1