How to load all images from folder? (Processing, Java)

EDIT: Coming back to this as a full stack developer with a masters degree is so funny. Hope it still helps somebody!

please help me. I want to load multiple images into this processing sketch without knowing the datanames. So that i can always just put .png images into the data folder and the program automatically loads them in. I've searched in some forums but didn't find anything except some code that I've already used but it doesn't run properly.

As soon as the program starts it gives me a NullPointerException at image();

This is the consoleoutput:

4096
D:\Program Files\processing-3.3.7\PROJECTS\Blendertutorial\data
[0] "1.png"
[1] "2.png"
[2] "3.png"
[3] "4.png"
[4] "5.png"
[5] "6.png"
[6] "7.png"

Also why is the folder.list(); output such a huge number? I only have 7 images in there...

import java.io.File;
String fileExtension = ".png";

java.io.File folder = new java.io.File(sketchPath("/PROJECTS/Blendertutorial/data"));
java.io.FilenameFilter extfilter = new java.io.FilenameFilter() {
  boolean accept(File dir, String name) {
    return name.toLowerCase().endsWith(fileExtension);
  }
};

PImage images;
String[] imageNames;
int i=0;
long folderInhalt = folder.length();

void setup(){
  size(500,500);
  println(folder.length());
  println(folder);
  printArray(imageNames);

  imageNames = folder.list(extfilter);
}

void draw(){
  if(mousePressed){
    images = loadImage(folder+"/"+imageNames[0]);
    println(images);
    println(imageNames[i]);
    delay(200);
    i++;
  }
  image(images,0,0);      //NULL POINTER EXCEPTION!
}

Solution

What happens in your code when draw() is called, but mousePressed is false?

Consider your code:

PImage images;
...
...
...

void draw(){
  if(mousePressed){
    images = loadImage(folder+"/"+imageNames[0]);
    println(images);
    println(imageNames[i]);
    delay(200);
    i++;
  }
  image(images,0,0);      //NULL POINTER EXCEPTION!
}

You have declared images but have not instantiated it.

In the case of mousePressed==false, images will remain null.

This behavior can explain your NullPointerException - you are invoking the image method with a null valued parameter.