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Floodfill is an problem that is well-solved recursively. It allows us an opportunity to think about the similarities and differences between creating a task for each sub-problem and wrapping all of your recursive calls in a single async. Further, we will harken back to this studio later in the semester when we discuss determinism.


In this studio, you will be using a parallel and recursive method to create an app that fills in a space with color, much like the paint bucket in GNU Image Manipulation Program.

FloodFillTool.png FloodFillShape.png




Demo Video

Mistakes to Avoid

Attention niels epting.svg Warning: Do NOT use == to compare colors. Use Objects.equals(a,b).
Attention niels epting.svg Warning: Do NOT finish more than you need to

Sequential Implementation Increased Stack Size Requirement

For this project, if you decide to first build a sequential version, you will need to change your VM Arguments slightly. In addition to the normal argument that needs to be pasted in for every assignment, you need to add -Xss4m. This will increase the size of the call stack, essentially increasing the maximum depth of your recursion.

The Core Questions

  • What are the tasks?
  • What is the data?
  • Is the data mutable?
  • If so, how is it shared?

Code To Use





Code To Implement

The class is composed of two methods: floodFill and floodFillKernel. The entire process in the kernel is triggered from the floodFill method.

	public static void floodFill(MutablePixels pixels, Color nextColor, int x, int y)
			throws InterruptedException, ExecutionException {
		Color prevColor = pixels.getColor(x, y);
		if (nextColor.equals(prevColor)) {
			// pass
		} else {
			finish(() -> {
				floodFillKernel(pixels, prevColor, nextColor, x, y);

This is so that the kernel can run recursively and asynchronously so the floodFill method can wrap the kernel in an all-inclusive finish.

class: FloodFiller.java Java.png
methods: floodFillKernel
package: floodfill.studio
source folder: src/main/java

The kernel should check if the given pixel is in bounds. If it is, it should check if the given pixel matches the targetColor (the color we wish to change). If it is, it should then replace the color with the replacementColor and asynchronously call the method again to repeat the process for the surrounding pixels. The purpose of the assignment is to understand recursion and how to make it work in parallel.

method: floodFillKernel Parallel.svg (parallel implementation required)

Testing Your Solution



class: FloodFillVizApp.java VIZ
package: floodfill.viz
source folder: src/visualization/java

JavaFX Setup

In order to run the visualization app you will need to setup JavaFX


A test suite exists but is limited and not all that exciting. Demo to an instructor to get credit for your studio locked in.

class: FloodFillTestSuite.java Junit.png
package: floodfill.studio
source folder: src/test/java