Hey guys. I have something for you. It’s my implementation of the quadtree, built on top of the Wikipedia Article.
Quadtree.java:
package org.matheusdev.arcengine.collision;
import java.util.List;
/**
*
* @author matheusdev
*
*/
public class Quadtree<E extends QuadtreeElement> {
private final QuadRect bounds;
private E[] elements;
private Quadtree<E> topLeft;
private Quadtree<E> topRight;
private Quadtree<E> botLeft;
private Quadtree<E> botRight;
public Quadtree(float size, int elemPerQuad) {
this(0, 0, size, elemPerQuad);
}
@SuppressWarnings("unchecked")
public Quadtree(float x, float y, float size, int elemPerQuad) {
bounds = new QuadRect(x, y, size);
elements = (E[])(new QuadtreeElement[elemPerQuad]);
}
protected boolean set(E e) {
for (int i = 0; i < maxElem(); i++) {
if (elements[i] == null) {
elements[i] = e;
return true;
}
}
return false;
}
public int maxElem() {
return elements.length;
}
public boolean insert(E e) {
if (!bounds.contains(e.getX(), e.getY())) {
return false;
}
if (set(e)) {
return true;
} else {
subdivide();
if (topRight.insert(e)) return true;
if (topLeft.insert(e)) return true;
if (botRight.insert(e)) return true;
if (botLeft.insert(e)) return true;
throw new ImpossibleException();
}
}
public void query(StaticRect range, List<E> results) {
if (!bounds.intersects(range)) {
return;
}
for (int i = 0; i < maxElem(); i++) {
if (elements[i] != null) {
if (range.contains(elements[i].getX(), elements[i].getY())) {
results.add(elements[i]);
}
}
}
if (!hasChildren()) {
return;
}
topLeft.query(range, results);
topRight.query(range, results);
botLeft.query(range, results);
botRight.query(range, results);
}
public boolean hasChildren() {
return topLeft != null;
}
/**
* <p>
* Subdivides this Quadtree into 4 other quadtrees.
* </p>
* <p>
* This is usually used, when this Quadtree already has an
* Element.
* </p>
* @return whether this Quadtree was subdivided, or didn't subdivide,
* because it was already subdivided.
*/
protected boolean subdivide() {
if (hasChildren()) {
return false;
}
float hs = bounds.s/2;
topLeft = new Quadtree<E>(bounds.x, bounds.y, hs, maxElem());
topRight = new Quadtree<E>(bounds.x+hs, bounds.y, hs, maxElem());
botLeft = new Quadtree<E>(bounds.x, bounds.y+hs, hs, maxElem());
botRight = new Quadtree<E>(bounds.x+hs, bounds.y+hs, hs, maxElem());
return true;
}
}
QuadtreeElement.java:
package org.matheusdev.arcengine.collision;
public interface QuadtreeElement {
public float getX();
public float getY();
}
I won’t post the SolidRect.java and the QuadRect.java code here, because these classes would then require Collidable.java and that would be too much for this topic 
The classes QuadRect.java and SolidRect.java are easy to implement anyways 
As you can see on the package name, this is code from my current work-in-progress engine ArcEngine. I hope, I’ll finish it sometime. It will be built ontop of libGDX with TWL and other stuff