Self-made Platform engine: Am I doing it wrong?

Game Mechanics
#1

Hi guys!

I’m currently playing around with java game development a bit.
I usually only write Servers in Java, so this is my first graphical application I am working on.
I learned the basics of how game update and render loops work, and got everything running smoothly so far.

There’s just one problem I came across:
Since I don’t use any framework (I love doing things on my own) I do not have the slightest idea of how I should implement a Platformer movement system.
My current approach looks like this:

// Pseudocode!
if (detectCollision(x, y + verticalVelocity) {
	// Moves this instance to the floor
	setToGround();
} else {
	computeVelocity();
	y += verticalVelocity;
}

if (detectCollision(x + horizontalVelocity, y) {
	// Moves this instance to the wall
	stopMovement();
} else {
	computeMovement();
	x += horizontalVelocity;
}

So what I’m basically doing is to iterate over a List of Blocks which define the collision in the level when detecting collisions.

As long as I don’t use walls which stop the horizontal movement of the player, everything works just fine.
But as soon as I move against a wall, everything goes foobar. I guess that’s because my collision detection is not entirely right:

//Actual code
public int detectCollision(Model model, int offX, int offY)
	{
		int colliding = 0;
		
		int mTop 	= (int) (y + offY);
		int mBot 	= (int) (y + height + offY);
		int mRight 	= (int) (x + width + offX);
		int mLeft 	= (int) (x + offX);
		int oTop	= (int) (model.y);
		int oBot	= (int) (model.y + model.height);
		int oRight	= (int) (model.x + model.width);
		int oLeft	= (int) (model.x);
		int mMidX	= (int) (x + (width / 2) + offX);
		int mMidY	= (int) (y + (height / 2) + offY);
		
		if ((mRight >= oLeft) && (mLeft <= oRight)) {
			if ((mBot >= oTop) && (mBot <= oBot)) {
				colliding += 8;
			}
			if ((mTop <= oBot) && (mTop >= oTop)) {
				colliding += 2;
			}
		}
		if ((mBot >= oTop) && (mTop <= oBot)) {
			if ((mRight >= oLeft) && (mRight <= oRight)) {
				colliding += 1;
			}
			if ((mLeft <= oRight) && (mLeft >= oLeft)) {
				colliding += 4;
			}
		}
				
		return colliding;
	}

What this basically does is returning a number from which I can read WHERE the collision is happening exactly.
Since the feedback is binary, there only is one possible combination for each situation.

This way, I can check if the instance is straight below (e.g. 8, in this case) my entity.
Anyway, this does not seem to work right.

So, my question is: Am I doing this right? Should I just continue optimising this simple JnR engine or am I doing it completely wrong?
If so, can anyone tell me a simple mechanic of how to do that right?

P.S. Sorry if this question was asked already, I used this forum’s search function but could not find any fitting topic.