Strange Collision Behaviour

I'm having some difficulty implementing very basic collision within libGDX. The update code for the "player" is as so:

private void updatePosition(float _dt)
{
    Vector2 _oldPos = _pos;

    _pos.add(_mov.scl(_dt*50));
    _bounds.setPosition(_pos.x-8, _pos.y-12);

    if(_game.getMap().checkCollision(_bounds))
    {
        System.out.println("Collision Detected");
        _pos = _oldPos;
        _bounds.setPosition(_pos.x-8, _pos.y-12);
    }
}

So, initially _oldPos is stored as the values of _pos position vector before applying any movement.

Movement is then performed by adding the movement vector _mov (multiplied by the delta-time * 50) to the position vector, the player's bounding rectange _bounds is then updated to this new position.

After this, the player's new bounding Rectangle is checked for intersections against every tile in the game's "map", if an intersection is detected, the player cannot move in that direction, so their position is set to the previous position _oldPos and the bounding rectangle's position is also set to the previous position.

Unfortunately, this doesn't work, the player is able to travel straight through tiles, as seen in this image:

So what is happening here? Am I correct in thinking this code should resolve the detected collision?

What is strange, is that replacing

_pos = _oldPos;

with (making the same move just made in reverse)

_pos.sub(_mov.scl(_dt*50));

Yields very different results, where the player still can travel through solid blocks, but encounters resistance.

This is very confusing, as the following statement should be true:

_oldPos == _pos.sub(_mov.scl(_dt*50));

---

A better solution for collision detection would be to have a Vector2 velocity, and every frame add velocity to position. You can have a method that tests if Up arrow key is pressed, add 1 (or whatever speed you would like) to velocity. And if down is pressed, subtract 1 (or whatever speed). Then you can have 4 collision rectangles on player, 1 on top of player, bottom, left, and right. You can say if(top collides with bounds){ if(velocity.y > 0){ set velocity.y = 0. } }

And do the same for down, left and right (eg... for bottom, make sure to test if(velocity.y < 0) instead of if(velocity.y > 0).

EDIT: You code is not working because you set oldPos = pos without instantiating a new Vector2. Which means when you add onto pos, it also changes oldPos. So say oldPos = new Vector2(pos);

---

try to test future position before move. If collision, don't move.

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