Top-down player collision with walls

Question

I am having trouble trying to think of how I am going to approach collisions in my game. My idea which sucked in the beginning because it would reduce performance dramatically has failed which was when I was updating the list of walls it would check collision with players but the player would just stop and wouldn't be able to move anymore. Collision not only has to work with player but with at least 15-20 other entities in the game as well. Does anyone have a good idea to check collision with a top down shooter?

Here is my update method with everything being updated:

public void update(float dt){

    inputHandler();

    //Update the player
    player.update(joystick1, joystick2, projectiles, dt);
    updateCamera();

    //Update them zombies
    for(int i = 0; i < enemies.size(); i++){
        Enemy e = enemies.get(i);
        e.update(player, dt);
        if(e.getDistanceFromPlayer() < 60){
            //check collision
            if(e.collided(player)){
                player.hit(e, particles);
            }
        }
    }

    //Update Projectiles
    for(int i = 0; i < projectiles.size(); i++){
        Projectile p = projectiles.get(i);
        p.update(dt);
        //Check if projectile has hit any enemies
        for(int j = 0; j < enemies.size(); j++){
            Enemy e = enemies.get(j);
            if(p.collided(e)){
                p.remove();
                e.hit(p);
                ParticleEmitter.createBloodParticle(particles, e.getX(), e.getY(), 5, 500, 10);
            }
        }
    }

    //Update Items
    for(int i = 0; i < items.size(); i++){
        Item item = items.get(i);
        if(item.collide(player)){
            if(item instanceof Ammo){
                player.getCurrentGun().addAmmo(item.getAmount());
            }
            if(item instanceof Health){
                player.addHealth(item.getAmount());
                if(player.getHealth() >= 100){
                    player.setHealth(100);
                }
            }
            item.remove();
        }
    }

    //Update them particles
    for(int i = 0; i < particles.size(); i++){
        Particle p = particles.get(i);
        p.update(dt);

    }

    int numberOfObjectInGame = 1 + walls.size() + enemies.size() + items.size() + particles.size() + projectiles.size();
    System.out.println(numberOfObjectInGame);

    logger.log();

}

I don't know when to check for collisions. Should it be done in the update method or in the player update method, or at some other point? Really confused at this point.

EDIT:

Ok so added 4 rectangle bounds to my player and had each one checked to see if they have collided with a wall. So right now it is working but i'm pretty sure this is horribly inefficient. Any way to make this more efficient...

Here is the code:

for(int i = 0; i < walls.size(); i++){
        Wall w = walls.get(i);
        if(Intersector.overlaps(w.getBounds(), up)){
            if(velocity.y < 0){
                velocity.y = 0;
            }
        }
        if(Intersector.overlaps(w.getBounds(), down)){
            if(velocity.y > 0){
                velocity.y = 0;
            }
        }
        if(Intersector.overlaps(w.getBounds(), left)){
            if(velocity.x < 0){
                velocity.x = 0;
            }
        }
        if(Intersector.overlaps(w.getBounds(), right)){
            if(velocity.x > 0){
                velocity.x = 0;
            }
        }
    }

It is checked to see if the player is colliding with every wall and the problem is this is only the player. This has to check with every moving entity in the game. Is this a good way to implement this into a game or should I look into something else?

Answer 1

First thing is that your player is sticking to the wall because whenever it collides, you set the velocity in that direction to be zero ( velocity.x=0;). zero means it stops moving in that direction, which is exactly whats happening. What you want to do is velocity.x=-velocity.x; or velocity.x*=-1;. This makes the player move in the opposite direction with the same velocity.

Second is that you have your collision checking code is inside the player object, so it only checks the player's collision. If each object reacts differently to collisions then you have to add the appropriate code in each object.

If they are all gonna bounce around, then keep a common code in your update method and loop through all the entities on stage, accessing their fields (up, down, ...) to check for each collision.

Something like this in your main update method:

Entity[] entities={...};
for(int j=0; i<entities.length; j++){
for(int i = 0; i < walls.size(); i++){
    Wall w = walls.get(i);
    if(Intersector.overlaps(w.getBounds(), entities[j].up)){
        if(entities[j].velocity.y < 0){
            entities[j].velocity.y = 0;
        }
    }
    .
    .
    .
}

}