Can't seem to make Collision detection work for this code
PVector pos_player;
    Bullet aBullet;
    Zombie zombie;
    void setup() {
      size(1200, 800);
      smooth();
      pos_player = new PVector( -100, -100, 0);
      aBullet = new Bullet();
      pos_player = new PVector( 200, 400, 0 );
      zombie = new Zombie();
    }

    void draw() {
      background(128);
      stroke(0);
      fill(0,255,0);
      rect( pos_player.x, pos_player.y, 50, 50);
      aBullet.simulate();
      aBullet.draw();
      zombie.simulate();
      zombie.draw();
     
       if( key == CODED ){
        if( keyCode == UP && keyPressed ){
         pos_player.y--;
        }
        if( keyCode == DOWN && keyPressed ){
          pos_player.y++;
        }
       }
}


    void keyPressed(){
    
      if( key == ' ' || key == 'f' ){
        aBullet = new Bullet();
      }
    }

    class Bullet{
      PVector velocity;
      PVector p,v;
     
      Bullet(){
        p = new PVector( pos_player.x+50, pos_player.y+25, 0 );
        v = new PVector(4,0,0);
      
      }
     
      void simulate(){
        p.x+=v.x;
        p.y+=v.y;
      }
      void draw(){
        pushStyle();
        fill(255,0,0);
        rectMode(CENTER);
        rect( p.x, p.y, 25, 25);
        popStyle(); 
      }
     
    }
   
    class Zombie {
      PVector p,v;
      Zombie(){
        p = new PVector( 1000,random(800),0);
        v = new PVector(-1,0,0);
      }
      void simulate(){
        p.x+=v.x;
        p.y+=v.y;
      }
      void draw(){
        pushStyle();
        fill(0,0,255);
        rectMode(CENTER);
        rect( p.x, p.y, 50, 50);
        popStyle(); 
      }
    }
 
Basically I want to be able to tell if the zombie touches any bullets the player fires. I also cant figure out how to make it so that when the bullet touches a zombie it kills it. Any help?

p.s zombie is blue bullet is red