Using arrayList pointers instead of multiple loops?

I am trying to write a program that contains many classes and in one class called "Dorm",I have an arrayList of Blocks,and in the "Block" class,I have an arrayList of Rooms,and in the "Room" class,I have an arrayList of "Students".
I am trying to access the number of available rooms(the rooms that at least have one empty space) through another class called the "Manager class". I have been told that I can just create another arrayList in the manager class to be used as a pointer and search up the empty rooms of the whole dormitory.

My question is,how is this going to work?

ps:This is what I wrote:

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public static void availableRooms() { //Shows the available rooms in the dormitory.
		
		Dorms dormitory = new Dorms();
    	Room room1 = new Room();
		
		for(int i=0;i<dormitory.getBlocks().size();i++)
			for(int j=0;j<Block.getRoomList().size();j++) {
				if(!(room1.getStudentList().get(room1.getRoomCapacity()).equals(null)))
					System.out.print("/t" + room1.getStudentList().get(i) + "/t");
				}
		
		
		
  	}

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My code isn't complete yet,so I'm not sure if it works...

Could you share your code/tentative? and clearly specify what's not working?

This being said, unless tied to specific constraints, one should make use of encapsulation and single responsibility principle (see SOLID on wiki) by keeping implementation details private and delegating tasks to the more relevant classes.

You may have something like:

class Dorm {
  private List<Block> blocks = ...
  ...
  public int getAvailableRooms() {
      int total = 0;
      for (Block b : blocks) {
        total += b.getAvailableRooms();
      }
      return total;
   }
 }

class Block {
  private List<Room> rooms = ....
  ...
  public int getAvailableRooms() {
    int total = 0;
    for (Room r : rooms) {
      if (! r.isFull()) {
         total++;
      }
    }
}

class Room {
   private int capacity = ...
   private List<Student> students = ..
   ...
   public boolean isFull() {
       return capacity == students.size();
   }
}

Where the Manager class, holding (an) instance(s) of Dorm, just make use of the getAvailableRooms() method which behind the scene delegate to the underlining Blocks and aggregate result... and so on.