Thread: acessing ArrayList

ok i need to make an method that adds a room on the specific floor
on each floor there can be maxRoomPerFloor rooms
if on floor are no rooms return false on other hand return true
@param floor is floor on witch room is added
@param r is room that is added

here is what i made so far

 
import java.util.ArrayList;
import java.util.List;
 
public class Hotel {
	String name;
	String address;
	List[] floors=new ArrayList[3]; /**floors in Hotel there are 2 floors plus a ground floor.**/
 
	int maxRoomPerFloor=4; /** max number rooms per floor*/
 
 
        public Hotel(String name, String address){
		this.name=name;
		this.address=address;
	}
 
public boolean addRoom(int floor, Room r){
		int roomPerFloor1;
		int roomPerFloor2;
		int roomPerFloor3;
		for(int i=0;i<floors.length;i++){
				if((floors[i]=floor)  && 
				   (roomPerFloor1<maxRoomPerFloor)&&
				   (roomPerFloor2<maxRoomPerFloor)&&
				   (roomPerFloor3<maxRoomPerFloor)){
					floors[i].add(r);
 
					switch (floor) {
		            case 1:  roomPerFloor1++;
		                     break;
		            case 2:  roomPerFloor2++;
                    		break;
		            case 3:  roomPerFloor3++;
                    		break; 
		            default: 
                    break;		
				    }
				}
				if((roomPerFloor1>maxRoomPerFloor) || 
				  (roomPerFloor2>maxRoomPerFloor) || 
				  (roomPerFloor3>maxRoomPerFloor)){
					return false;
				}
		}
		return true;
 
	}

in the first if statement if(floors[i]=floor) is giving me error saying canot convert from int to List, so how do i access the element in List?

(floors[i]=floor)

Does this do what you wanted? Did you want to compare floors[i] to floor, or set floors[i] to floor?

so how do i access the element in List?

The same way you access an element in any list.
myListName[indexNumberOfElementNeeded]
just like floors[i]

(roomPerFloor1<maxRoomPerFloor)&&
(roomPerFloor2<maxRoomPerFloor)&&
(roomPerFloor3<maxRoomPerFloor)

I see three problem with these lines of code. See the code just above those lines:

int roomPerFloor1;
int roomPerFloor2;
int roomPerFloor3;

When will 'NO VALUE' be less than maxRoomsPerFloor?

Originally Posted by jps

Does this do what you wanted? Did you want to compare floors[i] to floor, or set floors[i] to floor?


The same way you access an element in any list.
myListName[indexNumberOfElementNeeded]
just like floors[i]



I see three problem with these lines of code. See the code just above those lines:
When will 'NO VALUE' be less than maxRoomsPerFloor?

I wanted to set floors[i] to floor but it giving me error
yeah i know they have no value but i do that becouse if i set them to 0 they gonna reset every time i exit the for loop, so i think i need better implementation of this. i somehow need to check if room are full for every floor but dont know how to store them so they dont reset?

I wanted to set floors[i] to floor

Understood. Why is this inside an if statement as a stand alone unit. if ((floors[i]=floor) && (etc) && (etc) ) {
It just looks to add confusion to what the code is to accomplish. I would rather see something like: if ((floors[i]=floor > 0) && (etc) && (etc)) { ...you know, some conditional rather than a stand alone assignment.

I wanted to set floors[i] to floor but it giving me error
//.....
List[] floors=new ArrayList[3]

What data type does floors contain? What data type are you trying to add to floors? The compiler is complaining that the list holds type object and you are trying to add type int. There is a way to specify the list will hold a specific type (int in this case), rather than default object.

yeah i know they have no value but i do that becouse if i set them to 0 they gonna reset every time i exit the for loop, so i think i need better implementation of this

Yes I agree. As soon as you correct the type error on the list, the compiler is going to bark at you for all three uses of the variables which were not initialized. They simply must be given a value before you can compare them to anything. The variables (and the values they hold) are available anywhere the variable is in scope. Keeping that in mind, realize that when you execute the body of a loop, the loop is actually exited and re-entered at the top. Visualize it like jumping into the pool (loop start), swimming to the other end (loop body), and getting out of the pool to run back to the end where you jumped in (loop restart). You dry off while you are running outside the pool (variables scope was lost).

I hope I did not add to the confusion with the analogy, but RL duty calls and I must go. Good luck and hopefully I didn't cause more confusion.

Originally Posted by jps

Understood. Why is this inside an if statement as a stand alone unit. if ((floors[i]=floor) && (etc) && (etc) ) {
It just looks to add confusion to what the code is to accomplish. I would rather see something like: if ((floors[i]=floor > 0) && (etc) && (etc)) { ...you know, some conditional rather than a stand alone assignment.

Sorry actually im trying to compare them, so i found out i have to use .equals statement so it will be if((floors[i].equals(floor)) &&(...)&&(...) think is good like this

Originally Posted by jps

What data type does floors contain? What data type are you trying to add to floors? The compiler is complaining that the list holds type object and you are trying to add type int. There is a way to specify the list will hold a specific type (int in this case), rather than default object.

i get it. It can be set like this: int[] floors=new int[3] but if i do like this i will be unable to add Object type room to the Array

Originally Posted by jps

Yes I agree. As soon as you correct the type error on the list, the compiler is going to bark at you for all three uses of the variables which were not initialized. They simply must be given a value before you can compare them to anything. The variables (and the values they hold) are available anywhere the variable is in scope. Keeping that in mind, realize that when you execute the body of a loop, the loop is actually exited and re-entered at the top. Visualize it like jumping into the pool (loop start), swimming to the other end (loop body), and getting out of the pool to run back to the end where you jumped in (loop restart). You dry off while you are running outside the pool (variables scope was lost).

Yeah im aware of it and still i need to fix it. Im thinking what if i put them to Array?

i get it. It can be set like this: int[] floors=new int[3] but if i do like this i will be unable to add Object type room to the Array

Originally the code used an ArrayList. Now the code shown here uses an array. Just to be clear, an ArrayList can be declared with a type.

Yeah im aware of it and still i need to fix it. Im thinking what if i put them to Array?

Any type of variable declared inside the loop will have the same problem as any other type of variable declared inside the loop. To maintain the values the variable's scope must be extended beyond the loop.

Originally Posted by jps

Originally the code used an ArrayList. Now the code shown here uses an array. Just to be clear, an ArrayList can be declared with a type.

u mean ArrayList<Type> but it cannot be int, right just String and i need to cast it right?

Originally Posted by jps

Any type of variable declared inside the loop will have the same problem as any other type of variable declared inside the loop. To maintain the values the variable's scope must be extended beyond the loop.

So how do i solve this maybe add second inner for loop that counts rooms or
i solve it somehow with break statement to exit for and then increase counter?

u mean ArrayList<Type> but it cannot be int, right just String and i need to cast it right?

Yes. Also search "primitive types wrapper classes"

So how do i solve this maybe add second inner for loop that counts rooms or
i solve it somehow with break statement to exit for and then increase counter?

Think about how to solve the problem in terms of solving the problem before thinking in terms of code. Organize a list of steps into a step by step solution to the problem. Then write code to follow the steps. The steps will help guide what the code logic will look like.

What if I declare the roomPerFloor1,2,3 up in the beginning and initialize them in the constructor of hotel, i assume the constructor will be called only once and maybe replace switch with if but i belive it wont make a difference and maybe make a method that increases each variable?

Did you try it? Did it work?

Actually no, i added an main method and tryed to add a room but it giving me errors

public static void main(String[]args)throws Exception{
		Hotel hotel=new Hotel("Hilton","Brodway 1001");
		Room r=new Room(1,2,"view on beach");
		hotel.addRoom(1, r);
		for(int i=0;i<floors.length;i++){
		System.out.println(i);}
	}

error is stupid null pointer exception here in method addRoom

if((floors[i].equals(floor))

and here in main method

for(int i=0;i<floors.length;i++){

Did you try to println the variables and see which one(s) are null?
If you need more help you will need to post the code and error messages.

i solved the null pointer exceptions but now it seems it isnt working properly
the if statement if ((floors[i].equals(r)) was giving me null pointer so i changed it with if ((floors.equals(r)) where floor is: static List[] floors=new ArrayList[3];
now im not sure what it does, help

Any time you make changes to the code, post the new code with your question so that everyone is looking at the same code.