java multiple classes exercise : Here is the code below .

public class Country{

	private String name;
	private City [] cities;
	private int index =0;
	
	public Country (String n, int nrc){  // nrc as in number of cities
		name = n;
		cities = new City[nrc];
	}
	public boolean exists (City str){
		for(int i =0; i>index;i++){
			if(cities[i].equals(str)){
				return true;
			}
		} return false;
	}
	
	public void addCity (City str){
		if(str == null){
			System.out.print("City not initialized!");
		}
		if(exists(str)){
			System.out.print("City  exists!");
		}
		if(cities.length == index){
			System.out.print("Not enough space in array!");
		}
		cities[index++] = str;
	}
}


I want to know what " cities[index++] = str " is supposed to do. Anyone help? Thanks.