I'm just getting started in Java and am looking for advice on a good way to store nested sets of data. For example, I'm interested in storing city population data that can be accessed by looking up the city in a given state. (Note: eventually, other data will be stored with each city as well, this is just the first attempt at getting started.)
The current approach I'm using is to have a StateList Object which contains a HashMap that stores State Objects via a string key (i.e. HashMap<String, State>). Each State Object contains its own HashMap of City Objects keyed off the city name (i.e. HashMap<String, City>).
A cut down version of what I've come up with looks like this:
// TestPopulation.java
public class TestPopulation {
public static void main(String [] args) {
// build the stateList Object
StateList sl = new StateList();
// get a test state
State stateAl = sl.getState("AL");
// make sure it's there.
if(stateAl != null) {
// add a city
stateAl.addCity("Abbeville");
// now grab the city
City cityAbbevilleAl = stateAl.getCity("Abbeville");
cityAbbevilleAl.setPopulation(2987);
System.out.print("The city has a pop of: ");
System.out.println(Integer.toString(cityAbbevilleAl.getPopulation()));
}
// otherwise, print an error
else {
System.out.println("That was an invalid state");
}
}
}
// StateList.java
import java.util.*;
public class StateList {
// define hash map to hold the states
private HashMap<String, State> theStates = new HashMap<String, State>();
// setup constructor that loads the states
public StateList() {
String[] stateCodes = {"AL","AK","AZ","AR","CA","CO"}; // etc...
for (String s : stateCodes) {
State newState = new State(s);
theStates.put(s, newState);
}
}
// define method for getting a state
public State getState(String stateCode) {
if(theStates.containsKey(stateCode)) {
return theStates.get(stateCode);
}
else {
return null;
}
}
}
// State.java
import java.util.*;
public class State {
// Setup the state code
String stateCode;
// HashMap for cities
HashMap<String, City> cities = new HashMap<String, City>();
// define the constructor
public State(String newStateCode) {
System.out.println("Creating State: " + newStateCode);
stateCode = newStateCode;
}
// define the method for adding a city
public void addCity(String newCityName) {
City newCityObj = new City(newCityName);
cities.put(newCityName, newCityObj);
}
// define the method for getting a city
public City getCity(String cityName) {
if(cities.containsKey(cityName)) {
return cities.get(cityName);
}
else {
return null;
}
}
}
// City.java
public class City {
// Define the instance vars
String cityName;
int cityPop;
// setup the constructor
public City(String newCityName) {
cityName = newCityName;
System.out.println("Created City: " + newCityName);
}
public void setPopulation(int newPop) {
cityPop = newPop;
}
public int getPopulation() {
return cityPop;
}
}
This is working for me, but I'm wondering if there are gotchas that I haven't run into, or if there are alternate/better ways to do the same thing.
(P.S. I know that I need to add some more error checking in, but right now, I'm focused on trying to figure out a good data structure.)
(NOTE: Edited to change setPop() and getPop() to setPopulation() and getPopulation() respectively to avoid confucsion)
