My WorldProblem solution.

City.java

public class City implements Municipality{
	private int population;
	private String name;
	
	public City(String name, int population) {
		this.name = name;
		this.population = population;
	}

	@Override
	public int getPopulation() {
		return population;
	}

	@Override
	public String getString() {
		return name;
	}
}

Country.java

public class Country implements Municipality{
	private String name;
	private Municipality[] municipalities;
	
	public Country(String name, Municipality... entities) {
		this.name = name;
		municipalities = entities;
	}

	@Override
	public int getPopulation() {
		int result = 0;
		
		for(Municipality mun: municipalities) {
			result += mun.getPopulation();
		}
		
		return result;
	}

	@Override
	public String getString() {
		StringBuffer result = new StringBuffer();
		result.append(name);
		
		for(Municipality mun: municipalities) {
			result.append(" " + mun.getString());
		}
		
		return result.toString();
	}
}

District.java

public class District implements Municipality{
	private int population;
	private String name;
	
	public District(String name, int population) {
		this.name = name;
		this.population = population;
	}

	@Override
	public int getPopulation() {
		return population;
	}

	@Override
	public String getString() {
		return name;
	}

}

Main.java

public class Main {
	public static void main(String[] args) {
		World myWorld = new World(
				new Country("USA",
						new State("NY", 
							new City("New York", 8143197)),
						new City("LA", 3844829),
						new City("Chicago", 2842518),
						new District("Washington D.C.", 658893)),
				new Country("Germany",
						new City("Berlin", 3336026),
						new City("Hamburg", 1605606),
						new City("Munich", 831937)),
				new Country("Tamriel", 
						new State("Skyrim",
							new City("Dragonsreach", 580),
							new City("Falkreath", 390),
							new District("Sheogorath's Realm", 666)),
						new State("Morrowind", 
							new City("Seyda Neen", 46))));

	myWorld.printPopulation();
	
	System.out.println();
	
	myWorld.printWorld();
	}
}

Municipality.java

public interface Municipality {

	public String getString();
	
	public int getPopulation();
}

State.java

public class State implements Municipality{
	private String name;
	private Municipality[] municipalities;
	
	public State(String name, Municipality... entities) {
		this.name = name;
		municipalities = entities;
	}

	@Override
	public int getPopulation() {
		int result = 0;
		
		for(Municipality mun: municipalities) {
			result += mun.getPopulation();
		}
		
		return result;
	}

	@Override
	public String getString() {
		StringBuffer result = new StringBuffer();
		result.append(name);
		
		for(Municipality mun: municipalities) {
			result.append(" " + mun.getString());
		}
		
		return result.toString();
	}
}

World.java

public class World {
	private Country[] countries;

	public World(Country... entities) {
		countries = entities;
	}
	
	public void printPopulation() {
		int result = 0;
		
		for(Country c: countries) {
			result += c.getPopulation();
		}
		
		System.out.format("World Population: %d", result);
	}
	
	public void printWorld() {
		StringBuffer result = new StringBuffer();
		
		for(Country c: countries) {
			result.append(c.getString() + " ");
		}
		
		System.out.println("World to String: " + result.toString());
	}
}