The Composite Design Pattern

The composite pattern is used when a hierarchy of objects have common characters recursively happening along the hierarchy.

For example, the United Nation has countries, a country has states, a state has cities as its jurisdiction units, all of which have a name, area, population, etc. This can be implemented using the composite pattern.

public abstract class JurisdictionUnit {
      protected String name;
      protected long area;
      protected long population;

      public JurisdictionUnit(String n, long a, long p){
            name = n;
            area = a;
            population = p;
      }

      public abstract List<? extends JurisdictionUnit> getJurisdictionUnits();

     public String toString() {
            return ("Name: " + name + ", Area: " + area + ", Population: " + population);
      }
}

public class City extends JurisdictionUnit {
      public List<JurisdictionUnit> getJurisdictionUnits() {
            return new ArrayList<JurisdictionUnit>();
      }
}

public class State extends JurisdictionUnit{
      private ArrayList<City> cities;
      public State(String n, long a, long p, ArrayList<City> c) {
            super (n, a, p);
            cities = c;
      }

      public List<City> getJurisdictionUnits() {
            return cities;
      }
}

public class Country extends JurisdictionUnit{
      private ArrayList<State> states;
       public Country(String n, long a, long p, ArrayList<State> s) {
            super (n, a, p);
            states = s;
      }

      public List<State> getJurisdictionUnits() {
            return states;
      }

   
}

public class CountryCompositeDemo {
      public static void main(String[] args) {
            ArrayList<City> cityArray1 = new ArrayList<City>();
            cityArray1.add (new City ("Edan", 100L, 50L));
            cityArray1.add (new City ("Lincoln", 500L, 200L));

            State state1 = new State("StateOne", 600L, 250L, cityArray1);

            ArrayList<City> cityArray2 = new ArrayList<City>();
            cityArray1.add (new City ("Haloo", 100L, 50L));
            cityArray1.add (new City ("Orca", 500L, 200L));

            State state2 = new State("StateTwo", 600L, 250L, cityArray2);

            ArrayList<State> stateArray = new ArrayList<State>();
            stateArray.add(state1);
            stateArray.add(state2);

            Country luckyCountry = new Country("Lucky", 1200L, 500L, stateArray);

            System.out.println("The country: " + luckyCountry);
            System.out.println("The States :");
            for (State s : luckyCountry.getJurisdictionUnits()) {
                  System.out.println(s);
                  System.out.println("The Cities : ");
                  for (City c : s.getJurisdictionUnits()) {
                        System.out.println(c);
                  }
            }          
      }
}

A family tree can be another example. The first generation, second, third,..... generations can all have children, and all of them are persons, share the characters and behaviors of a person. The administration hierarchy of an organization is surely another example.

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