Sorting Maps

A TreeMap sorts its keys, using their own natural sort order.

  public static void main(String[] args) {
    Map<String,USState> states = new TreeMap<>();
    states.put("CO", new USState("CO","Colorado","Denver",5_607_154));
    states.put("CA", new USState("CA","California","Sacramento",39_536_653));
    states.put("NC", new USState("NC","North Carolina","Raleigh",10_273_419));
    states.put("ND", new USState("ND","North Dakota","Bismarck",755_393));
    states.put("NY", new USState("NY","New York","Albany",19_849_399 ));
    states.put("NJ", new USState("NJ","New Jersey","Trenton", 9_005_644));
    states.put("TX", new USState("TX","Texas","Austin", 28_304_596));
    states.put("AK", new USState("AK","Alaska","Juneau",739_795));
    states.put("WY", new USState("WY","Wyoming","Cheyenne",579_315));

    printStates(states);
  }

  private static void printStates(Map<String, USState> states) {
    Set<String> keys = states.keySet();
    for (String key : keys) {
      USState state = states.get(key);
      System.out.println(key+": "+state.getName() +", population "+state.getPopulation());
    }
  }
}

AK: Alaska, population 739795
CA: California, population 39536653
CO: Colorado, population 5607154
NC: North Carolina, population 10273419
ND: North Dakota, population 755393
NJ: New Jersey, population 9005644
NY: New York, population 19849399
TX: Texas, population 28304596
WY: Wyoming, population 579315

To sort the keys of other Map implementations, or to change the sort order, we can use a Comparator.

• Its compare takes two keys, which it uses to retrieve the mapped values from the map.

• It can then use properties of the retreived value objects to determine the sort order.

For this to work, the Comparator must have access to the Map object.

public class USStatePopulationComparator implements Comparator<String> {
  // Field to store a reference to a map
  private Map<String,USState> states;

  // Constructor requires a Map<String,USState> reference to put in the field
  public USStatePopulationComparator(Map<String,USState> states) {
    this.states = states;
  }

  @Override
  public int compare(String abbr1, String abbr2) {
    // compare uses keys to access objects whose properties define the order
    USState state1 = states.get(abbr1);
    USState state2 = states.get(abbr2);
    if (state1.getPopulation() > state2.getPopulation()) {
      return 1;
    }
    else if (state1.getPopulation() < state2.getPopulation()) {
      return -1;
    }
    return 0;
  }
}

Use the Comparator to sort the keys, then use the keys to retrieve the values.

Set<String> abbrevs = states.keySet();
// A Set can't be sorted, but a List can
List<String> keys = new ArrayList<>(abbrevs);
// Pass map reference to the comparator's constructor
Comparator<String> comp = new USStatePopulationComparator(states);
// Sort keys with the comparator.
keys.sort(comp);

// Use the sorted keys to access the values
System.out.println("== Sorted by population ==");
for (String abbrev : keys) {
  USState state = states.get(abbrev);
  System.out.println(abbrev+": "+state.getName()+", population "+state.getPopulation());
}

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