How to sort ArrayList like other sorted list?

Question

I have two ArrayList s. One is of the Double type and contains distances. The other one is of the String type and contains city types.

These are the lists:

private ArrayList<Double> distance_list = new ArrayList<Double>();
distance.add(123.43);
distance.add(450.43);
distance.add(230.65);

private ArrayList<String> city_list = new ArrayList<String>();
city_list.add("Munich");
city_list.add("Berlin");
city_list.add("Frankfurt");

Collections.sort(distance_list); //to get the closest distance first

How can I sort the city_list in the same order to the matching distances?

Answer 1

NavigableMap / SortedMap

Use a Map instead. A map pairs two items together, in a key-value relationship. Ask for the key to access its value partner.

Using a NavigableMap or SortedMap keeps, the keys in order.

NavigableMap< Double , String > map = new TreeMap<>() ;
map.add( 123.43d , "Munich" ) ;
…

You can then loop that map. See: How to Loop in NavigableMap in Java ;

for( Double key : map.keySet() )
{
    String city = map.get( key ) ;
    System.out.println( city + " = " + key ) ;
}

Here is a diagram graphic I made as an overview of the Map implementations bundled with Java 11. You can see SortedMap interface was added first, then later extended by NavigableMap interface.

Answer 2

You'll want to create a class that pairs both values and implements the Comparable interface, such as:

public class City implements Comparable<City> {

    private Double distance;
    private String name;

    // constructor, getters, setters

    @Override
    public int compareTo(City city) {
        return (double)(this.distance - city.getDistance());
    }
}

Then build a Treemap or some other Map type that keeps things sorted. I assume you wish to sort by distance.

//Create TreeMap
TreeMap<Double, String> cities = new TreeMap();

//Add cities
cities.put(123.43, "Munich");

The items will be sorted by the distances values!

Try it out and post your code if you need more help.

Answer 3

You can do it in 2 steps

• You create an englobing class
  • The class must implements Comparable (it is used in Collections.sort())
• Then you sort with Collections.sort()

class DistanceCity implements Comparable<DistanceCity>  {
    private Double distance;
    private String city;

    public DistanceCity(Double distance, String city) {
        this.distance = distance;
        this.city = city;
    }

    @Override
    public int compareTo(DistanceCity o) {
        return distance.compareTo(o.distance);
    }

    @Override
    public String toString() {
        return "DistanceCity{" + "distance=" + distance + ", city='" + city + "'}";
    }
}

private ArrayList<DistanceCity> liste;
liste = new ArrayList<>();
liste.add(new DistanceCity(123.43,"Munich"));
liste.add(new DistanceCity(450.43,"Berlin"));
liste.add(new DistanceCity(230.65,"Frankfurt"));

System.out.println(liste); // unordered
//[DistanceCity{distance=123.43, city='Munich'}, DistanceCity{distance=450.43, city='Berlin'}, DistanceCity{distance=230.65, city='Frankfurt'}]

Collections.sort(liste);

System.out.println(liste); // ordered
// [DistanceCity{distance=123.43, city='Munich'}, DistanceCity{distance=230.65, city='Frankfurt'}, DistanceCity{distance=450.43, city='Berlin'}]

Answer 4

A full implementation with Comparable and example

class City implements Comparable<City>{
  public String name = null;
  public Double distance = 0.0;

  public City(String name, Double distance){
    this.name = name;
    this.distance = distance;
  }

  @Override
  public int compareTo(City o){
    return this.distance.compareTo( o.distance );
  }

  @Override
  public String toString(){
    return this.name;
  }

}


public class CitiesDemo{
  public static void main (String[] args ){
  City munich = new City("Munich", 123.43);
  City berlin = new City("Berlin", 450.43);
  City frankfurt = new City("Frankfurt", 230.65);

  java.util.List<City> cities = new java.util.ArrayList<City>();
  cities.add( munich );
  cities.add( berlin );
  cities.add( frankfurt );
  java.util.Collections.sort( cities );
  System.out.println( cities );
  }

}

Will return:

~/ (main)$ javac CitiesDemo.java 

~/ (main)$ java CitiesDemo

[Munich, Frankfurt, Berlin]

~/ (main)$ 

Or, you can test it here: https://ideone.com/fbbLv6