How to sort an arraylist of objects by a property?

Question

Lets say you have an Arraylist of HockeyPlayer objects.

How could you sort that if they all have a variable int goalsScored. How could you sort them by goalsScored?

Answer 1

You can use Collections.sort with a custom Comparator<HockeyPlayer>.

    class HockeyPlayer {         public final int goalsScored;         // ...     };      List<HockeyPlayer> players = // ...      Collections.sort(players, new Comparator<HockeyPlayer>() {         @Override public int compare(HockeyPlayer p1, HockeyPlayer p2) {             return p1.goalsScored - p2.goalsScored; // Ascending         }      }); 

The comparision part can also be written this way :

players.sort(Comparator.comparingInt(HockeyPLayer::goalsScored)); 

Alternatively, you can make HockeyPlayer implementsComparable<HockeyPlayer>. This defines the natural ordering for all HockeyPlayer objects. Using a Comparator is more flexible in that different implementations can order by name, age, etc.

See also

• Java: What is the difference between implementing Comparable and Comparator?

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For completeness, I should caution that the return o1.f - o2.f comparison-by-subtraction shortcut must be used with extreme caution due to possible overflows (read: Effective Java 2nd Edition: Item 12: Consider implementing Comparable). Presumably hockey isn't a sport where a player can score goals in the amount that would cause problems =)

See also

• Java Integer: what is faster comparison or subtraction?

Answer 2

As @user6158055 suggets, it's one liner with Java 8, as follows:

Collections.sort(                 hockeyPlayerList,                 (player1, player2) -> player1.getGoalsScored()                         - player2.getGoalsScored()); 

Complete example to depict the same:

import java.util.ArrayList; import java.util.Collections; import java.util.List;  public class Main {      public static void main(String[] args) {         List<HockeyPlayer> hockeyPlayerList = new ArrayList<>();         hockeyPlayerList.add(new HockeyPlayer("A", 3));         hockeyPlayerList.add(new HockeyPlayer("D", 10));         hockeyPlayerList.add(new HockeyPlayer("B", 2));          System.out.println("Before Sort based on goalsScored\n");          hockeyPlayerList.forEach(System.out::println);          System.out.println("\nAfter Sort based on goalsScored\n");          Collections.sort(                 hockeyPlayerList,                 (player1, player2) -> player1.getGoalsScored()                         - player2.getGoalsScored());          hockeyPlayerList.forEach(System.out::println);     }      static class HockeyPlayer {          private String name;         private int goalsScored;          public HockeyPlayer(final String name, final int goalsScored) {             this.name = name;             this.goalsScored = goalsScored;         }          public String getName() {             return name;         }          public void setName(String name) {             this.name = name;         }          public int getGoalsScored() {             return goalsScored;         }          public void setGoalsScored(int goalsScored) {             this.goalsScored = goalsScored;         }          @Override         public String toString() {             return "HockeyPlayer [name=" + name + ", goalsScored="                     + goalsScored + "]";         }      } } 

Output:

Before Sort based on goalsScored  HockeyPlayer [name=A, goalsScored=3] HockeyPlayer [name=D, goalsScored=10] HockeyPlayer [name=B, goalsScored=2]  After Sort based on goalsScored  HockeyPlayer [name=B, goalsScored=2] HockeyPlayer [name=A, goalsScored=3] HockeyPlayer [name=D, goalsScored=10]