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?

• BalusC: Dupe: stackoverflow.com/questions/1814095/…, stackoverflow.com/questions/890254/sort-arraylist, stackoverflow.com/questions/1206073/… and more.

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?

---

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?

• polygenelubricants: Do self-goals count as negatives? Because if goalsScored is strictly positive, then the subtraction trick is fine.
• Matthew: Own goals count as a goal for the last person on the other team who touched the puck, so goalsScored would never be negative. Even if it was negative though I think the subtraction trick still works.
• Pshemo: return p1.goalsScored - p2.goalsScored; can be replaced by return Integer.compare(p1.goalsScored,p2.goalsScored); to avoid integer overflow if goalsScored can have negative value. Also since Java 8 this code can be replaced by Collections.sort(players, Comparator.comparingInt(h -> h.goalsScored));.
• user443346: @Pshemo great solution, fixed a lookalike problem with your solution.
• user1318194: @Pshemo thanks this was useful when my property to compare is a long - I could use return Long.compare(p1.getLongValue(), p2.getLongValue());

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]

Answer 3

Just one line with Java 8 :

Collections.sort(players, (p1, p2) -> p1.getGoalsScored() - p2.getGoalsScored());

• cammando: how to do the same for the property with string data type
• cammando: btw i found out a way to sort for string Collections.sort( people, (player1, player2) -> player1.getLastName().compareTo(player2.getLastName()));

Answer 4

Write a custom Comparator to do the job.

Answer 5

Use a generic Comparator like the Bean Comparator.

Answer 6

With Java 8 this is simple

Collections.sort(playList, Comparator.comparingInt(HockeyPLayer::goalsScored))

Answer 7

Java has a set of sort() methods for this sort of thing. See Collections.sort (and Comparable) for details.