39

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?

Arpit Aggarwal
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sieve411
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    Dupe: http://stackoverflow.com/questions/1814095/sorting-an-arraylist-of-contacts, http://stackoverflow.com/questions/890254/sort-arraylist, http://stackoverflow.com/questions/1206073/sorting-a-collection-of-objects and more. – BalusC Mar 29 '10 at 00:22

7 Answers7

96

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

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

Community
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polygenelubricants
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  • Do self-goals count as negatives? Because if `goalsScored` is strictly positive, then the subtraction trick is fine. – polygenelubricants Mar 29 '10 at 05:57
  • 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. – Matthew Nov 25 '14 at 05:46
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    `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));`. – Pshemo Aug 30 '15 at 22:20
  • @Pshemo great solution, fixed a lookalike problem with your solution. –  Sep 17 '15 at 06:51
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    @Pshemo thanks this was useful when my property to compare is a long - I could use `return Long.compare(p1.getLongValue(), p2.getLongValue());` –  Dec 05 '15 at 03:16
7

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]
Arpit Aggarwal
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5

Just one line with Java 8 : 

Collections.sort(players, (p1, p2) -> p1.getGoalsScored() - p2.getGoalsScored());
user6158055
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    how to do the same for the property with string data type – cammando Sep 09 '17 at 14:43
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    btw i found out a way to sort for string `Collections.sort( people, (player1, player2) -> player1.getLastName().compareTo(player2.getLastName()));` – cammando Sep 09 '17 at 14:58
3

Write a custom Comparator to do the job.

duffymo
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2

Use a generic Comparator like the Bean Comparator.

camickr
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1

With Java 8 this is simple

Collections.sort(playList, Comparator.comparingInt(HockeyPLayer::goalsScored))
Satyajit
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0

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

CWF
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