How can I sort the list of objects based on one of the object variables

Question

I need to sort the ArrayList films that are best rated for the worst (5 best - 1 worst). I use Collections.sort (), but the list comes out the same. what am I doing wrong

films.add(new Film().setRating(1)); films.add(new Film().setRating(2.5)); films.add(new Film().setRating(3.5)); films.add(new Film().setRating(4)); films.add(new Film().setRating(5)); films.add(new Film().setRating(1)); films.add(new Film().setRating(2)); films.add(new Film().setRating(3)); films.add(new Film().setRating(4)); Collections.sort(films, new Comparator<Film>() { @Override public int compare(Film o1, Film o2) { final double film1 = o1.getRating(); final double film2 = o2.getRating(); return film1 > film2? 1 : film1 < film2? -1 : 0; } }); 

Answer 1

Using the Double.compare method Double.compare fine:

 public static void main(String[] args) throws IOException, ClassNotFoundException { final List<Film> films = new ArrayList<>(); films.add(new Film().setRating(1)); films.add(new Film().setRating(2.5)); films.add(new Film().setRating(3.5)); films.add(new Film().setRating(4)); films.add(new Film().setRating(5)); films.add(new Film().setRating(1)); films.add(new Film().setRating(2)); films.add(new Film().setRating(3)); films.add(new Film().setRating(4)); System.out.println(films); Collections.sort(films, new Comparator<Film>() { @Override public int compare(Film o1, Film o2) { return Double.compare(o1.getRating(), o2.getRating()); } }); System.out.println(films); } 

Output:

 [1.0, 2.5, 3.5, 4.0, 5.0, 1.0, 2.0, 3.0, 4.0] [1.0, 1.0, 2.0, 2.5, 3.0, 3.5, 4.0, 4.0, 5.0] 

Film I used:

 private static final class Film { double rating; public double getRating() { return rating; } public Film setRating(double rating) { this.rating = rating; return this; } @Override public String toString() { return Double.toString(rating); } } 

I would suggest using Double.compare instead of your version or even o1.getRating - o2.getRating .

Is it either an implementation or your Film implementation is incorrect - maybe the installer in your example doesn’t install anything?

Answer 2

Instead, use the following Double method:

 public static int compare(double d1, double d2){} 

or like this:

 class Film implements Comparable<Film>{ double rating; @Override public int compareTo(Film o) { return Double.compare(rating, o.rating); } } 

Answer 3

Your code works very well on my computer:

 import java.util.ArrayList; import java.util.Collections; import java.util.Comparator; import java.util.List; public class SortFilms { public static void main(final String ... args) { new SortFilms().sort(); } public void sort() { final List<Film> films = new ArrayList<Film>(); films.add(new Film(1)); films.add(new Film(2.5)); films.add(new Film(3.5)); films.add(new Film(4)); films.add(new Film(5)); films.add(new Film(1)); films.add(new Film(2)); films.add(new Film(3)); films.add(new Film(4)); Collections.sort(films, new Comparator<Film>() { @Override public int compare(Film o1, Film o2) { final double film1 = o1.getRating(); final double film2 = o2.getRating(); return film1 > film2? 1 : film1 < film2? -1 : 0; } }); System.out.println(films); } private class Film { private final double rating; public Film(double rating) { this.rating = rating; } public String toString() { return "" + rating; } public Double getRating() { return rating; } } } 

Procudes:

 [1.0, 1.0, 2.0, 2.5, 3.0, 3.5, 4.0, 4.0, 5.0] 

Answer 4

There are two ways to solve this problem.

1. Implementation of the comparator interface comparison method in the second parameter of the Collections.sort method.

2. Providing your class implements the Comparable interface, and then implements the CompareTo method.

Here is the code for both solutions.

 public class ComparableTest implements Comparable < ComparableTest > { String name; int marks; @Override public int compareTo(ComparableTest o) { if (this.marks > o.marks) return 1; return -1; } public ComparableTest(String name, int marks) { this.name = name; this.marks = marks; } public String getName() { return name; } public int getMarks() { return marks; } } 

 package program; import java.util.Collections; import java.util.Comparator; import java.util.LinkedList; import java.util.List; import java.util.Random; public class TempList { public static void main(String[] args) { /*************************************************/ /****************Assigning Values*****************/ /*************************************************/ String[] names = { "A", "B", "C", "D", "E" }; List < ComparableTest > objects = new LinkedList < > (); /*************************************************/ /****************Printing Values******************/ /*************************************************/ for (String name: names) { objects.add(new ComparableTest(name, new Random().nextInt(100))); } System.out.println("Before Sorting"); for (ComparableTest object: objects) { System.out.println(object.name + " " + object.marks); } /*************************************************/ /*************Sorting based on marks**************/ /*************************************************/ Collections.sort(objects); System.out.println("\nSorting based on Marks"); for (ComparableTest object: objects) { System.out.println(object.name + " " + object.marks); } /*************************************************/ /*************Sorting based on names**************/ /*************************************************/ System.out.println("\nSorting based on Names"); Collections.sort(objects, new Comparator < ComparableTest > () { @Override public int compare(ComparableTest o1, ComparableTest o2) { if (o1.name.compareTo(o2.name) < 1) { return 1; } return -1; } }); for (ComparableTest object: objects) { System.out.println(object.name + " " + object.marks); } } }