So I have a HashMap looking like this :
HashMap<Movie, Float> movies;
It contains some movies with their global ratings as floats, and I want to sort the movies from the best to the worst.
I had a look in Collections.sort() but have no idea if I can do something with it...
It is not possible to sort a HashMap. If you need a sorted map take a look at TreeMap.
What about adding the rating value to the Movie class and let it implement Comparable?
public class Movie implements Comparable<Movie> {
private Float rating;
public Movie(Float rating) {
this.rating = rating;
}
public Float getRating() {
return rating;
}
public int compareTo(Movie param) {
return param.getRating().compareTo(rating);
}
@Override
public String toString() {
return String.valueOf(rating);
}
}
Then you can use your Movie class like this:
public static void main(String[] args) {
Set<Movie> movies = new HashSet<Movie>();
movies.add(new Movie(0.6f));
movies.add(new Movie(0.5f));
movies.add(new Movie(0.7f));
movies.add(new Movie(0.2f));
// Movie.class has to implement Comparable
System.out.println("First option:");
List<Movie> list = new ArrayList<Movie>(movies);
Collections.sort(list);
printMovies(list);
// Works without implementing Comparable in Movie.class
System.out.println("\nSecond option:");
List<Movie> secondList = new ArrayList<Movie>(movies);
Collections.sort(secondList, new Comparator<Movie>() {
public int compare(Movie movie1, Movie movie2) {
return movie2.getRating().compareTo(movie1.getRating());
}
});
printMovies(secondList);
}
private static void printMovies(List<Movie> list) {
for (Movie movie : list) {
System.out.println(movie);
}
}
Output:
First option:
0.7
0.6
0.5
0.2
Second option:
0.7
0.6
0.5
0.2
If you always want to sort the movies in the same way (from best to worse), I would choose the first option. If you always need different sort algorithms I would choose the second option, but even if your Movie class implements Comparable you can always provide a different Comparator as shown in the example.
HashMap is not the datastructure for what you want to achieve. you can read more about HashMap here for exmaple: http://docs.oracle.com/javase/1.5.0/docs/api/java/util/HashMap.html
what I suggest for you is to include your rating within your Movie object and make a Comperator that can compare your Movie objects based on rating. place the Movie's in a ArrayList, then you can sort it with the comperator and Collections.sort
use TreeMap. From this so answer
public class Testing {
public static void main(String[] args) {
HashMap<String,Double> map = new HashMap<String,Double>();
ValueComparator bvc = new ValueComparator(map);
TreeMap<String,Double> sorted_map = new TreeMap<String,Double>(bvc);
map.put("A",99.5);
map.put("B",67.4);
map.put("C",67.4);
map.put("D",67.3);
System.out.println("unsorted map: "+map);
sorted_map.putAll(map);
System.out.println("results: "+sorted_map);
}
}
class ValueComparator implements Comparator<String> {
Map<String, Double> base;
public ValueComparator(Map<String, Double> base) {
this.base = base;
}
// Note: this comparator imposes orderings that are inconsistent with equals.
public int compare(String a, String b) {
if (base.get(a) >= base.get(b)) {
return -1;
} else {
return 1;
} // returning 0 would merge keys
}
}
You can use the Guava library to do this - there is already an excellent answer on here explaining how to do this.
You could just use a TreeMap from the start. It is a sorted map, you just have to define a Comparator (in this case, use your floats)
If you want to stick to HashMaps, yes you can use Collections.sort. But again, you still have to learn how to use a Comparator.
HashMaps are unsorted by default. If you need ordering either use a TreeMap or simply sort into a List<Movie>
List<Movie> movies = new ArrayList<Movie>();
Collections.sort(movies, new Comparator<Movie>() {
public int compare(Movie m1, Movie m2) {
return m1.getRating() - m2.getRating();
}
});
