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Java 8 Comparator: How to Sort a List

Java 8 Comparator: How to Sort a List

In this article, we're going to see a few models on the best way to sort a List in Java 8. 

Sort a List of Strings Alphabetically 

List<String> cities = Arrays.asList(
       "Milan",
       "london",
       "San Francisco",
       "Tokyo",
       "New Delhi"
);
System.out.println(cities);
//[Milan, london, San Francisco, Tokyo, New Delhi]
cities.sort(String.CASE_INSENSITIVE_ORDER);
System.out.println(cities);
//[london, Milan, New Delhi, San Francisco, Tokyo]
cities.sort(Comparator.naturalOrder());
System.out.println(cities);
//[Milan, New Delhi, San Francisco, Tokyo, london]

 We've composed London with a lowercase "L" to more readily feature contrasts between Comparator.naturalOrder(), which returns a Comparator that sorts by putting capital letters first, and String.CASE_INSENSITIVE_ORDER, which returns a case-obtuse Comparator. 

Fundamentally, in Java 7, we were utilizing Collections.sort() that was tolerating a List and, in the end, a Comparator – in Java 8 we have the new List.sort(), which acknowledges a Comparator. 

Sort a List of Integers 

List<Integer> numbers = Arrays.asList(6, 2, 1, 4, 9);
System.out.println(numbers); //[6, 2, 1, 4, 9]
numbers.sort(Comparator.naturalOrder());
System.out.println(numbers); //[1, 2, 4, 6, 9]
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Sort a List by String Field 

How about we guess we have our Movie class and we need to sort our List by title. We can utilize Comparator.comparing() and pass a capacity that concentrates the field to use for arranging – title, in this model. 

List<Movie> movies = Arrays.asList(
        new Movie("Lord of the rings"),
        new Movie("Back to the future"),
        new Movie("Carlito's way"),
        new Movie("Pulp fiction"));
movies.sort(Comparator.comparing(Movie::getTitle));
movies.forEach(System.out::println);

 The Output will be: 

Movie{title='Back to the future'}
Movie{title='Carlito's way'}
Movie{title='Lord of the rings'}
Movie{title='Pulp fiction'}

 As you've likely seen, we haven't passed a Comparator, yet the List is accurately arranged. That is on the grounds that title, the separated field, is a String, and a String actualizes a Comparable interface. In the event that you look at the Comparator.comparing() execution, you will see that it calls compareTo on the separated key. 

return (Comparator<T> & Serializable)
            (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
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 Sort a List by Double Field 

Along these lines, we can utilize Comparator.comparingDouble() for contrasting twofold worth. In the model, we need to arrange our List of motion pictures by rating, from the most noteworthy to the least. 

List<Movie> movies = Arrays.asList(
        new Movie("Lord of the rings", 8.8),
        new Movie("Back to the future", 8.5),
        new Movie("Carlito's way", 7.9),
        new Movie("Pulp fiction", 8.9));
movies.sort(Comparator.comparingDouble(Movie::getRating)
                      .reversed());
movies.forEach(System.out::println);

 We utilized the turned around work on the Comparator so as to modify default normal request; that is, from most reduced to most noteworthy. Comparator.comparingDouble() utilizes Double.compare() in the engine. 

In the event that you have to look at int or long, you can utilize comparingInt() and comparingLong() individually. 

Sort a List With a Custom Comparator 

In the past models, we haven't indicated any Comparator since it wasn't vital, however how about we see a model where we characterize our own Comparator. Our Movie class has another field – "featured" – set utilizing the third constructor parameter. In the model, we need to sort the rundown with the goal that we have featured motion pictures at the highest priority on the List. 

List<Movie> movies = Arrays.asList(
        new Movie("Lord of the rings", 8.8, true),
        new Movie("Back to the future", 8.5, false),
        new Movie("Carlito's way", 7.9, true),
        new Movie("Pulp fiction", 8.9, false));
movies.sort(new Comparator<Movie>() {
    @Override
    public int compare(Movie m1, Movie m2) {
        if(m1.getStarred() == m2.getStarred()){
            return 0;
        }
        return m1.getStarred() ? -1 : 1;
     }
});
movies.forEach(System.out::println);
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 The outcome will be: 

Movie{starred=true, title='Lord of the rings', rating=8.8}
Movie{starred=true, title='Carlito's way', rating=7.9}
Movie{starred=false, title='Back to the future', rating=8.5}
Movie{starred=false, title='Pulp fiction', rating=8.9}

 We can, obviously, utilize a lambda articulation rather than Anonymous class, as pursues: 

movies.sort((m1, m2) -> {
    if(m1.getStarred() == m2.getStarred()){
        return 0;
    }
    return m1.getStarred() ? -1 : 1;
});

 We can likewise utilize Comparator.comparing() once more: 

movies.sort(Comparator.comparing(Movie::getStarred, (star1, star2) -> {
    if(star1 == star2){
         return 0;
    }
    return star1 ? -1 : 1;
}));

 In the last model, Comparator.comparing() takes the capacity to separate the way to use for arranging as the principal parameter, and a Comparator as the subsequent parameter. This Comparator utilizes the extricated keys for correlation; star1 and star2 are boolean and speak to m1.getStarred() and m2.getStarred() individually. 

Sort a List With Chain of Comparators 

In the last model, we need to have featured motion picture at the top and afterward sort by rating. 

List<Movie> movies = Arrays.asList(
        new Movie("Lord of the rings", 8.8, true),
        new Movie("Back to the future", 8.5, false),
        new Movie("Carlito's way", 7.9, true),
        new Movie("Pulp fiction", 8.9, false));
movies.sort(Comparator.comparing(Movie::getStarred)
                      .reversed()
                      .thenComparing(Comparator.comparing(Movie::getRating)
                      .reversed())
);
movies.forEach(System.out::println);

 And the output is: 

Movie{starred=true, title='Lord of the rings', rating=8.8}
Movie{starred=true, title='Carlito's way', rating=7.9}
Movie{starred=false, title='Pulp fiction', rating=8.9}
Movie{starred=false, title='Back to the future', rating=8.5}

 As you've seen, we first sort by featured and afterward by rating – both switched on the grounds that we need most noteworthy worth and genuine first.

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