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Java 8 Comparator: How to Sort a List
Want to sort your Lists? Look no further. Check out the variety of ways you can get your Lists just the way you want them.
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Mario Pio Gioiosa
CORE ·May. 20, 19 · Java Zone · TutorialLike [67]
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Join For FreeIn this article,were going to see several examples on how to sort a List in Java 8.
Sort a List of Strings Alphabetically
List 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]Weve written London with a lowercase "L" to better highlight differences betweenComparator.naturalOrder[], whichreturns a Comparator that sortsby placing capital letters first, andString.CASE_INSENSITIVE_ORDER, which returns a case-insensitive Comparator.
Basically, in Java 7, we were using Collections.sort[] that was accepting a List and, eventually, a Comparator in Java 8 we havethe newList.sort[], which acceptsa Comparator.
Sort a List of Integers
List 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]Sort a List by String Field
Lets suppose we have our Movie class and we want to sort our Listby title.We can use Comparator.comparing[]and pass a function that extracts the field to usefor sorting title, in this example.
List 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 youve probably noticed, we havent passed a Comparator, but theList is correctly sorted. Thats because title, the extracted field, is a String, and a String implements aComparable interface. If you peek at theComparator.comparing[] implementation, you will see that it callscompareToon the extracted key.
return [Comparator & Serializable] [c1, c2] -> keyExtractor.apply[c1].compareTo[keyExtractor.apply[c2]];Sort a List by DoubleField
In a similar way, we can use Comparator.comparingDouble[] for comparing double value. In the example, we want to order our List of movies by rating, from the highest to the lowest.
List 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 used thereversed function on the Comparatorin order to invert default natural order; that is, from lowest to highest. Comparator.comparingDouble[] uses Double.compare[] under the hood.
If you need to compare int or long, you can usecomparingInt[] andcomparingLong[] respectively.
Sort a ListWith a Custom Comparator
In the previous examples, we havent specified any Comparator since it wasnt necessary, but lets see an example in which we define our ownComparator. Our Movie class has a newfield starred set using the third constructor parameter. In the example, we want to sort the list so that wehave starred movies at the top of the List.
List 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[] { @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];The result 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, of course, use a lambda expression instead of Anonymous class, as follows:
movies.sort[[m1, m2] -> { if[m1.getStarred[] == m2.getStarred[]]{ return 0; } return m1.getStarred[] ? -1 : 1; }];We can also use Comparator.comparing[] again:
movies.sort[Comparator.comparing[Movie::getStarred, [star1, star2] -> { if[star1 == star2]{ return 0; } return star1 ? -1 : 1; }]];In the last example, Comparator.comparing[] takes the function to extract the key to usefor sorting as the first parameter, and a Comparator as thesecond parameter. This Comparator uses the extracted keys for comparison; star1 and star2are booleanand represent m1.getStarred[] and m2.getStarred[]respectively.
Sort a ListWith Chain ofComparators
In the last example, we want to have starred movie at the top and then sort by rating.
List 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 youve seen, we first sort by starredand then by rating both reversed because we want highest value and true first.
Topics:
java 8, sorting, lists, lambda expression
Published at DZone with permission of Mario Pio Gioiosa, DZone MVB. See the original article here.
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