Sort list based on priority from a map

Question

I have a simple map and need to create a list that is sorted based on the number in ascending order from a given list:

Map auto = new HashMap();
auto.put("Merc", 3);
auto.put("Citroen", 5);
auto.put("Opel", 10);
auto.put("BMW", 20);

List<String> given = new ArrayList<>();
given.add("Opel");
given.add("BMW");
given.add("Citroen");

So the given list needs to be sorted so that it will be in this order: Citroen, Opel, BMW. Was thinking of:

create another map then iterate through list
get number from first map
put the number as the key and the name as value in the new map
sort the map by key
iterate threw new map then add values to the list

This seems terrible :/, any suggestions and perhaps better data structures to use?

@mercury And why does Citrogen come after BMW,unless i lost counting :P :) — Kumar Abhinav, Aug 18 '14 at 20:13
@PeterLawrey: It looks like the OP wants to sort the members of the *list* based on their values in the map, *not* the keys of the map as a whole. It looks like your answer below misses that, too. — Louis Wasserman, Aug 18 '14 at 20:19
See this - http://stackoverflow.com/questions/109383/how-to-sort-a-mapkey-value-on-the-values-in-java — Random42, Aug 18 '14 at 20:25
@PeterLawrey in this case merc is not used. So if the map has 20 values and the list has 6 values it only needs to sort the 6 — Mercury, Aug 18 '14 at 20:55

Peter Lawrey · Accepted Answer · 2014-08-18T20:30:56.347

Using Java 8 you can do.

Map<String, Integer> auto = new HashMap<>();
auto.put("Merc", 3);
auto.put("Citroen", 5);
auto.put("Opel", 10);
auto.put("BMW", 20);

List<String> given = new ArrayList<>();
given.add("Opel");
given.add("BMW");
given.add("Citroen");

// to sort the selected elements.
given.sort(Comparator.comparing(auto::get));

// to sort all elements.
List<String> names = auto.entrySet().stream()
        .sorted(Comparator.comparing(Map.Entry::getValue))
        .map(Map.Entry::getKey)
        .collect(Collectors.toList());

Breaking this down

List<String> names = 
         // give the set of entries as a Stream.
         auto.entrySet().stream()
        // sort these entries, using the field returned by getValue()
        .sorted(Comparator.comparing(Map.Entry::getValue))
        // now sorted, turn each Entry into just the getKey()
        .map(Map.Entry::getKey)
        // now we have a stream of keys, turn this into a List<String>
        .collect(Collectors.toList());

Can you explain a little?\ – Kick Buttowski Aug 18 '14 at 20:22 — Kick Buttowski, Aug 18 '14 at 20:22

Volune · Answer 2 · 2014-08-18T20:38:41.573

5

Collections#sort

Collections.sort(given, new Comparator<String>() {
    @Override
    public int compare(String o1, String o2) {
        return auto.get(o1).compareTo(auto.get(o2));
    }
});

Or with lambda:

Collections.sort(given, (o1, o2) -> auto.get(o1).compareTo(auto.get(o2)));

Java 8 null-safe solution inspired from multiple answers

given.sort(Comparator.comparing((s) -> auto.getOrDefault(s, Integer.MAX_VALUE)));

edited Aug 18 '14 at 20:38

answered Aug 18 '14 at 20:20

Volune

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For the lamdba example, I would do `auto.getOrDefault(o1, Integer.MAX_VALUE)` in case of unmapped values. – NoDataFound Aug 18 '14 at 20:23

score 3 · Answer 3 · answered Aug 18 '14 at 20:19

3

With Java 8, you could just do

given.sort(Comparator.comparing(auto::get));

...and it's just that one-liner. Or with the Guava library you could do

Collections.sort(given, Ordering.natural().onResultOf(Functions.forMap(auto)));

answered Aug 18 '14 at 20:19

Louis Wasserman

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can you explain little? – Kick Buttowski Aug 18 '14 at 20:21
2

To explain, [Comparator.comparing](http://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparing-java.util.function.Function-) creates a Comparator implementation that applies the given function to each element and compares the results. In this case, `auto::get` will take a string from the list and return the number it's mapped to in `auto`, so the list will be sorted by these corresponding numbers. – Sean Van Gorder Aug 18 '14 at 20:34

score 0 · Answer 4 · answered Aug 18 '14 at 20:12

0

Create a Car class that implements Comparable and includes the name and priority.

Then you can sort lists with Collections.sort() directly.

answered Aug 18 '14 at 20:12

Kayaman

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Kumar Abhinav · Answer 5 · 2014-08-18T20:29:29.443

0

Map<String,Integer> auto = new HashMap<String,Integer>();
auto.put("Merc", 3);
auto.put("Citroen", 5);
auto.put("Opel", 10);
auto.put("BMW", 20);

Set<Map.Entry<String,Integer>> set = auto.entrySet();
List<Map.Entry<String,Integer>> list = new ArrayList<Map.Entry<String,Integer>>(set);
Collections.sort(list,new Comparator<Map.Entry<String,Integer>>(){

             @Override
            public int compare(Entry<String, Integer> o1,
                    Entry<String, Integer> o2) {

                return o1.getValue().compareTo(o2.getValue());
            }
});

Once you have List Objects of Map.Entry,you can extract key using Entry.getKey()

edited Aug 18 '14 at 20:29

answered Aug 18 '14 at 20:24

Kumar Abhinav

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Entry doesn't have a `compareTo` method. – Peter Lawrey Aug 18 '14 at 20:31
It implies that `given` is a map, but the question state that both are distinct (eg: a map and list). Also, while the question does not state it, the map may not have duplicate values while the list may. – NoDataFound Aug 18 '14 at 20:58

Sort list based on priority from a map

5 Answers5