通过 Java 8 中的键和值比较 hashmap 使用 lambda ZC1C425268E68385D14ZA74

Question

I am new to java 8 and would like to write a function which sorts hashmap by values and if values are the same sort by keys.

To sort the hashmap by values:

Map<String, Integer> map1 = new LinkedHashMap<>();
                 map.entrySet()
                .stream()
                .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
                .forEachOrdered(x -> map1.put(x.getKey(), x.getValue()));       
    map1.forEach((k,v) ->{ System.out.println(k +" "+v);} );

I have worked with Python 3 and it has sorting for both keys and values using mapSorted = sorted(map.items() , key = lambda item: (item[1], item[0])) . Is there something similar in Java 8?

Answer 1

The API you are looking forward to is Comparator#thenComparing . The reason why the implementation for such sorting is not straightforward is because of the type-inference.

The type inference needs some help which can be provided such as:

Comparator.comparing(Map.Entry<String, Integer>::getValue)
                    .reversed().thenComparing(Map.Entry::getKey)

Apart from which you shall ideally collect the output to a Map that preserves the order, otherwise, the sorting is a waste of computations. Hence something like this shall work:

LinkedHashMap<String, Integer> sortedMap = map.entrySet()
            .stream()
            .sorted(Comparator.comparing(Map.Entry<String, Integer>::getValue)
                    .reversed().thenComparing(Map.Entry::getKey))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                    (a, b) -> a, LinkedHashMap::new));

Answer 2

define your map

        HashMap<String, Integer>map1 = new HashMap();
        map1.put("aa",5);
        map1.put("bbb",2);
        map1.put("ccccc",2);
        map1.put("dddddd",3);

sort,if you want to compare with string your need defining it by yourself

        List<Entry<String, Integer>> collect = map1.entrySet().stream().sorted(new Comparator<Entry<String, Integer>>(){
            @Override
            public int compare(Entry<String, Integer> o1, Entry<String, Integer> o2) {
                int ll=0;
                if (o1.getValue()>o2.getValue()){
                    ll=-1;
                }
                else if(o1.getValue()<o2.getValue()){
                    ll=1;
                }
                else if (o1.getKey().length()>o2.getKey().length()) {
                    ll=-1;
                }
                else if (o1.getKey().length()<o2.getKey().length()) {
                    ll=1;
                };
                return ll;
            }
        }).collect(Collectors.toList());

the result like [aa=5, dddddd=3, ccccc=2, bbb=2]