CODEWITHSUNDEEP
java
kevin
kevin

Reputation: 81

What's the different between the result of HashMap.entrySet() and LinkedHashMap.entrySet(),do they have the same performance？

I know the different between the hashmap(arrays + linked) and linkedhashMap(keep the order when you put in it);

My question is do the entrySet and LinkedEntrySet has the same feature as HashMap and LinkedHashMap?

 Map<String,Integer> hashmap= new HashMap<>();
 Map<String,Integer> linkedmap = new LinkedHashMap<>();

 Set hashset = hashmap.entrySet();//EntrySet
 Set linkedset = linkedmap .entrySet();//LinkedEntrySet

// Here is my test code
@Test
public void mapTest(){
    Map<String,Integer> hashMap= new HashMap<>();
    Map<String,Integer> linkedHashMap= new LinkedHashMap<>();
    hashMap.put("1",1);
    hashMap.put("3",3);
    hashMap.put("2",2);
    hashMap.put("5",5);
    hashMap.put("8",8);
    hashMap.put("6",6);
    hashMap.put("7",7);
    hashMap.put("4",4);

    linkedHashMap.put("1",1);
    linkedHashMap.put("3",3);
    linkedHashMap.put("2",2);
    linkedHashMap.put("5",5);
    linkedHashMap.put("8",8);
    linkedHashMap.put("6",6);
    linkedHashMap.put("7",7);
    linkedHashMap.put("4",4);//LinkedHashMapwill keep the order

    Set hashSet = hashMap.entrySet();
    Set linkedSet= linkedHashMap.entrySet();//the linkedSetwill keep the order too???
    for (Object o : hashSet ) {
        System.out.println(o);
    }
    for (Object o : linkedSet) {
        System.out.println(o);
    }
}

Upvotes: 2

Views: 365

Answers (1)

Eran
Eran

Reputation: 394146

Looking at the code (Java 8), in both cases entrySet() returns an instance of an inner class of the corresponding Map implementation :

For LinkedHashMap :

public Set<Map.Entry<K,V>> entrySet() {
    Set<Map.Entry<K,V>> es;
    return (es = entrySet) == null ? (entrySet = new LinkedEntrySet()) : es;
}

For HashMap :

public Set<Map.Entry<K,V>> entrySet() {
    Set<Map.Entry<K,V>> es;
    return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
}

As you can see, they don't use neither LinkedHashSet nor HashSet. They have specific Set implementations.

And the reason they use specific internal implementations it that these Sets are backed by the respective Maps, so they don't have storage of their own.

Upvotes: 5

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