CODEWITHSUNDEEP
javajsoniteratorhashmap
Irgendw Pointer
Irgendw Pointer

Reputation: 1820

Why is data.entrySet().iterator() ordered in a different order, compared to the HashMap<Long, String> data?

I have input data (Timestamp, Json string) which I store in a HashMap<Long, String>. To process the JSON string I use an iterator.

I use Iterator<Entry<Long, String>> it = data.entrySet().iterator(); to get the entrySet as iterator.

My problem is now that the HashMap is organized as follows:

 123,{String1} 
 124,{String2} 
 125,{String3}

Better expressed: I add A to the HashMap, Z to the HashMap, E to the HashMap, I to the HashMap. I expect the order A, Z, E, I and I want the same order in the iterator.

If I put it in an iterator I get:

124, {String2}
123, {String1}
125, {String3}

The order of the itarator is different from the HashMap order. For me the order of the iterator has to be exactly the same as the order of the HashMap. What I'm doing wrong, that the iterator is changing the order of the data?

Upvotes: 1

Views: 1041

Answers (4)

Sanoop
Sanoop

Reputation: 1022

Additional to the above findings,
Even you can use LinkedHashMap as given below,LinkedHashMap will make sure values stored in the order of insertion.

public static void main(String[] args) {
    Map<Integer, String> map = new LinkedHashMap<Integer, String>();
    Map<Integer, String> map2 = new HashMap<Integer, String>();
    map.put(1, "oneone");
    map.put(3, "twoone");
    map.put(2, "threeone");
    map2.put(1, "oneone");
    map2.put(3, "twoone");
    map2.put(2, "threeone");
    for(Entry<Integer, String> entry:map.entrySet()){
        System.out.println(entry.getKey());

    }

    for(Entry<Integer, String> entry:map2.entrySet()){
        System.out.println(entry.getKey());

    }

}

Here result will be,

1
3
2
1
2
3

Upvotes: 2

Smutje
Smutje

Reputation: 18153

Excerpt from the java.util.Map API doc: "The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not."

Upvotes: 1

Narendra Pathai
Narendra Pathai

Reputation: 41955

HashMap documentation says that

This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

So you should not rely on the order while you are iterating views entrySet or keySet or values.

Some implementations of Map such as TreeMap provide ordering guarantee as @Oleg S and @Smutje have already suggested.

Upvotes: 1

Oleg Sklyar
Oleg Sklyar

Reputation: 10082

HashMap is not organised in any particular order, or more precisely, the order is not guaranteed. If you want an ordered map, use SortedMap with a TreeMap implementation providing a Comparator if you need to override the default ordering of keys.

Upvotes: 3

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