java.util.Map中的迭代器

Question

Created a simple java application with a Map to store some data and then iterate and print the key value.

Map<String,String> myMap = new HashMap<String,String>();
    myMap.put("1", "1");
    myMap.put("2", "2");
    myMap.put("3", "3");
    myMap.put("4", "4");

    Iterator<String> it1 = myMap.keySet().iterator();
    while(it1.hasNext()){
        String key = it1.next();
        System.out.println("Map key: "+key + "    Map Value:"+myMap.get(key));
       /* if(key.equals("2")){
            myMap.put("1","4");
            //myMap.put("4", "4");
        }*/
    }

Even I tried the change the put method with different combination like

    myMap.put("4", "4");
    myMap.put("1", "1");
    myMap.put("3", "3");
    myMap.put("2", "2");

But I couldn't understood why my program always display the first 3 object in descending. I couldn't trace what sequencing the iterator uses..

Map key: 3    Map Value:3
Map key: 2    Map Value:2
Map key: 1    Map Value:1
Map key: 4    Map Value:4

Answer 1

The order in which you'll receive elements from a HashMap is not guaranteed. The only guarantees that it makes is that every insertion of a duplicate item is rejected (thus, leaving you with a collection of unique items).

If you want both guarantees, you'll want to use a LinkedHashMap instead, which guarantees insertion order.

Answer 2

HashMap does not gurantee ordering of elements

Use LinkedHashMap(entry order) or TreeMap (sorted order) if ordering is requrired.

Answer 3

Because HashMap use Hashing function for storing elements.. order is decide by Hashing function..
In simple word Hasing function return memory place where element well be store.

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