How to iterate hashmap in reverse order in Java

Question

I am trying this for some hour but not finding any best approach to achieve iteration of hashmap in reverse order, this is the hashmap I have.

      Map

        

Answer 1

Perhaps you need a NavigableMap, like a TreeMap.

Answer 2

    TreeMap<Integer, String> map = new TreeMap<Integer, String>();
    map.put(1, "abc1");
    map.put(2, "abc2");
    map.put(3, "abc3");
    NavigableMap<Integer, String> nmap = map.descendingMap();
    for (NavigableMap.Entry<Integer, String> entry : nmap.entrySet()) {
        System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue());
    }

An implementation of NPE idea.

Answer 3

But treemap also gives in asecding order, what i want is in descending order.

Implement a Comparator that will compare it reverse than natural order and then just iterate normally you will have reverse iteration

Answer 4

I've found that the Iterators obtained from Java Hashtable via: Hashtable.values().iterator() and Hashtable.keys().asIterator() are both in reverse order by default. One oddity, The values().iterator has a first final value of "0" which I didn't add when populating it.

Answer 5

Use insted:

new TreeMap<>(Collections.reverseOrder())

and you will get what you want.

Answer 6

You can use TreeMap#descendingKeySet method.

Map<Integer, List<String>> map = new TreeMap<Integer, List<String>>();

for(Integer key : map.descendingKeySet()) {
    List<String> value = map.get(key);
    List<Map<String,?>> security = new LinkedList<Map<String,?>>();  
    for(int ixy = 0; ixy < value.size()-1; ixy++){
        security.add(createItem(value.get(ixy), value.get(ixy+1))); 
    }
    adapter.addSection(Integer.toString(key), new SimpleAdapter(getApplicationContext(), security, R.layout.list_complex, new String[] { ITEM_TITLE, ITEM_CAPTION }, new int[] { R.id.list_complex_title, R.id.list_complex_caption }));
} 

Reference:

https://docs.oracle.com/javase/8/docs/api/java/util/TreeMap.html#descendingKeySet--