How to iterate hashmap in reverse order in Java

问题

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

             for(Integer key : map.keySet()) {
                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 }));  
            }

I have seen example of TreeMap as well,

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

But treemap also gives in ascending order, what I want is in descending order.

回答1:

Hashmap does not have specific order. But you can use TreeMap.

Perhaps this simple example can help you :

Map<Integer, String> map = new TreeMap<Integer, String>();
        map.put(1, "abc1");
        map.put(2, "abc2");
        map.put(3, "abc3");

        ArrayList<Integer> keys = new ArrayList<Integer>(map.keySet());
        for(int i=keys.size()-1; i>=0;i--){
            System.out.println(map.get(keys.get(i)));
        }

回答2:

best approach to acheive iteration of hashmap in reverse order

HashMap does not define any particular ordering of its element. Therefore the "reverse" order isn't defined either.

For a TreeMap, you can use descendingMap().

回答3:

A HashMap doesn't maintain eny order between keys.

A TreeMap orders its keys by their natural order, or by the order imposed by a comparator that you pass when constructing the map. So if you want to have Integer keys ordered in reverse order, construct the TreeMap this way:

Map<Integer, List<String>> sortedMap = 
    new TreeMap<Integer, List<String>>(Collections.reverseOrder());

回答4:

Map<Integer, List<String>> sortedMap = new TreeMap<Integer, List<String>>(Collections.reverseOrder());

Collections.reverseOrder() keeps the map sorted in descending order.

回答5:

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--

回答6:

You can't iterate over a HashMap in reverse because of this:

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.

What you should use is a LinkedHashMap:

This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map. (A key k is reinserted into a map m if m.put(k, v) is invoked when m.containsKey(k) would return true immediately prior to the invocation.)

回答7:

The hashmap is not an ordered collection. Use TreeMap instead, which has descendingKeySet for reverse iteration. See the javadocs. LinkedHashMap is also a good choice.

回答8:

    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.

回答9:

Perhaps you need a NavigableMap, like a TreeMap.

回答10:

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

回答11:

Use insted:

new TreeMap<>(Collections.reverseOrder())

and you will get what you want.

来源：https://stackoverflow.com/questions/10596132/how-to-iterate-hashmap-in-reverse-order-in-java

标签

java

hashmap