Accessing the last entry in a Map

How to move a particular HashMap entry to Last position?

For Example, I have HashMap values like this:

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

map= {Not-Specified 1, test 2, testtest 3};

"Not-Specified" may come in any position. it may come first or in the middle of the map. But i want to move the "Not-Specified" to the last position.

How can I do that? thanks in advance.

Answers


To answer your question in one sentence:

Per default, Maps don't have a last entry, it's not part of their contract.


And a side note: it's good practice to code against interfaces, not the implementation classes (see Effective Java by Joshua Bloch, Chapter 8, Item 52: Refer to objects by their interfaces).

So your declaration should read:

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

(All maps share a common contract, so the client need not know what kind of map it is, unless he specifies a sub interface with an extended contract).


Possible Solutions
Sorted Maps:

There is a sub interface SortedMap that extends the map interface with order-based lookup methods and it has a sub interface NavigableMap that extends it even further. The standard implementation of this interface, TreeMap, allows you to sort entries either by natural ordering (if they implement the Comparable interface) or by a supplied Comparator.

You can access the last entry through the lastEntry method:

NavigableMap<String,Integer> map = new TreeMap<String, Integer>();
// add some entries
Entry<String, Integer> lastEntry = map.lastEntry();
Linked maps:

There is also the special case of LinkedHashMap, a HashMap implementation that stores the order in which keys are inserted. There is however no interface to back up this functionality, nor is there a direct way to access the last key. You can only do it through tricks such as using a List in between:

Map<String,String> map = new LinkedHashMap<String, Integer>();
// add some entries
List<Entry<String,Integer>> entryList =
    new ArrayList<Map.Entry<String, Integer>>(map.entrySet());
Entry<String, Integer> lastEntry =
    entryList.get(entryList.size()-1);
Proper Solution:

Since you don't control the insertion order, you should go with the NavigableMap interface, i.e. you would write a comparator that positions the Not-Specified entry last.

Here is an example:

final NavigableMap<String,Integer> map = 
        new TreeMap<String, Integer>(new Comparator<String>() {
    public int compare(final String o1, final String o2) {
        int result;
        if("Not-Specified".equals(o1)) {
            result=1;
        } else if("Not-Specified".equals(o2)) {
            result=-1;
        } else {
            result =o1.compareTo(o2);
        }
        return result;
    }

});
map.put("test", Integer.valueOf(2));
map.put("Not-Specified", Integer.valueOf(1));
map.put("testtest", Integer.valueOf(3));
final Entry<String, Integer> lastEntry = map.lastEntry();
System.out.println("Last key: "+lastEntry.getKey()
         + ", last value: "+lastEntry.getValue());

Output:

Last key: Not-Specified, last value: 1

Solution using HashMap:

If you must rely on HashMaps, there is still a solution, using a) a modified version of the above comparator, b) a List initialized with the Map's entrySet and c) the Collections.sort() helper method:

    final Map<String, Integer> map = new HashMap<String, Integer>();
    map.put("test", Integer.valueOf(2));
    map.put("Not-Specified", Integer.valueOf(1));
    map.put("testtest", Integer.valueOf(3));

    final List<Entry<String, Integer>> entries =
        new ArrayList<Entry<String, Integer>>(map.entrySet());
    Collections.sort(entries, new Comparator<Entry<String, Integer>>(){

        public int compareKeys(final String o1, final String o2){
            int result;
            if("Not-Specified".equals(o1)){
                result = 1;
            } else if("Not-Specified".equals(o2)){
                result = -1;
            } else{
                result = o1.compareTo(o2);
            }
            return result;
        }

        @Override
        public int compare(final Entry<String, Integer> o1,
            final Entry<String, Integer> o2){
            return this.compareKeys(o1.getKey(), o2.getKey());
        }

    });

    final Entry<String, Integer> lastEntry =
        entries.get(entries.size() - 1);
    System.out.println("Last key: " + lastEntry.getKey() + ", last value: "
        + lastEntry.getValue());

}

Output:

Last key: Not-Specified, last value: 1


HashMap doesn't have "the last position", as it is not sorted.

You may use other Map which implements java.util.SortedMap, most popular one is TreeMap.


A SortedMap is the logical/best choice, however another option is to use a LinkedHashMap which maintains two order modes, most-recently-added goes last, and most-recently-accessed goes last. See the Javadocs for more details.


move does not make sense for a hashmap since its a dictionary with a hashcode for bucketing based on key and then a linked list for colliding hashcodes resolved via equals. Use a TreeMap for sorted maps and then pass in a custom comparator.


When using numbers as the key, I suppose you could also try this:

        Map<Long, String> map = new HashMap<>();
        map.put(4L, "The First");
        map.put(6L, "The Second");
        map.put(11L, "The Last");

        long lastKey = 0;
        //you entered Map<Long, String> entry
        for (Map.Entry<Long, String> entry : map.entrySet()) {
            lastKey = entry.getKey();
        }
        System.out.println(lastKey); // 11

In such scenario last used key is usually known so it can be used for accessing last value (inserted with the one):

class PostIndexData {
    String _office_name;
    Boolean _isGov;
    public PostIndexData(String name, Boolean gov) {
        _office_name = name;
        _isGov = gov;
    }
}
//-----------------------
class KgpData {
    String _postIndex;
    PostIndexData _postIndexData;
    public KgpData(String postIndex, PostIndexData postIndexData) {
        _postIndex = postIndex;
        _postIndexData = postIndexData;;
    }
}

public class Office2ASMPro {
    private HashMap<String,PostIndexData> _postIndexMap = new HashMap<>();
    private HashMap<String,KgpData> _kgpMap = new HashMap<>();
...
private void addOffice(String kgp, String postIndex, String officeName, Boolean gov) {
            if (_postIndexMap.get(postIndex) == null) {
                _postIndexMap.put(postIndex, new PostIndexData(officeName, gov));
            }
            _kgpMap.put( kgp, new KgpData(postIndex, _postIndexMap.get(postIndex)) );
        }

Find missing all elements from array
        int[] array = {3,5,7,8,2,1,32,5,7,9,30,5};
        TreeMap<Integer, Integer> map = new TreeMap<>();
        for(int i=0;i<array.length;i++) {
            map.put(array[i], 1);
        }
        int maxSize = map.lastKey();
        for(int j=0;j<maxSize;j++) {
            if(null == map.get(j))
                System.out.println("Missing `enter code here`No:"+j);
        }

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