Java LinkedHashMap get first or last entry

The semantics of LinkedHashMap are still those of a Map, rather than that of a LinkedList. It retains insertion order, yes, but that's an implementation detail, rather than an aspect of its interface.

The quickest way to get the "first" entry is still entrySet().iterator().next(). Getting the "last" entry is possible, but will entail iterating over the whole entry set by calling .next() until you reach the last. while (iterator.hasNext()) { lastElement = iterator.next() }

edit: However, if you're willing to go beyond the JavaSE API, Apache Commons Collections has its own LinkedMap implementation, which has methods like firstKey and lastKey, which do what you're looking for. The interface is considerably richer.

One more way to get first and last entry of a LinkedHashMap is to use toArray() method of Set interface.

But I think iterating over the entries in the entry set and getting the first and last entry is a better approach.

The usage of array methods leads to warning of the form " ...needs unchecked conversion to conform to ..." which cannot be fixed [but can be only be suppressed by using the annotation @SuppressWarnings("unchecked")].

Here is a small example to demonstrate the usage of toArray() method:

    public static void main(final String[] args) {
        final Map<Integer,String> orderMap = new LinkedHashMap<Integer,String>();
        orderMap.put(6, "Six");
        orderMap.put(7, "Seven");
        orderMap.put(3, "Three");
        orderMap.put(100, "Hundered");
        orderMap.put(10, "Ten");

        final Set<Entry<Integer, String>> mapValues = orderMap.entrySet();
        final int maplength = mapValues.size();
        final Entry<Integer,String>[] test = new Entry[maplength];
        mapValues.toArray(test);

        System.out.print("First Key:"+test[0].getKey());
        System.out.println(" First Value:"+test[0].getValue());

        System.out.print("Last Key:"+test[maplength-1].getKey());
        System.out.println(" Last Value:"+test[maplength-1].getValue());
    }

    // the output geneated is :
    First Key:6 First Value:Six
    Last Key:10 Last Value:Ten

Using Java8 stream this can be done very easily:

LinkedHashMap<String, Integer> linkedHashMap = new LinkedHashMap<>();
linkedHashMap.put("A", 1);
linkedHashMap.put("B", 2);
linkedHashMap.put("C", 3);
linkedHashMap.put("D", 4);

//First entry
Map.Entry<String, Integer> firstEntry = linkedHashMap.entrySet().stream().findFirst().get();

//Last entry
Map.Entry<String, Integer> lastEntry = linkedHashMap.entrySet().stream().skip(linkedHashMap.size() - 1).findFirst().get();

For first element use entrySet().iterator().next() and stop iterating after 1 iteration. For last one the easiest way is to preserve the key in a variable whenever you do a map.put.

        import java.util.Arrays;
        import java.util.LinkedHashMap;
        import java.util.List;
        import java.util.Map;

        public class Scratch {
           public static void main(String[] args) {

              // Plain java version

              Map<String, List<Integer>> linked = new LinkedHashMap<>();
              linked.put("a", Arrays.asList(1, 2, 3));
              linked.put("aa", Arrays.asList(1, 2, 3, 4));
              linked.put("b", Arrays.asList(1, 2, 3, 4, 5));
              linked.put("bb", Arrays.asList(1, 2, 3, 4, 5, 6));

              System.out.println("linked = " + linked);

              String firstKey = getFirstKey(linked);
              System.out.println("firstKey = " + firstKey);
              List<Integer> firstEntry = linked.get(firstKey);
              System.out.println("firstEntry = " + firstEntry);

              String lastKey = getLastKey(linked);
              System.out.println("lastKey = " + lastKey);
              List<Integer> lastEntry = linked.get(lastKey);
              System.out.println("lastEntry = " + lastEntry);



           }

           private static String getLastKey(Map<String, List<Integer>> linked) {
              int index = 0;
              for (String key : linked.keySet()) {
             index++;
             if (index == linked.size()) {
                return key;
             }
              }
              return null;
           }

           private static String getFirstKey(Map<String, List<Integer>> linked) {
              for (String key : linked.keySet()) {
             return key;
              }
              return null;
           }
        }

right, you have to manually enumerate keyset till the end of the linkedlist, then retrieve the entry by key and return this entry.

public static List<Fragment> pullToBackStack() {
    List<Fragment> fragments = new ArrayList<>();
    List<Map.Entry<String, Fragment>> entryList = new ArrayList<>(backMap.entrySet());
    int size = entryList.size();
    if (size > 0) {
        for (int i = size - 1; i >= 0; i--) {// last Fragments
            fragments.add(entryList.get(i).getValue());
            backMap.remove(entryList.get(i).getKey());
        }
        return fragments;
    }
    return null;
}