Move Item in LinkedHashMap

Question

Is there a way to move an item within a LinkedHashMap? Specifically, I'd like to move an item to the first position. I know that if I wanted to add an item to the end, I could just do:

LinkedHashMap<String,Integer> items = new LinkedHashMap<String,Integer>();
//...add some items
int i = items.get("removedItem");
items.remove("removedItem");
items.put("removedItem",i);

Possible duplicate of http://stackoverflow.com/questions/7679819/how-to-add-element-at-specific-index-position-in-linkedhashmap — wassgren, Dec 30 '14 at 20:38
No. Even though `LinkedHahsMap` respects insertion order on iteration, it still implements `Map` for which there is no such method as "put an item in first position". Among collections, only a `List` can do that reliably. — fge, Dec 30 '14 at 20:40

score 4 · Answer 1 · answered Dec 30 '14 at 20:50

For the case of moving items to the front, you could create a new LinkedHashMap with your preferred order. e.g.

LinkedHashMap<K,V>  newMap = new LinkedHashMap<K,V>(oldMap.size());
V valueIWantToBeFirst= oldMap.remove(keyIWantToBeFirst);
newMap.put(keyIWantToBeFirst, valueIWantToBeFirst);
newMap.putAll(oldMap);  // keeps previous order for all remaining entries

score 1 · Accepted Answer · answered Dec 30 '14 at 21:34

1

It you want to keep the same Map instance, you can do it in 3 lines like this.

Map<Integer, String> map = new LinkedHashMap<>();
map.put(1, "One");
map.put(2, "Two");
map.put(3, "Three");
map.put(4, "Four");

Map<Integer, String> copy = new LinkedHashMap<>(map);
map.keySet().retainAll(Collections.singleton(3));
map.putAll(copy);

System.out.println(map);

answered Dec 30 '14 at 21:34

Paul Boddington

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In actuality, my `LinkedHashMap` is `>`. Will the append the `ArrayList` for `3` contained in `copy` onto the `ArrayList` in `map`? – Adam_G Dec 30 '14 at 23:10
The `ArrayList` for a key in `map` is the same `ArrayList` as the one in `copy`, so it shouldn't make any difference. – Paul Boddington Dec 30 '14 at 23:19
did you mean key or value? – Adam_G Dec 30 '14 at 23:24
I meant value, sorry. – Paul Boddington Dec 30 '14 at 23:26

user9506553 · Answer 3 · 2018-03-17T07:41:10.570

public void sortList() {

    ArrayList<String> objectsToOrder=new ArrayList<>(object.keySet());

    ArrayList<String> orderedObjects=new ArrayList<>();
    orderedObjects.add("A");
    orderedObjects.add("B");
    orderedObjects.add("C");
    orderedObjects.add("D");
    orderedObjects.add("E");

    LinkedHashMap<String,ArrayList<QueryRow>>  orderedlist = new LinkedHashMap<String,ArrayList<QueryRow>>(object.size());

  for(int i=0;i<orderedObjects.size();i++){
      if(objectsToOrder.contains(orderedObjects.get(i))){
          ArrayList<QueryRow> valueIWantToBeFirst= object.remove(orderedObjects.get(i));
          orderedlist.put(orderedObjects.get(i),valueIWantToBeFirst);
      }
  }

    orderedlist.putAll(new LinkedHashMap<String, ArrayList<QueryRow>>(object));

    object.clear();

    object.putAll(new LinkedHashMap<String, ArrayList<QueryRow>>(orderedlist));

}

Move Item in LinkedHashMap

3 Answers3

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