Whatsit Whatsit - 7 months ago 68
Java Question

Java Ordered Map

In Java, Is there an object that acts like a Map for storing and accessing key/value pairs, but can return an ordered list of keys and an ordered list of values, such that the key and value lists are in the same order?

So as explanation-by-code, I'm looking for something that behaves like my fictitious OrderedMap:

OrderedMap<Integer, String> om = new OrderedMap<>();
om.put(0, "Zero");
om.put(7, "Seven");

String o = om.get(7); // o is "Seven"
List<Integer> keys = om.getKeys();
List<String> values = om.getValues();

for(int i = 0; i < keys.size(); i++)
Integer key = keys.get(i);
String value = values.get(i);
Assert(om.get(key) == value);


The SortedMap interface (with the implementation TreeMap) should be your friend.

The interface has the methods:

  • keySet() which returns a set of the keys in ascending order
  • values() which returns a collection of all values in the ascending order of the corresponding keys

So this interface fulfills exactly your requirements. However, the keys must have a meaningful order. Otherwise you can used the LinkedHashMap where the order is determined by the insertion order.