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Java Ordered Map


Question

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);
}
2016/01/04
1
327
1/4/2016 12:17:01 PM

Accepted Answer

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.

2016/10/14
404
10/14/2016 9:28:22 PM


LinkedHashMap maintains the order of the keys.

java.util.LinkedHashMap appears to work just like a normal HashMap otherwise.

2015/01/05

I think the closest collection you'll get from the framework is the SortedMap

2009/03/19

You can leverage NavigableMap interface that may be accessed and traversed in either ascending or descending key order. This interface is intended to supersede the SortedMap interface. The Navigable map is usually sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time.

There are three most useful implementations of it: TreeMap, ImmutableSortedMap, and ConcurrentSkipListMap.

TreeMap example:

TreeMap<String, Integer> users = new TreeMap<String, Integer>();
users.put("Bob", 1);
users.put("Alice", 2);
users.put("John", 3);

for (String key: users.keySet()) {
  System.out.println(key + " (ID = "+ users.get(key) + ")");
}

Output:

Alice (ID = 2)
Bob (ID = 1)
John (ID = 3)
2014/06/22

I think the SortedMap interface enforces what you ask for and TreeMap implements that.

http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedMap.html http://java.sun.com/j2se/1.5.0/docs/api/java/util/TreeMap.html

2009/03/19

Since Java 6 there is also non-blocking thread-safe alternative to TreeMap. See ConcurrentSkipListMap.

2012/06/06

Source: https://stackoverflow.com/questions/663374
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