Java Class that implements Map and keeps insertion order?

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I'm looking for a class in java that has key-value association, but without using hashes. Here is what I'm currently doing:

  1. Add values to a Hashtable.
  2. Get an iterator for the Hashtable.entrySet().
  3. Iterate through all values and:
    1. Get a Map.Entry for the iterator.
    2. Create an object of type Module (a custom class) based on the value.
    3. Add the class to a JPanel.
  4. Show the panel.

The problem with this is that I do not have control over the order that I get the values back, so I cannot display the values in the a given order (without hard-coding the order).

I would use an ArrayList or Vector for this, but later in the code I need to grab the Module object for a given Key, which I can't do with an ArrayList or Vector.

Does anyone know of a free/open-source Java class that will do this, or a way to get values out of a Hashtable based on when they were added?

Thanks!

I suggest a LinkedHashMap or a TreeMap. A LinkedHashMap keeps the keys in the order they were inserted, while a TreeMap is kept sorted via a Comparator or the natural Comparable ordering of the elements.

Since it doesn't have to keep the elements sorted, LinkedHashMap should be faster for most cases; TreeMap has O(log n) performance for containsKey, get, put, and remove, according to the Javadocs, while LinkedHashMap is O(1) for each.

If your API that only expects a predictable sort order, as opposed to a specific sort order, consider using the interfaces these two classes implement, NavigableMap or SortedMap. This will allow you not to leak specific implementations into your API and switch to either of those specific classes or a completely different implementation at will afterwards.

Java Class that implements Map and keeps insertion order?, How to preserve insertion order in HashMap? [duplicate] · java hashmap. This question already has answers here: Java Class that implements  Java Map Keep Insertion Order – If you are looking to do implementation of Map using java where ordering of the object needs to be preserved then map is not a right option as this class does not makes any guarantees of the order of the object. I will suggest to use LinkedHashMap or TreeMap.

LinkedHashMap will return the elements in the order they were inserted into the map when you iterate over the keySet(), entrySet() or values() of the map.

Map<String, String> map = new LinkedHashMap<String, String>();

map.put("id", "1");
map.put("name", "rohan");
map.put("age", "26");

for (Map.Entry<String, String> entry : map.entrySet()) {
    System.out.println(entry.getKey() + " = " + entry.getValue());
}

This will print the elements in the order they were put into the map:

id = 1
name = rohan 
age = 26 

How to preserve insertion order in HashMap?, I'm looking for a class in java that has key-value association, but without using hashes. Here Hashtable based on when they were added? Get a Map.Entry for the iterator. Create an object of type Module (a custom class) based on the value. Add the class to a JPanel. Show the panel. The problem with this is that I do not have control over the order that I get the values back, so I cannot display the values in the a given order (without hard-coding the order).

If an immutable map fits your needs then there is a library by google called guava (see also guava questions)

Guava provides an ImmutableMap with reliable user-specified iteration order. This ImmutableMap has O(1) performance for containsKey, get. Obviously put and remove are not supported.

ImmutableMap objects are constructed by using either the elegant static convenience methods of() and copyOf() or a Builder object.

Java Class that implements Map and keeps insertion order , public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,​V> This implementation differs from HashMap in that it maintains a doubly-​linked Note that insertion order is not affected if a key is re-inserted into the map. A LinkedHashMap keeps the keys in the order they were inserted, while a TreeMap is kept sorted via a Comparator or the natural Comparable ordering of the elements.

You can maintain a Map (for fast lookup) and List (for order) but a LinkedHashMap may be the simplest. You can also try a SortedMap e.g. TreeMap, which an have any order you specify.

LinkedHashMap (Java Platform SE 7 ), The HashMap class does not maintain the order of the elements. The LinkedHashMap class implements a doubly linked list so that it can object created in this way maintains the insertion order in which the elements were  You can use LinkedHashMap to main insertion order in Map The important points about Java LinkedHashMap class are: It contains onlyunique elements. A LinkedHashMap contains values based on the key 3.It may have one null key and multiple null values. 4.It is same as HashMap instead maintains insertion order

I don't know if it is opensource, but after a little googling, I found this implementation of Map using ArrayList. It seems to be pre-1.5 Java, so you might want to genericize it, which should be easy. Note that this implementation has O(N) access, but this shouldn't be a problem if you don't add hundreds of widgets to your JPanel, which you shouldn't anyway.

How to maintain insertion order of the elements in Java HashMap , This implementation differs from HashMap in that it maintains a doubly-linked list is normally the order in which keys were inserted into the map (insertion-order). Map; public class LinkedHashMapDemo { public static void main(String  You'll get a list of Map.Entry<K,V> pairs, which always get iterated in the same order. That order is the same as the order by which you put the items in. Alternatively, use the java.util.SortedMap, where the keys must either have a natural ordering or have it specified by a Comparator.

LinkedHashMap in Java, Yes, you should use Map over a specific implementation whenever your backing store and keep track of the key to ensure uniqueness. What you actually want to do is create your own interface/class which LinkedHashMap is the only java map that has the insertion order feature that you're looking for. Une LinkedHashMap contient des valeurs en fonction de la clé 3.Il peut avoir une clé nulle et des valeurs nulles multiples. 4.Il est identique à HashMap maintient l'ordre d'insertion . public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>

How to communicate that insertion order matters in a map , Here's a guide for selecting the proper implementation of a Set, List , or Map . LinkedHashSet - insertion order; LinkedHashMap - insertion order of keys (by  Java Class that implements Map and keeps insertion order? simple name and class name in Java Class? How do two conjugate elements of a group have the same order?

Java Practices->Choosing the right Collection, In Java, we can use LinkedHashMap to keep the insertion order. IntStream; public class JavaHashMap { public static void main(String[] args)  Implementations of Map. There are several classes that implement the Java Map but three major and general-purpose implementations are HashMap, TreeMap, and LinkedHashMap. Let’s see the characteristics and behaviors of each implementation with an example program. HashMap Class. The most common class that implements the Java Map interface is

Comments
  • You don't need to use entryset/map.entry. you can iterate over keys and values by using hashtable.keys as an enumeration or by using hashtable.keyset.iterator.
  • I took the liberty to change the title, since not using hashes is not actually the problem, but keeping the insertion order.
  • Similar Question, Java Ordered Map
  • This won't work for me because, as per javadocs, this only gives ordered values (through the values() call). Is there a way to get ordered Map.Entry instances?
  • @CoryKendall: Does TreeMap not work? It is supposed to be sorted by keys, not by values.
  • My mistake, I thought Sets were unsorted.
  • Please note: The sorting of a TreeMap is based on the natural order of the keys: "The map is sorted according to the natural ordering of its keys". The LinkedHashMap is sorted bij insert order. Big difference!
  • @AlexR: That is only true if the LinkedHashMap was created using the special constructor which is provided for that purpose. By default, iteration is in insertion order.