Maps in Java

Concrete maps in Java JDK 1.8

class HashMap<K,V> 
    extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable

class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable

class TreeMap<K,V>
    extends AbstractMap<K,V>
    implements NavigableMap<K,V>, Cloneable, java.io.Serializable

class LinkedHashMap<K,V>
    extends HashMap<K,V>
    implements Map<K,V>

Below is the summary of differences between HashMap, LinkedHashMap, TreeMap, and Hashtable in Java:

(Source: javarevisited.blogspot.com)

• If we need to get the key back in insertion order, then use LinkedHashMap.
• If we need to get the key back in their true/natural order, then use TreeMap.
• Otherwise, HashMap is best because it is typically faster and requires less overhead.

An example of using LinkedHashMap, TreeMap and HashMap in Java
HashMap

        /* HashMap: Put, Get, Remove is O(1); Not synchronized;
           Allow null key and value; Not guaranteed order.
         */
        Map<Integer, String> hashMap = new HashMap<>();
        hashMap.put(2,"Peter");
        hashMap.put(1,"Marry");
        hashMap.put(3,"Gayle");
        hashMap.put(null, null);
        hashMap.put(null, "Unknown");
        hashMap.put(null, null);

        System.out.println("Size of hashMap: " +hashMap.size());

        System.out.println("Look up a value:" + hashMap.get(1));

        System.out.println("Look up by null key " + hashMap.get(null));

        // iterate through all entries
        System.out.println("Order of Entries in HashMap - Not guaranteed");
        for(Map.Entry<Integer, String> entry : hashMap.entrySet()) {
            System.out.println("Key:" + entry.getKey() + " Value:" + entry.getValue());
        }

Actually HashMap can’t keep duplicates within it. Whenever there is a duplicate key added to the map it overrides the existing one. So each time there will be only one key available with that name. Output in console showed below:

Size of hashMap: 4
Look up a value:Marry
Look up by null key null
Order of Entries in HashMap - Not guaranteed
Key:null Value:null
Key:1 Value:Marry
Key:2 Value:Peter
Key:3 Value:Gayle

TreeMap

        /* TreeMap: Put, Get, Remove is O(log(n)); Not synchronized;
           NOT Allow null key and value; Sorted in order
         */

        Map<Integer, String> treeMap = new TreeMap<>();
        treeMap.put(2, "Jack");
        treeMap.put(3, "Haney");
        treeMap.put(0, "Barron");
        System.out.println("Order of Entries in TreeMap - Sorted in natural order");
        for(Map.Entry<Integer, String> entry : treeMap.entrySet()) {
            System.out.println("Key:" + entry.getKey() + " Value:" + entry.getValue());
        }

        //Using TreeMap to create a sorted hashMap, sorting keys on natural order
        SortedMap<Integer, String> sortedMap = new TreeMap<>(Comparator.naturalOrder());
        // TreeMap is not allow null Key and Value so remove all null entries in hashMap
        hashMap.remove(null);
        sortedMap.putAll(hashMap);
        System.out.println("Order of entries in TreeMap from the existing hashMap");
        System.out.println(sortedMap);

Noted that TreeMap is an implementation of SortedMap and keeps keys in their natural order or a custom order specified by Comparator provided while creating TreeMap.
Output in console:

Order of Entries in TreeMap - Sorted in natural order
Key:0 Value:Barron
Key:2 Value:Jack
Key:3 Value:Haney
Order of entries in TreeMap from the existing hashMap
{1=Marry, 2=Peter, 3=Gayle}

LinkedHashMap

        /* LinkedHashMap: Put, Get, Remove is O(1); Not synchronized;
           Allow null key and value; Sorted basing on the insertion order.
        */
        Map<Integer, String> linkedHashMap = new LinkedHashMap<>();
        linkedHashMap.put(2, "Jack");
        linkedHashMap.put(3, "Haney");
        linkedHashMap.put(0, "Barron");
        linkedHashMap.put(null, null);
        System.out.println("Order of Entries in LinkedHashMap");
        for(Map.Entry<Integer, String> entry : linkedHashMap.entrySet()) {
            System.out.println("Key:" + entry.getKey() + " Value:" + entry.getValue());
        }

        //Using LinkedHashMap to create copy of a Map in Java
        Map<Integer, String> copy = new LinkedHashMap<>(sortedMap);

Output in console:

Order of Entries in LinkedHashMap
Key:2 Value:Jack
Key:3 Value:Haney
Key:0 Value:Barron
Key:null Value:null
Order of Entries in a copy Map created by LinkedHashMap
{1=Marry, 2=Peter, 3=Gayle}

To sum up, all concrete classes offer a key -> value and a way to iterate through the key. The most important distinction between theses classes is the time guarantees and the ordering of the keys.

References:

• Cay S. Horstmann, Gray Cornel, “Collection”, in Core Java Volume I- Fundamentals, chapter 13, 8th Edition, Sun Microsystems Press.
• Maurice Naftalin and Philip Wadler, “Maps”, in Java Generics and Collections, chapter 16, 2007 O’Reilly Media.
• Oracle Document, Retrieved at http://docs.oracle.com/javase/tutorial/collections/interfaces/index.html
• http://javarevisited.blogspot.com/2015/08/difference-between-HashMap-vs-TreeMap-vs-LinkedHashMap-Java.html
• Java67.com, Retrieved at How to Sort HashMap in Java based on Keys and Values