Answer by roottraveller for Difference between HashMap, LinkedHashMap and TreeMap

All three classes HashMap, TreeMap and LinkedHashMap implements java.util.Map interface, and represents mapping from unique key to values.

HashMap

1. A HashMap contains values based on the key.

2. It contains only unique elements.

3. It may have one null key and multiple null values.

4. It maintains no order.

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

LinkedHashMap

1. A LinkedHashMap contains values based on the key.
2. It contains only unique elements.
3. It may have one null key and multiple null values.

4. It is same as HashMap instead maintains insertion order. //See class deceleration below

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

TreeMap

1. A TreeMap contains values based on the key. It implements the NavigableMap interface and extends AbstractMap class.
2. It contains only unique elements.
3. It cannot have null key but can have multiple null values.

4. It is same as HashMap instead maintains ascending order(Sorted using the natural order of its key.).

   public class TreeMap<K,V> extends AbstractMap<K,V> implements NavigableMap<K,V>, Cloneable, Serializable

Hashtable

1. A Hashtable is an array of list. Each list is known as a bucket. The position of bucket is identified by calling the hashcode() method. A Hashtable contains values based on the key.
2. It contains only unique elements.
3. It may have not have any null key or value.
4. It is synchronized.

5. It is a legacy class.

   public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable

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Ref: http://javarevisited.blogspot.in/2015/08/difference-between-HashMap-vs-TreeMap-vs-LinkedHashMap-Java.html