LinkedHashMap

LinkedHashMap is a Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order).

Example:

import java.util.LinkedHashMap;
import java.util.Set;
import java.util.Iterator;
import java.util.Map;
public class LinkedHashMapDemo {
    public static void main(String args[]) {
         // HashMap Declaration
         LinkedHashMap<Integer, String> lhmap = 
                 new LinkedHashMap<Integer, String>();

         //Adding elements to LinkedHashMap
         lhmap.put(22, "Abey");
         lhmap.put(33, "Dawn");
         lhmap.put(1, "Sherry");
         lhmap.put(2, "Karon");
         lhmap.put(100, "Jim");

         // Generating a Set of entries
         Set set = lhmap.entrySet();

         // Displaying elements of LinkedHashMap
         Iterator iterator = set.iterator();
         while(iterator.hasNext()) {
            Map.Entry me = (Map.Entry)iterator.next();
            System.out.print("Key is: "+ me.getKey() + 
                    "& Value is: "+me.getValue()+"\n");
         }
    }
}

Output:

Key is: 22& Value is: Abey
Key is: 33& Value is: Dawn
Key is: 1& Value is: Sherry
Key is: 2& Value is: Karon
Key is: 100& Value is: Jim

TreeMap

TreeMap is Red-Black tree based NavigableMap implementation. It is sorted according to the natural ordering of its keys.
TreeMap class implements Map interface similar to HashMap class. The main difference between them is that HashMap is an unordered collection while TreeMap is sorted in the ascending order of its keys. TreeMap is unsynchronized collection class which means it is not suitable for thread-safe operations until unless synchronized explicitly.

Example:

import java.util.TreeMap;
import java.util.Set;
import java.util.Iterator;
import java.util.Map;

public class Details {

   public static void main(String args[]) {

      /* This is how to declare TreeMap */
      TreeMap<Integer, String> tmap = 
             new TreeMap<Integer, String>();

      /*Adding elements to TreeMap*/
      tmap.put(1, "Data1");
      tmap.put(23, "Data2");
      tmap.put(70, "Data3");
      tmap.put(4, "Data4");
      tmap.put(2, "Data5");

      /* Display content using Iterator*/
      Set set = tmap.entrySet();
      Iterator iterator = set.iterator();
      while(iterator.hasNext()) {
         Map.Entry mentry = (Map.Entry)iterator.next();
         System.out.print("key is: "+ mentry.getKey() + " & Value is: ");
         System.out.println(mentry.getValue());
      }

   }
}

Output:

key is: 1 & Value is: Data1
key is: 2 & Value is: Data5
key is: 4 & Value is: Data4
key is: 23 & Value is: Data2
key is: 70 & Value is: Data3

TreeSet

TreeSet is similar to HashSet except that it sorts the elements in the ascending order while HashSet doesn’t maintain any order. TreeSet allows null element but like HashSet it doesn’t allow. Like most of the other collection classes this class is also not synchronized, however it can be synchronized explicitly like this: 
SortedSet s = Collections.synchronizedSortedSet(new TreeSet(...));


Example:



import java.util.TreeSet;
public class TreeSetExample {
     public static void main(String args[]) {
         // TreeSet of String Type
         TreeSet<String> tset = new TreeSet<String>();

         // Adding elements to TreeSet<String>
         tset.add("ABC");
         tset.add("String");
         tset.add("Test");
         tset.add("Pen");
         tset.add("Ink");
         tset.add("Jack");

         //Displaying TreeSet
         System.out.println(tset);

         // TreeSet of Integer Type
         TreeSet<Integer> tset2 = new TreeSet<Integer>();

         // Adding elements to TreeSet<Integer>
         tset2.add(88);
         tset2.add(7);
         tset2.add(101);
         tset2.add(0);
         tset2.add(3);
         tset2.add(222);
         System.out.println(tset2);
    }
 }

Output: You can see both the TreeSet have been sorted in ascending order implicitly.

[ABC, Ink, Jack, Pen, String, Test]
[0, 3, 7, 88, 101, 222]




Hashmap

HashMap is a Map based collection class that is used for storing Key & value pairs. This class makes no guarantees as to the order of the map. It is similar to the Hashtable class except that it is unsynchronized and permits nulls(null values and null key).

HashMap maintains key and value pairs and often denoted as HashMap<Key, Value> or HashMap<K, V>. HashMap implements Map interface. HashMap is similar to Hashtable with two exceptions – HashMap methods are unsynchornized and it allows null key and null values unlike Hashtable. It is used for maintaining key and value mapping.

It is not an ordered collection which means it does not return the keys and values in the same order in which they have been inserted into the HashMap. It neither does any kind of sorting to the stored keys and Values. You must need to import java.util.HashMap or its super class in order to use the HashMap class and methods.

Example:

import java.util.HashMap;
import java.util.Map;
import java.util.Iterator;
import java.util.Set;
public class Details {

   public static void main(String args[]) {

      /* This is how to declare HashMap */
      HashMap<Integer, String> hmap = new HashMap<Integer, String>();

      /*Adding elements to HashMap*/
      hmap.put(12, "Nabin");
      hmap.put(2, "Rabin");
      hmap.put(7, "Sabin");
      hmap.put(49, "Rashmi");
      hmap.put(3, "Kripa");

      /* Display content using Iterator*/
      Set set = hmap.entrySet();
      Iterator iterator = set.iterator();
      while(iterator.hasNext()) {
         Map.Entry mentry = (Map.Entry)iterator.next();
         System.out.print("key is: "+ mentry.getKey() + " & Value is: ");
         System.out.println(mentry.getValue());
      }

      /* Get values based on key*/
      String var= hmap.get(2);
      System.out.println("Value at index 2 is: "+var);

      /* Remove values based on key*/
      hmap.remove(3);
      System.out.println("Map key and values after removal:");
      Set set2 = hmap.entrySet();
      Iterator iterator2 = set2.iterator();
      while(iterator2.hasNext()) {
          Map.Entry mentry2 = (Map.Entry)iterator2.next();
          System.out.print("Key is: "+mentry2.getKey() + " & Value is: ");
          System.out.println(mentry2.getValue());
       }

   }
}

Output:

key is: 49 & Value is: Rashmi
key is: 2 & Value is: Rabin
key is: 3 & Value is: Kripa
key is: 7 & Value is: Sabin
key is: 12 & Value is: Nabin
Value at index 2 is: Rabin
Map key and values after removal:
Key is: 49 & Value is: Rashmi
Key is: 2 & Value is: Rabin
Key is: 7 & Value is: Sabin
Key is: 12 & Value is: Nabin