SatyaCodes

Java- LinkedHashMap

LinkedHashMap – Internal implementation

  • LinkedHashMap is just an extension of HashMap , internally uses HashTable+DoublyLinkedList

  • It has two references head and **tail **which will keep track of the latest object inserted and the first object inserted

1.Each node in a LinkedHashMap needs to have information about previous node and next node as the order in which they are accessed in important. The structure is as follows.

Structure of Node

class Entry<K,V> extends HashMap.Node<K,V> {
        Entry<K,V> before, after;
        Entry(int hash, K key, V value, Node<K,V> next) {
            super(hash, key, value, next);
   }

2.After inserting map.put(“one”, “AAA”);,Bucket with 16 capacity is created, hashcode & index will be caluculated.Here one is added, head and tail will refer to it.

3.On adding map.put(“two”, “BBB”);, inserted in 8th bucket & it is next to -one”, so head one, tail two, before & after will linked, like this all elements will be added & Insertion order will be marinated.

4.If any collision, new element will be added in same Bucket location with LinkedList& points to next.

public class HashMapDemo {

	public static void main(String[] args) {
 LinkedHashMap<String, String> map = new LinkedHashMap<>();
 map.put("one", "AAA"); //7 -> []
 map.put("two", "BBB"); // 
 map.put("two", "ZZZ");
 map.put("three", "CCC");
 map.put("four", "DDD");
 map.put(null, "XXX");	
 System.out.println(map);
	}
}
{one=AAA, two=ZZZ, three=CCC, four=DDD, null=XXX}

So here Insertion Order is preserved, elements the order they added, same order they will store

public class LinkedHashMapDemo {
    public static void main(String[] args) {
        LinkedHashMap h = new LinkedHashMap();
        h.put("one", "Satya");
        h.put("two", "Ravi");
        h.put("three", "Rakesh");
        h.put("four", "Surya");
        System.out.println(h);//  Insertion Order Preserved

    System.out.println("adding exsting key:" + h.put("two", "Madhu"));
        System.out.println("All keys : " + h.keySet());
        System.out.println("All Values : " + h.values());
        System.out.println("Both Key-Values\n---------");

        Set s = h.entrySet();
        Iterator it = s.iterator();
        while (it.hasNext()) {
            Map.Entry m = (Map.Entry) it.next();
            System.out.println(m.getKey() + "\t : " + m.getValue());
        }
    }
}
{one=Satya, two=Ravi, three=Rakesh, four=Surya}
adding exsting key:Ravi
All keys : [one, two, three, four]
All Values : [Satya, Madhu, Rakesh, Surya]
Both Key-Values
---------
one  : Satya
two  : Madhu
three    : Rakesh
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