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LinkedHashMap is a Map class that extends HashMap class. This class stores list of entries(key-value pairs)in the order in which they were inserted. i.e. the first element to enter the LinkedHashMap occupies the first position, the second to enter the Map occupies the second position and so on.

Important features of LinkedHashSet

• A null object can be stored as a key in a LinkedHashMap.
• A null object can be stored as a value in a LinkedHashMap.
• Unlike HashMap, LinkedHashMap gives a guarantee about the order of its elements.







A simple constructor of LinkedHashMap




LinkedHahMap()
This constructor creates an empty LinkedHashMap to hold a particular type of key-value pair objects. Example -

LinkedHashMap <String,Integer> tm = new LinkedHashMap <String,Integer>();


This constructor example creates a LinkedHashMap to hold key-value pairs of String and Integer objects.






Some methods in a LinkedHashMap.




Methods	Description
void clear()	Removes all the key/value pairs from the HashMap.
int size()	Returns the total number of key-value pairs in a HashMap.
boolean isEmpty()	Returns true if HashMap has no key-value pair in it.
V get(Object key)	Returns the value associated with a specified key.
put(K key, V value)	Puts the specified key and its associated value in the HashMap.
boolean containsKey(Object key)	Returns true if HashMap contains a particular key, otherwise false.
Set<Map.Entry<K,V>> entrySet()	Returns the Set containing Map.Entry objects.






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Creating a LinkedHashMap

In this example we will create a LinkedHashMap to store objects in the pair of key and value, where key is a String object and value is an Integer object. By default, the map entries(key-value pairs) in the LinkedHashMap will be ordered as per their insertion order.

import java.util.*;


class LinkedHashMapDemo
{
public static void main(String... ar)
{
LinkedHashMap<String,Integer> lhm = new LinkedHashMap<String,Integer>();

lhm.put("A", 1000);
lhm.put("C", 4000);
lhm.put("B", 2000);
lhm.put("F", 6000);
lhm.put("D", 5000);

System.out.println(lhm);

System.out.println("Iterating LinkedHashMap using Map.Entry and for-each loop");
Set<Map.Entry<String,Integer>> set= lhm.entrySet();


for(Map.Entry<String,Integer> mapE : set)
{
	System.out.print(mapE.getKey() + " : ");
	System.out.println(mapE.getValue());
}


System.out.println("Value with the key B is "+ lhm.get("B"));
System.out.println("Value with the key F is "+ lhm.get("F"));

}

}

Output is :

{A=1000, C=4000, B=2000, F=6000, D=5000}
Iterating LinkedHashMap using Map.Entry and for-each loop
A : 1000
C : 4000
B : 2000
F : 6000
D : 5000
Value with the key B is 2000
Value with the key F is 6000

Program Analysis

• We have created a LinkedHashMap, to store objects in the form of String and Integer objects.
• Method entrySet() returns a Set of the entries in the Map, in the form of objects of type Map.Entry.
• Inside for-each loop we have called methods of Map.Entry like getKey() and getValue() to iterate over each map entry which is in pair of key and value.
• Method getKey() of Map.Entry returns the key information of a particular may entry, while the method getValue() returns the value associated with this key.






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