In this article, we will discuss LinkedHashMap class – one of the Map implemented classes in detail

Key points about LinkedHashMap:

• LinkedHashMap is exactly same as that of HashMap
• but preserves insertion-order
• whereas HashMap stores entries in random-order

LinkedHashMap:

• LinkedHashMap is implementation class of Map interface (i.e.; LinkedHashMap implements Map)
• This is introduced in Java 1.4 version
• LinkedHashMap uses combination of LinkedList & hashtable to store Map entries (i.e.; key-value pairs)
• LinkedHashMap allows only unique keys but there is no restriction on values which can be duplicated
• At any time, LinkedHashMap contains only unique keys
• Insertion-order is maintained
• While iterating through LinkedHashMap, we will get map entries as per insertion-order
• Allows NULL insertion for key but maximum of only one
• Also, allows NULL insertion for values without any upper limit i.e.; we can insert null value against any unique key
• Without generics, LinkedHashMap allows to insert any type of Key/Values;
• With generics, it is type-bounded except, if we take both key-value as Objects within angle brackets
• LinkedHashMap is non-synchronized
• Present in java.util package and extends java.util.HashMap implements java.util.Map interface
• Also, implements java.lang.Cloneable, java.io.Serializable marker interfaces which provides special ability to LinkedHashMap (provided by JVM at run time) like,
• java.lang.Cloneable: to create a duplicate object or to clone an object
• java.io.Serializable: to transfer objects across network

Source: Team BenchResources.Net

LinkedHashMap constructors:

LinkedHashMap hs = new LinkedHashMap();

• creates an empty LinkedHashMap object of size 16
• with default fill ratio 0.75

LinkedHashMap hs = new LinkedHashMap(int initialCapacity);

• creates an empty LinkedHashMap object of specified size (or initial capacity)
• with default fill ratio 0.75

LinkedHashMap hs = new LinkedHashMap(int initialCapacity, float loadFactor);

• creates an empty LinkedHashMap object of specified size (or initial capacity)
• with specified fill ratio (for example 0.85)

LinkedHashMap hs = new LinkedHashMap(Collection c);

• created an equivalent LinkedHashMap object for the specified collection
• it is basically used for inter-conversion between collection objects

LinkedHashMap hs = new LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder);

• creates an empty LinkedHashMap object of specified size (or initial capacity)
• with specified fill ratio (for example 0.85)
• along with specified ordering mode like true for access-order and false for insertion-order

Fill ratio (or Load factor)

• Fill ratio is also known as Load factor
• This factor determines when to increase the size of LinkedHashMap automatically
• For example, for the 1^st two constructors the default load factor is 75 à which means after filling 75 % of LinkedHashMap, new LinkedHashMap of bigger size will be created
• For 3^rd constructor, programmer can define load factor while creating LinkedHashMap object. If programmer define it to be 0.95, then after filling 95% of LinkedHashMap, size of LinkedHashMap will be increased automatically
• The value of Load factor should be in between 0 to 1.0

LinkedHashMap examples:

 LinkedHashMapAddAndRemove.java

package in.bench.resources.java.collection;

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

public class LinkedHashMapAddAndRemove {

	public static void main(String[] args) {

		// creating LinkedHashMap object of type <Integer, String>
		LinkedHashMap<Integer, String> lhm = 
				new LinkedHashMap<Integer, String>();

		// adding key-value pairs to LinkedHashMap object
		lhm.put(1, "Google");
		lhm.put(2, "Facebook");
		lhm.put(3, "Yahoo");
		lhm.put(4, "Amazon");
		lhm.put(5, "Reddit");

		System.out.println("Printing all"
				+ " key-value pairs inside {}\n" + lhm + "\n");

		System.out.println("\nIterating using keySet\n");

		// Iterating key-pairs using keySet
		Set<Integer> keys = lhm.keySet();
		for(Integer key : keys) {
			System.out.println(key + "  " + lhm.get(key));
		}

		System.out.println("\n\nIterating using Map Entry interface\n");

		// Iterating key-pairs using Map entry
		Set set = lhm.entrySet();
		Iterator iterator = set.iterator();

		while(iterator.hasNext()) {

			Map.Entry mapEntry = (Map.Entry)iterator.next();
			System.out.println(mapEntry.getKey() + "  "
					+ mapEntry.getValue());
		}

		// removing map entry at 4th position
		System.out.println("\n\nEntry removed at 4th position : "
				+ lhm.remove(4));
	}
}

Output:

Printing all key-value pairs inside {}
{1=Google, 2=Facebook, 3=Yahoo, 4=Amazon, 5=Reddit}


Iterating using keySet

1  Google
2  Facebook
3  Yahoo
4  Amazon
5  Reddit


Iterating using Map Entry interface

1  Google
2  Facebook
3  Yahoo
4  Amazon
5  Reddit


Entry removed at 4th position : Amazon

Difference between HashMap and LinkedHashMap ?

• The main difference between HashMap and LinkedHashMap is insertion-order
• HashMap doesn’t maintain insertion-order and print values in random-order while iterating
• whereas LinkedHashMap maintains insertion-order as seen in above example
• Note: All methods of LinkedHashMap is non-synchronized

How to make LinkedHashMap synchronized ?

• It can be easily converted into synchronized LinkedHashMap
• Using utility method synchronizedMap(lhm); of java.util.Collections class

  Map map = Collections.synchronizedMap(lhm);
  

 

References:

• https://docs.oracle.com/javase/tutorial/collections/intro/
• https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Map.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Map.Entry.html
• https://docs.oracle.com/javase/tutorial/collections/interfaces/map.html
• https://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html
• https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html
• http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashMap.html
• https://docs.oracle.com/javase/8/docs/api/java/util/LinkedHashMap.html

 

Happy Coding !!
Happy Learning !!