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:

Happy Coding !!
Happy Learning !!

BenchResources.Net

Java tutorial for beginners

LinkedHashMap class

Key points about LinkedHashMap:

LinkedHashMap:

LinkedHashMap constructors:

LinkedHashMap hs = new LinkedHashMap();

LinkedHashMap hs = new LinkedHashMap(int initialCapacity);

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

LinkedHashMap hs = new LinkedHashMap(Collection c);

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

Fill ratio (or Load factor)

LinkedHashMap examples:

LinkedHashMapAddAndRemove.java

Difference between HashMap and LinkedHashMap ?

How to make LinkedHashMap synchronized ?

Key points about LinkedHashMap:

LinkedHashMap:

LinkedHashMap constructors:

LinkedHashMap hs = new LinkedHashMap();

LinkedHashMap hs = new LinkedHashMap(int initialCapacity);

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

LinkedHashMap hs = new LinkedHashMap(Collection c);

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

Fill ratio (or Load factor)

LinkedHashMap examples:

LinkedHashMapAddAndRemove.java

Difference between HashMap and LinkedHashMap ?

How to make LinkedHashMap synchronized ?

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