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

1. Key points about HashMap:

HashMap based on hashcode of keys where
- keys are unique
- values can be duplicate

2. HashMap:

HashMap is the implementation class of Map interface (i.e.; HashMap implements Map)
HashMap uses hashtable to store key-value pairs (which is known as map entry)
HashMap allows only unique keys but there is no restriction on values which can be duplicated
At any time, HashMap contains only unique keys
Insertion-order is NOT maintained
While iterating through HashMap, we will get map entries in random-order, as against 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, HashMap allows to insert any type of key and values;
With generics, it is type-bounded except, if we take both key-value as Objects within angle brackets
HashMap is non-synchronized
Search operation is faster i.e.; searching any element from HashMap is faster, as it uses hashing to store elements
Present in java.util package and extends java.util.AbstractMap implements java.util.Map interface
Also, implements java.lang.Cloneable, java.io.Serializable marker interfaces which provides special ability to HashMap (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

3. HashMap constructors:

3.1 HashMap hm = new HashMap();

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

3.2 HashMap hs = new HashMap(int initialCapacity);

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

3.3 HashMap hs = new HashMap(int initialCapacity, float loadFactor);

creates an empty HashMap object of specified size (or initial capacity)
and specified fill ratio (for example 0.85)

3.4 HashMap hs = new HashMap(Map m);

creates an equivalent HashMap object for specified map
it is basically used for inter-conversion between map objects

4. Fill ratio (or Load factor):

Fill ratio is also known as Load factor
This factor determines when to increase the size of HashMap automatically
For example, for the 1^st two constructors the default load factor is 0.75 –> which means after filling 75% of original HashMap, new HashMap of bigger size will be created
For 3^rd constructor, programmer can define load factor while creating HashMap object. If programmer defined it to be 0.95, then after filling 95% of HashMap, size of HashMap will be increased automatically
The value of Load factor should be in between 0 to 1.0

5. HashMap examples:

HashMapAddAndRemove.java

package in.bench.resources.java.collection;

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

public class HashMapAddAndRemove {

	public static void main(String[] args) {

		// creating HashMap object of type <Integer, String>
		HashMap<Integer, String> hm = new HashMap<Integer, String>();

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

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

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

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

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

		// Iterating key-pairs using Map entry
		Set set = hm.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 : "
				+ hm.remove(4));
	}
}

Output:

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

Iterating using keySet

1  Google
3  Yahoo
2  Facebook
5  Reddit
4  Amazon

Iterating using Map Entry interface

1  Google
3  Yahoo
2  Facebook
5  Reddit
4  Amazon

Entry removed at 4th position : Amazon

Note: All methods of HashMap is non-synchronized

Q) How to make HashMap synchronized ?

HashMap can be easily converted into synchronized HashMap
Using utility method synchronizedMap(hm); of java.util.Collections class

Map map = Collections.synchronizedMap(hm);

References:

Happy Coding !!
Happy Learning !!

BenchResources.Net

Java tutorial for beginners

Java – HashMap class with examples

1. Key points about HashMap:

2. HashMap:

3. HashMap constructors:

3.1 HashMap hm = new HashMap();

3.2 HashMap hs = new HashMap(int initialCapacity);

3.3 HashMap hs = new HashMap(int initialCapacity, float loadFactor);

3.4 HashMap hs = new HashMap(Map m);

4. Fill ratio (or Load factor):

5. HashMap examples:

HashMapAddAndRemove.java

Q) How to make HashMap synchronized ?

Related Articles:

References:

Related

1. Key points about HashMap:

2. HashMap:

3. HashMap constructors:

3.1 HashMap hm = new HashMap();

3.2 HashMap hs = new HashMap(int initialCapacity);

3.3 HashMap hs = new HashMap(int initialCapacity, float loadFactor);

3.4 HashMap hs = new HashMap(Map m);

4. Fill ratio (or Load factor):

5. HashMap examples:

HashMapAddAndRemove.java

Q) How to make HashMap synchronized ?

Related Articles:

References:

Related

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