What is a HashMap in Java?

HashMap is a very powerful data structure in Java that allows you to store key-value pairs. It is very similar to a Hashtable but it is not synchronized. HashMap allows you to store null values as well as duplicate keys. In this article, we will discuss how to initialize HashMap in Java.

What is a HashMap?

A HashMap is a data structure that uses a hash function to map keys to values. A hash function is any function that can be used to map data of arbitrary size to fixed-size values. The values returned by a hash function are called hash values, hash codes, digests, or simply hashes.

HashMap is a part of the Java Collections Framework. It was introduced in Java 1.2 and is very popular.

A HashMap cannot contain duplicate keys and each key can map to at most one value. If you try to insert a duplicate key, the old value will be replaced with the new one.

HashMap works on the principle of hashing. It uses a hash function to calculate the index where the key will be stored. The index is calculated by using the hashCode() method. This index is called the bucket and it stores all the entries (key-value pairs) that have the same hashCode().

A HashMap can have only one null key and multiple null values.

What are the benefits of using a HashMap?

When you want to store data in key, value format, HashMap is the obvious choice. The HashMap class in Java provides the basic implementation of the Map interface of Java by storing the data in (Key, Value) pairs to access them by an index of another type (e.g. an Integer).

Java HashMap allows null keys and values.HashMap is a part of Java’s collection framework and implements the Map interface.

Some benefits of using a HashMap are:
-HashMap is very fast. It outperforms other Map implementations like TreeMap and LinkedHashMap for large inputs.
-Our code is more readable if we use methods from Map instead of directly accessing fields from objects.
-We can use any object as a key or value in HashMap, even primitive types are allowed(autoboxing).

How does a HashMap work?

The HashMap class uses a hash table to implement the Map interface. This allows the execution time of basic operations, such as get( ) and put( ), to remain constant even for large sets.

The HashMap class provides three constructors.
-HashMap( ): This creates an empty hash table with the default initial capacity (16) and default load factor (0.75).
-HashMap(int capacity): This creates an empty hash table with the specified initial capacity and default load factor (0.75).
-HashMap(int capacity, float fillRatio): This creates an empty hash table with specified initial capacity and fill ratio.

How to initialize a HashMap in Java

HashMap is a part of the Java Collections Framework.Maps store key value pairs. The basic idea is to associate a key with a value. When you need the value, you can provide the key and retrieve the value from the HashMap. In this article, we’ll show you a few different ways to initialize a HashMap in Java.

How to create a HashMap?

There are four ways to create a HashMap in java.

• 1. using new operator
• 2. using reflection
• 3. using clone() method
• 4. using deserialization
  How to initialize a HashMap?
  There are four ways to initialize HashMap in java. 1) Initialization with add() method 2) Initialization by putting all elements of same type 3) Initialization using Arrays.asList() 4) Initialization using Collection.addAll()
  How to put values in a HashMap?
  
  We can initialize a HashMap in three ways –

Initializing HashMap during declaration –
HashMap myMap = new HashMap<>();
myMap.put(1, “John”);
myMap.put(2, “Smith”);
myMap.put(3, “Steve”);

Initializing HashMap with another Map –
HashMap myNewMap = new HashMap<>(myMap);

Initializing Hashmap with values() method of the Map interface –
HashMap myLastMapt = new Hashmap<>(myNewMap.values());

How to get values from a HashMap?

There are multiple ways of iterating or looping through a Map in Java.
1) We can use the keySet() method to get all the keys from the HashMap and then use the get() method corresponding to each key to retrieve the value.
2) We can also use the entrySet() method which returns a Set of Map.Entry objects. Each Entry in this set is essentially a key-value pair.

3) Another way of looping through a HashMap is by using Java 8 forEach and lambda expression.
4) If you are using Java 8 or above, we can use stream API to iterate over HashMap values as well.

Conclusion

HashMap is a part of Java’s collection framework and implements Map interface.

It also inherits AbstractMap class.

The HashMap in Java is not synchronized, which means it is not thread-safe. However, we can use Collections.synchronizedMap(hashMap) to get a synchronized hashmap.

HashMap allows one null key and multiple null values.