Difference Between Map and HashMap

Java provides different types of data structures like Set, Vector, Array, Tree, Map, and HashMap. Map and HashMap are the two important data structure because they work on key-value concept. In this section, we will discuss the major differences between Map and HashMap.

It is used to insert, update, remove the elements. Whereas the HashMap is a non-synchronized class of Java collection framework that contains null values and keys.

Map Interface

It is important to note that Map is an interface, meaning it defines a set of methods that must be implemented by concrete classes. It represents a collection of key-value pairs, where each key is unique and maps to a corresponding value. It provides pre-defined methods that manipulate the elements of the map. These operations are insert, remove, update and retrieving elements. Its implementing classes are as follows:

• HashMap
• Hashtable
• TreeMap
• ConcurrentHashMap
• LinkedHashMap

Key Characteristics of Map Interface:

Key-Value Pairing: Maps associate keys with values, allowing quick retrieval of a value based on its corresponding key.

Unique Keys: Each key in a map must be unique. Duplicate keys are not allowed.

Various Implementations: Java offers several implementations of the Map interface, each with its own unique characteristics and use cases.

Implementation of the Map Interface

The Map interface can be implemented by using the implementing class. Consider the following example:

File Name: MapExample.java

Output:

Nick:2
Abhi:1
Paul:4
Ketty:3

HashMap Class

HashMap is one of the most commonly used implementations of the Map interface in Java. It is a non-synchronized class of Java collection. It stores key-value pairs in a hash table, allowing for fast retrieval of values based on associated keys. It allows us to store the null values and null keys. The underlying data structure of a HashMap consists of an array of linked lists, where each element in the array is a bucket that can hold multiple key-value pairs.

Key Characteristics of HashMap

Fast Lookup: HashMap provides constant-time performance for basic operations like get() and put(), making it efficient for large datasets.

Unordered: The order of key-value pairs in a HashMap is not guaranteed. It does not maintain any particular order of elements.

Allows Null Keys and Values: Unlike some other implementations of Map, HashMap allows both null keys and null values.

Not Thread-Safe: By default, HashMap is not synchronized and is not thread-safe. If multiple threads access a HashMap concurrently and at least one of the threads modifies the map structurally, it must be synchronized externally.

Implementation of the HashMap Class

HashMap class can be declared as follows:

Consider the below example to implement the HashMap:

Output:

Iterating Hashmap...
1 Chris
2 Morris
3 Sam
4 Cruise

Key Differences Between Map and HashMap

Some key differences between the Map and HashMap are as follows:

• The Map is an interface, and HashMap is a class of the Java collection framework.
• The Map interface can be implemented by using its implementing classes. In comparison, the HashMap class implements the Map interface.
• The Map contains unique key-pair values. But, the HashMap can hold duplicate values.
• The Map does not allow null values. But the HashMap can have one null key and multiple values.
• The Map has two implementations, which are HashMap and TreeMap. Whereas HashMap implements Map interface and extends AbstractMap class.
• There is no difference between the Map and HashMap objects.
• Since Map is an interface, it does not inherently provide thread safety. However, some implementations of Map, such as ConcurrentHashMap, offer built-in thread safety. On the other hand, HashMap is not thread-safe by default.

Here's a tabular representation summarizing the key differences between Map and HashMap in Java:

The table highlights the main distinctions between the Map interface and the HashMap class in Java, covering aspects such as implementation details, handling of null keys and values, and thread safety. Understanding these differences is crucial for selecting the appropriate data structure based on the specific requirements of a Java application.

Conclusion

In summary, while Map is an interface that defines the behavior of a collection of key-value pairs, HashMap is a concrete implementation of that interface with specific characteristics regarding performance, ordering, and thread safety. Understanding these distinctions is essential for Java developers to choose the appropriate data structure based on their specific requirements and constraints.