Java HashMap

Java HashMap is used to store data in key-value pairs where each key is unique. It allows fast operations like searching, updating, and deleting data using keys. It is one of the most commonly used classes in the Java Collection Framework.

In this chapter, you will learn about the Java HashMap class, its features, and examples.

What is HashMap in Java?

Java HashMap class implements the Map interface and is used to store key-value pairs. Each key must be unique, and if a duplicate key is inserted, the old value is replaced with the new value.

HashMap is part of the java.util package and allows one null key and multiple null values. It is not synchronized and provides fast performance for basic operations.

It extends the AbstractMap class and implements the Map interface.

Hierarchy of HashMap Class

HashMap is a part of the Java Collection Framework. It extends the AbstractMap class and implements the Map interface.

The Map interface further extends other interfaces in the hierarchy, making HashMap a key component for storing key-value pairs in Java.

The following image shows the hierarchy of the HashMap class:

HashMap Class Declaration

Let's see the declaration for java.util.HashMap class.

HashMap Class Parameters

Let's see the Parameters for java.util.HashMap class.

• K: It is the type of keys maintained by this map.
• V: It is the type of mapped values.

Constructors of Java HashMap Class

Java HashMap class provides different constructors to create and initialize a HashMap.

1. HashMap()

It is used to construct a default HashMap.

Here is the syntax:

2. HashMap(Map m)

It is used to construct a HashMap containing the elements of the specified Map.

Here is the syntax:

3. HashMap(int capacity)

It is used to construct a HashMap with the specified initial capacity.

Here is the syntax:

4. HashMap(int capacity, float loadFactor)

It is used to construct a HashMap with the specified capacity and load factor.

Here is the syntax:

Methods of Java HashMap Class

The following table shows the commonly used methods of the HashMap class:

Creating a Simple HashMap

A HashMap is used to store key-value pairs. We can add elements using the put() method and traverse them using entrySet().

Example

In this example, we are creating a HashMap, adding elements, and traversing it.

Output:

Iterating Hashmap...
1 Mango
2 Apple
3 Banana
4 Grapes

Handling Duplicate Keys in HashMap

HashMap does not allow duplicate keys. If a duplicate key is added, the old value is replaced with the new value.

Example

In this example, we are adding a duplicate key, which replaces the previous value.

Output:

1 Grapes
2 Apple
3 Banana

Adding Elements in HashMap

Elements can be added using methods like put(), putIfAbsent(), and putAll().

Example

In this example, we are adding elements using different methods like put(), putIfAbsent(), and putAll().

Output:

100 Amit
101 Vijay
102 Rahul
103 Gaurav
104 Ravi

Removing Elements from HashMap

Elements can be removed using remove() methods based on key or key-value pair.

Example

In this example, we are removing elements using remove() methods.

Output:

101 Vijay
103 Gaurav

Replacing Elements in HashMap

We can replace values using replace() and replaceAll() methods.

Example

In this example, we are replacing values using replace() and replaceAll() methods.

Output:

100 Ajay
101 Ajay
102 Ajay

Difference Between HashSet and HashMap

HashSet stores only values, whereas HashMap stores key-value pairs (entries). The following table shows the main differences between HashSet and HashMap based on the different aspects: