How to Create HashMap in Java: Syntax and Examples

To create a HashMap in Java, import java.util.HashMap and instantiate it using new HashMap<KeyType, ValueType>(). This creates a collection that stores key-value pairs for fast data retrieval.

Syntax

The basic syntax to create a HashMap is:

• HashMap<KeyType, ValueType>: Defines the types of keys and values stored.
• new HashMap<>(): Creates a new empty HashMap instance.

java

HashMap<KeyType, ValueType> map = new HashMap<>();

Example

This example shows how to create a HashMap, add key-value pairs, and print the map.

java

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("apple", 3);
        map.put("banana", 5);
        map.put("orange", 2);

        System.out.println("HashMap contents: " + map);
    }
}

Output

HashMap contents: {orange=2, banana=5, apple=3}

Common Pitfalls

Common mistakes when creating or using HashMaps include:

• Not importing java.util.HashMap, causing compilation errors.
• Using raw types without specifying key and value types, which leads to warnings and unsafe code.
• Assuming HashMap maintains insertion order; it does not. Use LinkedHashMap if order matters.
• Using mutable objects as keys without proper hashCode() and equals() implementations.

java

/* Wrong: raw type usage */
HashMap map = new HashMap(); // Avoid raw types

/* Right: specify types */
HashMap<String, Integer> map = new HashMap<>();

Quick Reference

Operation	Syntax	Description
Create HashMap	HashMap map = new HashMap<>();	Creates an empty HashMap.
Add entry	map.put(key, value);	Adds or updates a key-value pair.
Get value	map.get(key);	Retrieves value by key, or null if not found.
Check key	map.containsKey(key);	Checks if key exists.
Remove entry	map.remove(key);	Removes key and its value.

Key Takeaways

Use HashMap<KeyType, ValueType> to create a map with specific key and value types.

Always import java.util.HashMap before using it.

HashMap does not keep the order of entries; use LinkedHashMap if order matters.

Avoid raw types to prevent unsafe code and warnings.

Use put() to add entries and get() to retrieve values.