Java HashMap

A HashMap is an important part of the Java collections framework and implements the Map interface. It stores items as key-value pairs, where each key is unique and is used to fetch the associated value. It is one of the most commonly used data structures for storing key-value pairs. It offers fast lookups, insertions, and deletions, making it an excellent choice for many programming tasks where performance matters.

Key characteristics:

• Null Support: Allows a single null key and multiple null values.
• Order: Does not maintain any insertion or sorted order of elements.
• Thread Safety: Not synchronized by default; not safe for concurrent access without external synchronization.
• Performance: Provides constant-time performance for .get() and .put() operations in average cases.
• Key Uniqueness: Keys must be unique; inserting a duplicate key will overwrite the existing value.

Creating a HashMap

The HashMap class in Java is part of the java.util package:

import java.util.HashMap;

HashMap<KeyDataType, ValueDataType> myHashMap = new HashMap<KeyDataType, ValueDataType>();

In the syntax:

• KeyDataType: The data type of the keys to be inserted.
• ValueDataType: The data type of the values to be inserted.

Accessing Items in a HashMap

The .get() method can be used to access the value of a key in a HashMap.

Syntax

hashmap_name.get(key);

Parameters:

• key: The key whose value is to be accessed.

Return value:

The .get() method returns the value of the given key.

Example

This example demonstrates the usage of the .get() method to access the value of a key in a HashMap:

import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap
    HashMap<String, String> courseTeacher = new HashMap<String, String>();

    // Add items to the HashMap
    courseTeacher.put("History", "Ben");
    courseTeacher.put("Mathematics", "Jeanette");
    courseTeacher.put("Physics", "Lily");

    // Access the value of a key
    System.out.println(courseTeacher.get("Physics"));
  }
}

Copy to clipboard
Copy to clipboard

Here is the output:

Lily

Copy to clipboard
Copy to clipboard

Adding Items to a HashMap

The .put() method is used to add items to a HashMap.

Syntax

hashmap_name.put(key, value);

Parameters:

• key: The key to be added.
• value: The value for the key to be added.

Example

This example demonstrates the usage of the .put() method to add items to a HashMap:

import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap
    HashMap<String, String> courseTeacher = new HashMap<String, String>();

    // Add items to the HashMap
    courseTeacher.put("History", "Ben");
    courseTeacher.put("Mathematics", "Jeanette");
    courseTeacher.put("Physics", "Lily");

    // Print the HashMap
    System.out.println(courseTeacher);
  }
}

Copy to clipboard
Copy to clipboard

Here is the output:

{Mathematics=Jeanette, History=Ben, Physics=Lily}

Copy to clipboard
Copy to clipboard

Removing Items from a HashMap

There are two methods that can be used to remove items from a HashMap:

• .remove(): This method can be used to delete an item from a HashMap.
• .clear(): This method can be used to delete all items from a HashMap.

Syntax

Here is the syntax for these methods:

hashmap_name.remove(key);
hashmap_name.clear();

Parameters:

• key: The key to be deleted along with its value.

Example

This example demonstrates the usage of the .remove() and .clear() methods to delete items from a HashMap:

import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap
    HashMap<String, String> courseTeacher = new HashMap<String, String>();

    // Add items to the HashMap
    courseTeacher.put("History", "Ben");
    courseTeacher.put("Mathematics", "Jeanette");
    courseTeacher.put("Physics", "Lily");

    // Remove an item from the HashMap
    courseTeacher.remove("Physics");

    // Print the HashMap
    System.out.println(courseTeacher);

    // Remove all items from the HashMap
    courseTeacher.clear();

    // Print the HashMap
    System.out.println(courseTeacher);
  }
}

Copy to clipboard
Copy to clipboard

Here is the output:

{Mathematics=Jeanette, History=Ben}
{}

Copy to clipboard
Copy to clipboard

Iterating Over a HashMap

There are three methods that can be used to iterate over a HashMap:

• .keySet(): Used to iterate over the keys in a HashMap.
• .values(): Used to iterate over the values in a HashMap.
• .entrySet(): Used to iterate over the items in a HashMap.

Syntax

Here is the syntax for these methods:

hashmap_name.keySet();
hashmap_name.values();
hashmap_name.entrySet();

Example

This example demonstrates the usage of the for-each loop to iterate over a HashMap:

import java.util.HashMap;
import java.util.Map;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap
    HashMap<String, String> courseTeacher = new HashMap<String, String>();

    // Add items to the HashMap
    courseTeacher.put("History", "Ben");
    courseTeacher.put("Mathematics", "Jeanette");
    courseTeacher.put("Physics", "Lily");

    // Iterate over the keys in the HashMap
    for (String i : courseTeacher.keySet()) {
      System.out.println(i);
    }

    // Iterate over the values in the HashMap
    for (String i : courseTeacher.values()) {
      System.out.println(i);
    }

    // Iterate over the items in the HashMap
    for (Map.Entry<String, String> entry: courseTeacher.entrySet()) {
      System.out.print(entry);
      System.out.print("\n");
    }
  }
}

Copy to clipboard
Copy to clipboard

In the example, Map.Entry is a nested interface within the Map interface that represents a key-value pair stored in a map.

Here is the output:

Mathematics
History
Physics
Jeanette
Ben
Lily
Mathematics=Jeanette
History=Ben
Physics=Lily

Copy to clipboard
Copy to clipboard

Frequently Asked Questions

1. How does a HashMap work internally?

A HashMap uses an array of buckets, and each bucket is a linked list or a balanced tree (from Java 8 onward). Keys are hashed to determine the bucket index, and values are stored accordingly.

2. Is HashMap thread-safe?

No. HashMap is not synchronized. For thread-safe operations, use Collections.synchronizedMap() or ConcurrentHashMap.

3. How to increase the performance of a HashMap?

• Choose an appropriate initial capacity and load factor.
• Use immutable keys with properly implemented hashCode() and equals() methods.