Hashtable in Java

1. Introduction to Hashtable

Hashtable is a legacy Java class that implements the Map interface. It stores data as key-value pairs and is synchronized, making its individual operations thread-safe.

Syntax

Hashtable<KeyType, ValueType> table = new Hashtable<>();

Example

import java.util.Hashtable;

public class Main {
    public static void main(String[] args) {
        Hashtable<Integer, String> students = new Hashtable<>();

        students.put(1, "Mani");
        students.put(2, "Arun");
        students.put(3, "Priya");

        System.out.println(students);
    }
}

The output contains all three key-value pairs. The order is not guaranteed.

2. Creating a Hashtable

Hashtable<String, Integer> scores = new Hashtable<>();

Creates an empty Hashtable.

3. Adding Elements Using put()

The put() method adds or updates a key-value pair.

Hashtable<Integer, String> students = new Hashtable<>();

students.put(1, "Mani");
students.put(2, "Arun");
students.put(1, "Priya");

System.out.println(students.get(1)); // Priya

4. Accessing Values Using get()

Returns the value associated with a key.

Hashtable<Integer, String> students = new Hashtable<>();
students.put(1, "Mani");

System.out.println(students.get(1)); // Mani

5. Checking Keys Using containsKey()

Hashtable<Integer, String> students = new Hashtable<>();
students.put(1, "Mani");

System.out.println(students.containsKey(1)); // true

Checks whether a key exists.

6. Checking Values Using containsValue()

Hashtable<Integer, String> students = new Hashtable<>();
students.put(1, "Mani");

System.out.println(students.containsValue("Mani")); // true

Checks whether a value exists.

7. Removing Elements Using remove()

Hashtable<Integer, String> students = new Hashtable<>();
students.put(1, "Mani");
students.put(2, "Arun");

students.remove(1);

System.out.println(students.containsKey(1)); // false

Removes the entry associated with the specified key.

8. Updating Values Using replace()

Hashtable<Integer, String> students = new Hashtable<>();
students.put(1, "Mani");

students.replace(1, "Arun");

System.out.println(students.get(1)); // Arun

Updates the value when the key exists.

9. Finding the Size Using size()

Hashtable<Integer, String> students = new Hashtable<>();
students.put(1, "Mani");
students.put(2, "Arun");

System.out.println(students.size()); // 2

Returns the number of key-value pairs.

10. Checking Whether a Hashtable Is Empty

Hashtable<String, Integer> scores = new Hashtable<>();

System.out.println(scores.isEmpty()); // true

The isEmpty() method checks whether the table contains entries.

11. Removing All Entries Using clear()

Hashtable<Integer, String> students = new Hashtable<>();
students.put(1, "Mani");

students.clear();

System.out.println(students.size()); // 0

Removes all key-value pairs.

12. Iterating Through a Hashtable

Use entrySet() to access keys and values.

Hashtable<Integer, String> students = new Hashtable<>();
students.put(1, "Mani");
students.put(2, "Arun");

for (var entry : students.entrySet()) {
    System.out.println(entry.getKey() + " = " + entry.getValue());
}

The iteration order is not guaranteed.

13. Getting All Keys and Values

Hashtable<Integer, String> students = new Hashtable<>();
students.put(1, "Mani");
students.put(2, "Arun");

System.out.println(students.keySet());
System.out.println(students.values());

• keySet() returns the keys.
• values() returns the values.

14. Handling Duplicate Keys and Values

Keys must be unique, but values can be duplicated.

Hashtable<Integer, String> students = new Hashtable<>();

students.put(1, "Mani");
students.put(1, "Arun");
students.put(2, "Arun");

System.out.println(students.size()); // 2
System.out.println(students.get(1)); // Arun

Adding an existing key replaces its previous value.

15. Does Hashtable Allow Null Keys and Values?

No. Hashtable does not allow null keys or null values. Attempting to add either throws a NullPointerException.

Hashtable<String, String> map = new Hashtable<>();

map.put("language", "Java");
// map.put(null, "Java"); // NullPointerException
// map.put("language", null); // NullPointerException

16. Setting a Default Value Using getOrDefault()

Hashtable<String, Integer> scores = new Hashtable<>();
scores.put("Mani", 90);

System.out.println(scores.getOrDefault("Arun", 0)); // 0

Returns the default value if the key is absent.

17. Adding Entries Using putIfAbsent()

Hashtable<Integer, String> students = new Hashtable<>();

students.put(1, "Mani");
students.putIfAbsent(1, "Arun");
students.putIfAbsent(2, "Priya");

System.out.println(students.get(1)); // Mani

Adds the mapping only if the key is absent.

18. Hashtable vs HashMap vs ConcurrentHashMap

19. Important Hashtable Methods

20. Time Complexity of Hashtable

Note: Hash collisions can affect performance.

Conclusion

Hashtable is a legacy Java collection that stores key-value pairs and synchronizes its individual operations.

Key points:

• Keys are unique; values can be duplicated.
• Null keys and values are not allowed.
• Iteration order is not guaranteed.
• Individual methods are synchronized, but multi-step operations may still require external synchronization.
• For modern concurrent applications, ConcurrentHashMap is generally preferred.

Audience Engagement: Have you used Hashtable, HashMap, or `ConcurrentHashMap in a Java project? Which would you choose for a multithreaded application, and why?