Dictionary Class in Java

The Dictionary class serves as a foundation for creating associative data structures in Java. Associative data structures, also known as dictionaries or maps, allow us to store values (the “values”) associated with unique identifiers (the “keys”). Think of it as a real-world dictionary where we look up a word (the key) to find its corresponding definition (the value). The Dictionary class provides an organized way to manage these associations, making it an essential tool in many programming scenarios.

Throughout this guide, we’ll take you through the essential aspects of the Dictionary class, whether you’re a novice programmer or a seasoned developer looking to expand your toolkit.

Dictionary Class Features

Before we dive into the technical details, let’s highlight some of the key features that make the Dictionary class indispensable:

  • Key-Value Association: Dictionaries allow us to associate values with unique keys, enabling efficient data retrieval based on these keys.
  • Dynamic Sizing: Unlike arrays, dictionaries can dynamically adjust their size to accommodate new key-value pairs.
  • Flexibility: Dictionaries can store values of different data types and offer a flexible way to structure your data.

Now that we’ve gained an overview of the Dictionary class, let’s dive deeper into its functionalities and understand how it can simplify your Java programming tasks.

Creating and Using a Dictionary in Java

To start using the Dictionary class, we need to import it from the java.util package and create an instance of it. Following is an example of how we can create a Dictionary and perform basic operations:

import java.util.Dictionary;
import java.util.Hashtable;

public class DictionaryDemo {
    public static void main(String[] args) {
        // Create a new Dictionary instance
        Dictionary<Integer, String> dictionary = new Hashtable<>();

        // Add key-value pairs to the dictionary
        dictionary.put(1, "Apple");
        dictionary.put(2, "Banana");
        dictionary.put(3, "Orange");

        // Retrieve a value using a key
        String fruit = dictionary.get(2);
        System.out.println("Fruit at key 2: " + fruit);


Fruit at key 2: Banana

In the example above, we import the Dictionary class from the java.util package and create a new instance using the Hashtable implementation. We then add key-value pairs to the dictionary and retrieve a value using a specific key.

Common Dictionary Applications

Dictionaries are incredibly versatile and find application in various programming scenarios. Following are some common use cases:

  • Data Indexing: Dictionaries are used for indexing and searching through large datasets efficiently.
  • Configuration Management: Dictionaries can store configuration settings with easily retrievable keys.
  • Caching: In memory-intensive applications, dictionaries serve as caches, storing frequently accessed data.
  • Language Translation: Dictionaries are integral to language translation applications, associating words between languages.

Advanced Usage: Implementing Custom Dictionaries

If you’re curious about how dictionaries work under the hood, you can implement your own custom dictionary using arrays or other data structures. Following is a simplified example:

public class CustomDictionary<K, V> {
    private int capacity;
    private Object[] keys;
    private Object[] values;
    private int size;

    public CustomDictionary(int capacity) {
        this.capacity = capacity;
        keys = new Object[capacity];
        values = new Object[capacity];

    public void put(K key, V value) {
        if (size == capacity) {
            throw new IllegalStateException("Dictionary is full");
        keys[size] = key;
        values[size] = value;

    public V get(K key) {
        for (int i = 0; i < size; i++) {
            if (key.equals(keys[i])) {
                return (V) values[i];
        return null; // Key not found

This class can be used the same as the Dictionary class we’ve used above.


public class Main {
    public static void main(String[] args) {
        // Create a new CustomDictionary instance with a capacity of 3
        CustomDictionary<String, Integer> dictionary = new CustomDictionary<>(3);

        // Add key-value pairs to the dictionary
        dictionary.put("apple", 5);
        dictionary.put("banana", 8);
        dictionary.put("orange", 12);

        // Retrieve values using keys
        int apples = dictionary.get("apple");
        int bananas = dictionary.get("banana");
        int oranges = dictionary.get("orange");

        // Print the retrieved values
        System.out.println("Number of apples: " + apples);
        System.out.println("Number of bananas: " + bananas);
        System.out.println("Number of oranges: " + oranges);

The output is not given here. You can execute this code in your computer and print the output in the comment section. 🙂

It's important to note that the Dictionary class is now considered to be outdated and is not recommended for use in new applications. Instead, the Map interface and its various implementations, such as HashMap and TreeMap, are preferred for key-value storage in Java. However, the Dictionary class is still included in the Java API for backward compatibility, and may be useful in certain situations where it is necessary to maintain compatibility with older code.

FAQs about the Dictionary Class in Java

Is the Dictionary class part of the latest Java versions?

The Dictionary class is available in Java, but newer implementations like HashMap and LinkedHashMap are preferred due to enhanced features.

Can I store duplicate keys in a Dictionary?

Dictionaries require unique keys for each value. Duplicate keys will overwrite existing entries.

What happens if I try to retrieve a value with a nonexistent key?

If you attempt to retrieve a value using a key that doesn’t exist in the dictionary, you’ll typically get a null value.

Are there other implementations of dictionaries in Java?

Yes, besides the Hashtable implementation, you can use HashMap, LinkedHashMap, and TreeMap based on your requirements.

Can I use objects as keys in a Dictionary?

Yes, you can use objects as keys, but they must correctly implement the equals() and hashCode() methods.

Is the Dictionary class thread-safe?

The Hashtable implementation of the Dictionary class is thread-safe, ensuring data integrity in multithreaded scenarios.

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