15.5.2.4. Hashtable

Today we will discuss the concept of a Hashtable in Java. A Hashtable is a synchronized collection. Can anyone tell me what they think 'synchronized' means in this context?

Does it mean that it can handle multiple threads at the same time?

Exactly! 'Synchronized' allows multiple threads to access the data safely. However, what do you think could be a downside of this?

Maybe it makes it slower since it has to manage access from multiple threads?

Good point! It does incur some performance overhead in single-threaded situations. Now, why do you think Hashtable is still around, despite newer solutions?

Maybe for legacy reasons? Like maintaining old code?

Correct! Legacy systems often continue to use it. In fact, it has been around since Java's early days! Let's move on to its behavior with null values.

So, does anyone know how Hashtable handles null keys or values?

I think it does not allow them at all!

That's right! Unlike HashMap, you cannot have null keys or values in a Hashtable. This is crucial to remember when deciding which map to use. Why do you think that restriction might exist?

Maybe to avoid confusion when looking up values?

Exactly! Null keys could lead to ambiguity. Let’s now talk about how you would implement a Hashtable.

Let’s look at how to implement a Hashtable. You can create a Hashtable using Hashtable<KeyType, ValueType> hashtable = new Hashtable<>();. Can anyone give an example?

For instance, I could create one to store student IDs and names: Hashtable<Integer, String> students = new Hashtable<>();

Perfect! So how would you add a student’s name by ID?

students.put(1, 'Alice');! Right?

Exactly! And how would you retrieve that name?

students.get(1); would return 'Alice'.

Fantastic! Now we have an understanding of basic interactions with Hashtable. Let's summarize what we've learned.