# 20.4 Mutable vs. Immutable Types

## Immutable Data Types

{% embed url="<https://www.youtube.com/watch?index=6&list=PL8FaHk7qbOD637Q-6p7nn5dKz1tK6WAjJ&v=fHuU2zeBwIs>" %}

An immutable data type is one for which an instance cannot change in any observable way after instantiation.

Examples:

* Mutable: ArrayDeque, Percolation.
* Immutable: Integer, String, Date.

```java
public class Date {
   public final int month;
   public final int day;
   public final int year;
   private boolean contrived = true;
   public Date(int m, int d, int y) {
       month = m; day = d; year = y;
   }
}
```

The `final` keyword will help the compiler ensure immutability.

* `final` variable means you may assign a value once (either in the constructor of the class or in the initializer), but after it can never change.
* `final` is neither sufficient nor necessary for a class to be immutable.

### Advantages vs. Disadvantages of Immutability:

Advantage: Less to think about; avoids bugs and makes debugging easier.

* Analogy: Immutable classes have some buttons you can press / windows you can look inside. Results are ALWAYS the same, no matter what.

Disadvantage: Must create a new object anytime anything changes.

* Example: String concatenation is slow!

<figure><img src="/files/qw6FLS6JQxrXS3AwJmJy" alt=""><figcaption></figcaption></figure>

## Mutable Types

{% embed url="<https://www.youtube.com/watch?index=7&list=PL8FaHk7qbOD637Q-6p7nn5dKz1tK6WAjJ&v=7lqlRGO_mGY>" %}

### **Mutable HashSet Keys**

In principle, we can create a HashSet\<List>.

Weird stuff happens if:

* We insert a List into a HashSet.
* We later mutate that List.

**Key Point**: Never mutate (modify) an object being used as a key. Incorrect results arise, and the item gets lost. [The slides for the Hashing II Lecture ](https://docs.google.com/presentation/d/1U_-RQCJB3j9B-k-kY8I4nS-FuxIvO8EgVIrOthx2InU/edit#slide=id.g2165b69ef3f_0_291)provide a very thorough visual example of this point.

### Actual Implementation of HashSet/Map in Java

In this last section, Professor Hug gives a quick walkthrough of the code for HashSets and HashMaps in Java's official implementation.

{% embed url="<https://youtu.be/pc5SDicUqWo>" %}


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://cs61b-2.gitbook.io/cs61b-textbook-2025/20.-hashing-ii/20.4-mutable-vs.-immutable-types.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.