What is mutable?
Mutable means that something can be changed after it has been created. In programming, a mutable object lets you modify its contents-add, remove, or update data-without having to make a brand‑new object.
Let's break it down
Think of a mutable object like a notebook you can write in, erase, and rewrite. An immutable object is like a printed page: once it’s printed, you can’t alter the words without printing a new page. In many languages, lists, dictionaries, and most custom objects are mutable, while strings, numbers, and tuples are often immutable.
Why does it matter?
Mutable objects let you work with data efficiently because you don’t need to copy everything each time you make a change. However, because they can be altered from anywhere in the code, they can also cause unexpected bugs, especially when multiple parts of a program think they own the same data.
Where is it used?
- Storing collections that need frequent updates (e.g., a list of user messages).
- Managing state in user interfaces (e.g., a UI component’s current settings).
- Caching results that may be refreshed later.
- Working with databases or files where records are edited in place.
Good things about it
- Performance: Updating in place avoids the overhead of creating new copies.
- Flexibility: You can easily add, remove, or modify items as your program runs.
- Memory efficient: Less memory is used because the same object is reused.
Not-so-good things
- Side effects: Changing an object in one place can unintentionally affect other parts of the code that reference the same object.
- Harder to debug: Tracking where a mutation happened can be tricky.
- Thread safety: In multi‑threaded programs, mutable objects need extra care (locks, synchronization) to avoid race conditions.