Before discussing the difference between mutable and immutable objects, it's important that we properly define what an Object is in Object-Oriented Programming (OOP). This will serve as one of the foundations/basic concepts to fully understand the topic.
So to start, an Object is a piece of data that contains its own:
- identity (unique name),
- states (characteristics or attributes) and,
- behaviors (methods).
For example, assume that we are going to adopt a dog (or a cat!). After adoption, we would give that dog a name to call it (to uniquely identify it-- especially when we take it outside for a walk). Our dog has different attributes, including but not limited to, breed, color, and age. Finally, the dog may perform several functions like eating, sleeping, and barking.
Let’s have a look at another example of an object:
The above figure illustrates a house having a unique house number plate. It also has certain features like roofColor and doorColor, along with certain functions like the doorOpens and doorShuts.
To extend our understanding, it's also crucial that we understand that an object is a specific "realization" of a class. You can describe a class as a logical entity which is responsible for creating individual objects. You may create multiple objects from a single class-- but it's imperative that a class always comes first whenever an object has to be created. It is a blueprint of sorts. Therefore, an object can also be defined as an instance of a class.
Keeping the above discussion in mind, let’s dive into the details of mutable and immutable objects.