What does it mean for a class to be immutable?

In programming, when we say a class is immutable, it means that once an object of that class is created, its state cannot be changed. In other words, after instantiation, the object's fields or properties remain constant and cannot be altered.

Here are the steps to make a class immutable:

1. Declare the class as final: A final class cannot be extended (inherited). This ensures the methods of the class cannot be overridden.

2. Make all fields private: This prevents direct access and modification of the fields.

3. Don't provide "setter" methods: Setter methods are used to modify field values. By not providing these methods, you prevent modification of the fields.

4. Make all mutable fields final: This ensures that the fields can be assigned only once.

5. Initialize all fields via a constructor: This allows you to set the values of the fields at the time of object creation.

6. Ensure exclusive access to any mutable components: If your class has any fields which refer to mutable objects, ensure that these objects cannot be changed. This can be done by not providing methods that modify the mutable objects. Also, you should not share references to the mutable objects. Instead, you could create new copies of the objects when necessary.

By following these steps, you can create a class that is immutable. This can be useful in multi-threaded environments, as immutable objects are thread-safe.