What does immutability mean in object design, and what are its benefits?

What does immutability mean in object design, and what are its benefits?

• A. Mutable objects cannot be changed after creation.
• B. Immutable objects cannot change after creation; benefits include simplicity, thread-safety, and easier reasoning.
• C. Immutability means objects cannot have methods.
• D. Immutability prevents any state in the program.

Answer: (B)

Immutability means an object's state is fixed at creation and cannot be changed afterward. If you need a different state, you create a new object with the new values. This doesn't stop an object from having methods or from existing in a program with other mutable state; it just means that particular object won't mutate its own fields. The benefits are clear: it makes reasoning about code easier because once you have a reference to an immutable object, you know its state won't change unexpectedly. It also helps with thread-safety: multiple threads can safely access the same instance without worrying about concurrent mutations, often reducing or eliminating the need for synchronization. In practice, this leads to simpler, more predictable code and can enable optimizations like caching or memoization since the data stays the same. For example, a Point class with coordinates x and y can be immutable by storing these values once and not providing any setters. To represent a moved point, you create a new Point with the new coordinates instead of altering the original one. The other statements miss the essence: immutability isn’t about forbidding methods, nor about preventing all state in the program, and mutable objects can still exist alongside immutable ones.

Immutability means an object's state is fixed at creation and cannot be changed afterward. If you need a different state, you create a new object with the new values. This doesn't stop an object from having methods or from existing in a program with other mutable state; it just means that particular object won't mutate its own fields.

The benefits are clear: it makes reasoning about code easier because once you have a reference to an immutable object, you know its state won't change unexpectedly. It also helps with thread-safety: multiple threads can safely access the same instance without worrying about concurrent mutations, often reducing or eliminating the need for synchronization. In practice, this leads to simpler, more predictable code and can enable optimizations like caching or memoization since the data stays the same.

For example, a Point class with coordinates x and y can be immutable by storing these values once and not providing any setters. To represent a moved point, you create a new Point with the new coordinates instead of altering the original one.

The other statements miss the essence: immutability isn’t about forbidding methods, nor about preventing all state in the program, and mutable objects can still exist alongside immutable ones.