Immutable interface


In object-oriented programming, "immutable interface" is a pattern for designing an immutable object. The immutable interface pattern involves defining a type which does not provide any methods which mutate state. Objects which are referenced by that type are not seen to have any mutable state, and appear immutable.

Example

Java

Consider a Java class which represents a two-dimensional point.

public class Point2D

The class Point2D is mutable: its state can be changed after construction, by invoking either of the setter methods or setY).
An immutable interface for Point2D could be defined as:

public interface ImmutablePoint2D

By making Point2D implement ImmutablePoint2D, client code could now reference a type which does not have mutating methods, and thus appears immutable. This is demonstrated in the following example:

ImmutablePoint2D point = new Point2D; // a concrete instance of Point2D is referenced by the immutable interface
int x = point.getX; // valid method call
int y = point.setX; // compile error: the method setX does not exist on type ImmutablePoint2D

By referencing only the immutable interface, it is not valid to call a method which mutates the state of the concrete object.

Advantages

public void mutate
An alternative to the immutable interface pattern is the immutable wrapper pattern.
Persistent data structures are effectively immutable while allowing modified views of themselves.