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 or functions 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 2 dimensional point.

public class Point2D {
    private int x;
    private int y;
    public Point2D(int x, int y) { this.x = x; this.y = y; }

    public int getX() { return this.x; }
    public int getY() { return this.y; }

    public void setX(int newX) { this.x = newX; }
    public void setY(int newY) { this.y = newY; }
}

The class Point2D is mutable: its state can be changed after construction, by invoking either of the setter methods (

setX()

or

setY()

). An immutable interface for Point2D could be defined as:

public interface ImmutablePoint2D {
    public int getX();
    public int getY();
}

By making Point2D implement ImmutablePoint2D, client code could now reference a type which does not have mutating methods, and thus appears immutable.