Design of immutable and mutable objects in Java

Question

My problem concerns an API design.

Let's say I'm designing a vector (math/physics meaning). I would like to have both an immutable implemenation and a mutable one.

I have then my vector that looks like this:

public interface Vector {
  public float getX(); public float getY();
  public X add(Vector v);
  public X subtract(Vector v);
  public X multiply(Vector v);
  public float length();
}

I wonder how I can ensure to have both a mutable and an immutable implementation. I don't really like java.util.List's approach (allowing mutability by default) and the UnsupportedOperationException() that Guava's immutable implementation has.

How can I design a "perfect" interface or abstract class Vector with both these implementations?

I've thought about something like this:

public interface Vector {
  ...
  public Vector add(Vector v);
  ...
}
public final class ImmutableVector implements Vector {
  ...
  public ImmutableVector add(Vector v) {
    return new ImmutableVector(this.x+v.getX(), this.y+v.getY());
  }
  ...
}
public class MutableVector implements Vector {
  ...
  public MutableVector add(Vector v) {
    this.x += v.getX();
    this.y += v.getY();
    return this;
  }
  ...
}

So all in all, I would like to check if this approach has flagrant design flaws, which are they and what should I do tho fix these?

---

Notes: the "vector" stuff is an example of a more general use case. For the sake of my question I could have chosen to rewrite the List interface or anything else. Please focus on the more general use case.

---

Final choice, after answers below, based on Joda-time as someone explained but now edited:

/** Basic class, allowing read-only access. */
public abstract class ReadableVector {
  public abstract float getX(); public abstract float getY();
  public final float length() {
    return Vectors.length(this);
  }
  // equals(Object), toString(), hashCode(), toImmutableVectors(), mutableCopy()
}
/** ImmutableVector, not modifiable implementation */
public final class ImmutableVector extends ReadableVector implements Serializable {
  // getters
  // guava-like builder methods (copyOf, of, etc.)
}
/** Mutable implementation */
public class Vector extends ReadableVector implements Serializable {
  // fields, getters and setters
  public void add (ReadableVector v) {/* delegate to Vectors */}
  public void subtract(ReadableVector v) {/* delegate to Vectors */}
  public void multiply(ReadableVector v) {/* delegate to Vectors */}
}
/** Tool class containing all the logic */
public final class Vectors {
  public static ImmutableVector add(ReadableVector v1, ReadableVector v2) {...}
  public static void addTo(Vector v1, ReadableVector v2) {...}
  ...
}

I changed Vector from an interface to a abstract class because basically a vector shouldn't be anything else.

Thank you to everyone.

Answer 1

I do not think there is anything evidently wrong with your design. I find it perfectly valid. There are few things that I would take into account if I were you:

• Reckless users may write code for the interface Vector thinking their implementations are always mutable.
• Immutability typically means more objects and a performance penalty due to the need to put more and more objects in the heap and forces the garbage collection to do more work. If your application will need to do many "add" operations you may need to pay the price. But hey, that's the whole purpose of having a mutable version, right?
• Also, if you are writing for a multithreading environment, you will still need to synchronize access to share variables of type Vector when you are not sure of implementation above all if you want to ensure that the implementation can be switched without consequences. This, again, proves that it can be hard to write code oblivious of implementation details.
• Although I argued a bit with @Paulo Eberman in other post, I do believe he is totally right. I think it is best to have two separate interfaces, one for immutable objects, and one for mutable (which could extend this latter).

Of course most of this points are arguable, these are just my opinions.

Answer 2

I think this design is not very good. Having mutable arithmetical objects is not good if even you have them explicitly marked as mutable. Additionally, I wouldn't put vector operations in the class vector. Because now you have only addition and multiplication and tomorrow you will want something else and your class will grow and grow as you will add this or what vector operation. If I were you, I would create an immutable vector like this

public class Vector {

    private Double X;
    private Double Y;

    public Vector(Double x, Double y) {
        X = x;
        Y = y;
    }

    public Double getX() {
        return X;
    }

    public Double getY() {
        return Y;
    }
}

and then I would create a class for doing basic vector operations:

public class BaseVectorAlgebra {

    public static Vector add(Vector arg1, Vector arg2) {
        return new Vector(arg1.getX() + arg2.getX(), arg1.getY() + arg2.getY());
    }

}

This way you will have an easy way to extend the system without touching existing classes and without introducing mutability, which just complicate things.

UPDATE:

If you still want to go with mutable vectors, then I would add SetX and SetY setters into Vector class, but put mutability decision into BaseVectorAlgebra like this:

public static Vector addInto(Vector arg1, Vector arg2) {
    arg1.setX(arg1.getX() + arg2.getX());
    arg1.setY(arg1.getY() + arg2.getY());

    return arg1;
}

But really I don't like mutability here as it introduces unnecessary complications

Answer 3

As a user of your Vector library, I would not like to have one add implementation which modifies the current Object and another add implementation (of the same interface) which returns a new one.

Better have a clear set of methods which do not modify the current object, and then have additional methods in the mutable vector which do modify the current object.

Answer 4

Your idea is fine, but it's hardly perfect.

You've left out generics.

You assume that arithmetic operations such as addition and subtraction are defined for the types your Vector is holding, which may not be true. (Generics might help with that.)

I don't know how useful an immutable vector is in the context of mathematics and physics.

A perfect API would have an analogous Matrix class, since you'll need to do linear algebra for math and physics.

I'd have a look at Apache's common math library for inspiration. It's the heir to JAMA. I find that looking at successful designs and implementations by my betters is a good way to learn.