Building big, immutable objects without using constructors having long parameter lists

I have some big (more than 3 fields) objects that can and should be immutable. Every time I run into that case I tend to create constructor abominations with long parameter lists.

It doesn't feel right, it is hard to use, and readability suffers.

It is even worse if the fields are some sort of collection type like lists. A simple addSibling(S s) would ease the object creation so much but renders the object mutable.

What do you guys use in such cases?

I'm on Scala and Java, but I think the problem is language agnostic as long as the language is object oriented.

Solutions I can think of:

1. "Constructor abominations with long parameter lists"
2. The Builder Pattern

Well, you want both an easier to read and immutable object once created?

I think a fluent interface CORRECTLY DONE would help you.

It would look like this (purely made up example):

final Foo immutable = FooFactory.create()
    .whereRangeConstraintsAre(100,300)
    .withColor(Color.BLUE)
    .withArea(234)
    .withInterspacing(12)
    .build();

I wrote "CORRECTLY DONE" in bold because most Java programmers get fluent interfaces wrong and pollute their object with the method necessary to build the object, which is of course completely wrong.

The trick is that only the build() method actually creates a Foo (hence you Foo can be immutable).

FooFactory.create(), whereXXX(..) and withXXX(..) all create "something else".

That something else may be a FooFactory, here's one way to do it....

You FooFactory would look like this:

// Notice the private FooFactory constructor
private FooFactory() {
}

public static FooFactory create() {
    return new FooFactory();
}

public FooFactory withColor( final Color col ) {
    this.color = color;
    return this;
}

public Foo build() {
    return new FooImpl( color, and, all, the, other, parameters, go, here );
}

In Scala 2.8, you could use named and default parameters as well as the copy method on a case class. Here's some example code:

case class Person(name: String, age: Int, children: List[Person] = List()) {
  def addChild(p: Person) = copy(children = p :: this.children)
}

val parent = Person(name = "Bob", age = 55)
  .addChild(Person("Lisa", 23))
  .addChild(Person("Peter", 16))

Well, consider this on Scala 2.8:

case class Person(name: String, 
                  married: Boolean = false, 
                  espouse: Option[String] = None, 
                  children: Set[String] = Set.empty) {
  def marriedTo(whom: String) = this.copy(married = true, espouse = Some(whom))
  def addChild(whom: String) = this.copy(children = children + whom)
}

scala> Person("Joseph").marriedTo("Mary").addChild("Jesus")
res1: Person = Person(Joseph,true,Some(Mary),Set(Jesus))

This does have its share of problems, of course. For instance, try making espouse and Option[Person], and then getting two persons married to each other. I can't think of a way to solve that without resorting to either a private var and/or a private constructor plus a factory.

Here are a couple of more options:

Option 1

Make the implementation itself mutable, but separate the interfaces that it exposes to mutable and immutable. This is taken from the Swing library design.

public interface Foo {
  X getX();
  Y getY();
}

public interface MutableFoo extends Foo {
  void setX(X x);
  void setY(Y y);
}

public class FooImpl implements MutableFoo {...}

public SomeClassThatUsesFoo {
  public Foo makeFoo(...) {
    MutableFoo ret = new MutableFoo...
    ret.setX(...);
    ret.setY(...);
    return ret; // As Foo, not MutableFoo
  }
}

Option 2

If your application contains a large but pre-defined set of immutable objects (e.g., configuration objects), you might consider using the Spring framework.