Static factory methods vs traditional constructors

IMPORTANT: This blog has been moved to jlordiales.me. The content here might be outdated. To see this post on the new blog go to http://jlordiales.me/2012/12/26/static-factory-methods-vs-traditional-constructors

I’ve previously talked a little bit about the Builder Pattern, a useful pattern to instantiate classes with several (possibly optional) attributes that results in easier to read, write and maintain client code, among other benefits. Today, I’m going to continue exploring object creation techniques but this time for a more general case.

Take the following example, which is by no means a useful class other than to make my point. We have a RandomIntGenerator class that, as the name suggests, generates random int numbers. Something like:

public class RandomIntGenerator {
    private final int min;
    private final int max;

    public int next() {...}
}

Our generator takes a minimum and maximum and then generates random numbers between those 2 values. Notice that the two attributes are declared final so we have to initialize them either on their declaration or in the class constructor. Let’s go with the constructor:

    public RandomIntGenerator(int min, int max) {
        this.min = min;
        this.max = max;
    }

Now, we also want to give our clients the possibility to specify just a minimum value and then generate random values between that minimum and the max possible value for ints. So we add a second constructor:

    public RandomIntGenerator(int min) {
        this.min = min;
        this.max = Integer.MAX_VALUE;
    }

So far so good, right? But in the same way that we provided a constructor to just specify the minimum value, we want to do the same for just the maximum. We’ll just add a third constructor like:

    public RandomIntGenerator(int max) {
        this.min = Integer.MIN_VALUE;
        this.max = max;
    }

If you try that, you’ll get a compilation error that goes: Duplicate method RandomIntGenerator(int) in type RandomIntGenerator. What’s wrong?

The problem is that constructors, by definition, have no names. As such, a class can only have one constructor with a given signature in the same way that you can’t have two methods with the same signature (same return type, name and parameters type). That is why when we tried to add the RandomIntGenerator(int max) constructor we got that compilation error, because we already had the RandomIntGenerator(int min) one.

Is there something we can do in cases like this one? Not with constructors but fortunately there’s something else we can use: static factory methods, which are simply public static methods that return an instance of the class. You’ve probably used this technique without even realizing it. Have you ever used Boolean.valueOf? It looks something like:

    public static Boolean valueOf(boolean b) {
        return (b ? TRUE : FALSE);
    }

Applying static factories to our RandomIntGenerator example, we could get:

public class RandomIntGenerator {
    private final int min;
    private final int max;

    private RandomIntGenerator(int min, int max) {
        this.min = min;
        this.max = max;
    }
    
    public static RandomIntGenerator between(int max, int min) {
        return new RandomIntGenerator(min, max);
    }
    
    public static RandomIntGenerator biggerThan(int min) {
        return new RandomIntGenerator(min, Integer.MAX_VALUE);
    }
    
    public static RandomIntGenerator smallerThan(int max) {
        return new RandomIntGenerator(Integer.MIN_VALUE, max);
    }

    public int next() {...}
}

Note how the constructor was made private to ensure that the class is only instantiated through its public static factory methods. Also note how your intent is clearly expressed when you have a client with RandomIntGenerator.between(10,20) instead of new RandomIntGenerator(10,20)

It’s worth mentioning that this technique is not the same as the Factory method Design Pattern from the Gang of Four. Any class can provide static factory methods instead of, or in addition to, constructors. So what are the advantages and disadvantages of this technique?

We already mentioned the first advantage of static factory methods: unlike constructors they have names. This has two direct consequences,

1. We can provide a meaningful name for our constructors.
2. We can provide several constructors with the same number and type of parameters, something that as we saw earlier we can’t do with class constructors.

Another advantage of static factories is that, unlike constructors, they are not required to return a new object every time they are invoked. This is extremely useful when working with immutable classes to provide constant objects for common used values and avoid creating unnecessary duplicate objects. The Boolean.valueOf code that I showed previously illustrates this point perfectly. Notice that this static method returns either TRUE or FALSE, both immutable Boolean objects.

A third advantage of static factory methods is that they can return an object of any subtype of their return type. This gives you the possibility to change the return type freely without affecting clients. Moreover, you can hide implementation classes and have an interface-based API, which is usually a really good idea. But I think this can be better seen by an example.

Remember the RandomIntGenerator at the beginning of this post? Let’s make that a little bit more complicated. Imagine that we now want to provide random generators not just for integers but for other data-types like String, Double or Long. They are all going to have a next() method that returns a random object of a particular type, so we could start with an interface like:

public interface RandomGenerator<T> {
    T next();
}

Our first implementation of the RandomIntGenerator now becomes:

class RandomIntGenerator implements RandomGenerator<Integer> {
    private final int min;
    private final int max;

    RandomIntGenerator(int min, int max) {
        this.min = min;
        this.max = max;
    }

    public Integer next() {...}
}

We could also have a String generator:

class RandomStringGenerator implements RandomGenerator<String> {
    private final String prefix;

    RandomStringGenerator(String prefix) {
        this.prefix = prefix;
    }

    public String next() {...}
}

Notice how all the classes are declared package-private (default scope) and so are their constructors. This means that no client outside of their package can create instances of these generators. So what do we do? Tip: It starts with “static” and ends with “methods”.
Consider the following class:

public final class RandomGenerators {
    // Suppresses default constructor, ensuring non-instantiability.
    private RandomGenerators() {}
    
    public static final RandomGenerator<Integer> getIntGenerator() {
        return new RandomIntGenerator(Integer.MIN_VALUE, Integer.MAX_VALUE);
    }
    
    public static final RandomGenerator<String> getStringGenerator() {
        return new RandomStringGenerator("");
    }
}

RandomGenerators is just a noninstantiable utility class with nothing else than static factory methods. Being on the same package as the different generators this class can effectively access and instantiate those classes. But here comes the interesting part. Note that the methods only return the RandomGenerator interface, and that’s all the clients need really. If they get a RandomGenerator<Integer> they know that they can call next() and get a random integer.
Imagine that next month we code a super efficient new integer generator. Provided that this new class implements RandomGenerator<Integer> we can change the return type of the static factory method and all clients are now magically using the new implementation without them even noticing the change.

Classes like RandomGenerators are quite common both on the JDK and on third party libraries. You can see examples in Collections (in java.util), Lists, Sets or Maps in Guava. The naming convention is usually the same: if you have an interface named Type you put your static factory methods in a noninstantiable class named Types.

A final advantage of static factories is that they make instantiating parameterized classes a lot less verbose. Have you ever had to write code like this?

Map<String, List<String>> map = new HashMap<String, List<String>>();

You are repeating the same parameters twice on the same line of code. Wouldn’t it be nice if the right side of the assign could be inferred from the left side? Well, with static factories it can. The following code is taken from Guava’s Maps class:

  public static <K, V> HashMap<K, V> newHashMap() {
    return new HashMap<K, V>();
  }

So now our client code becomes:

Map<String, List<String>> map = Maps.newHashMap();

Pretty nice, isn’t it? This capability is known as Type inference. It’s worth mentioning that Java 7 introduced type inference through the use of the diamond operator. So if you’re using Java 7 you can write the previous example as:

Map<String, List<String>> map = new HashMap<>();

The main disadvantage of static factories is that classes without public or protected constructors cannot be extended. But this might be actually a good thing in some cases because it encourages developers to favor composition over inheritance.

To summarize, static factory methods provide a lot of benefits and just one drawback that might actually not be a problem when you think about it. Therefore, resist the urge to automatically provide public constructors and evaluate if static factories are a better fit for your class.

The ins and outs of immutability

IMPORTANT: This blog has been moved to jlordiales.me. The content here might be outdated. To see this post on the new blog go to http://jlordiales.me/2012/12/24/the-ins-and-outs-of-immutability

So in my first post I talked a little bit about the builder pattern and I mentioned a really powerful but yet overlooked concept: immutability.

What is an immutable class? It’s simply a class whose instances can’t be modified. Every value for the class’ attributes is set on their declaration or in its constructor and they keep those values for the rest of the object’s life-cycle. Java has quite a few immutable classes, such as String, all the boxed primitives (Double, Integer, Float, etc), BigInteger and BigDecimal among others. There is a good reason for this: immutable classes are easier to design, implement and use than mutable classes. Once they are instantiated they can only be in one state so they are less error prone and, as we’ll see later in this post, they are more secure.

How do you ensure that a class is immutable? Just follow these 5 simple steps:

1. Don’t provide any public methods that modify the object’s state, also known as mutators (such as setters).
2. Prevent the class from being extended. This doesn’t allow any malicious or careless class to extend our class and compromise its immutable behavior. The usual and easier way to do this is to mark the class as final, but there’s another way that I’ll mention in this post.
3. Make all fields final. This is a way to let the compiler enforce point number 1 for you. Additionally, it clearly lets anyone who sees your code know that you don’t want those fields to change their values once they are set.
4. Make all fields private. This one should be pretty obvious and you should follow it regardless of whether you’re taking immutability into consideration or not, but I’m mentioning this just in case.
5. Never provide access to any mutable attribute. If your class has a mutable object as one of its properties (such as a List, a Map or any other mutable object from your domain problem) make sure that clients of your class can never get a reference to that object. This means that you should never directly return a reference to them from an accessor (e.g., a getter) and you should never initialize them on your constructor with a reference passed as parameter from a client. You should always make defensive copies in this case.

That’s a lot of theory and no code, so lets see what a simple immutable class looks like and how it deals with the 5 steps I mentioned before:

public class Book {
    private final String isbn;
    private final int publicationYear;
    private final List reviews;
    private Book(BookBuilder builder) {
        this.isbn = builder.isbn;
        this.publicationYear = builder.publicationYear;
        this.reviews = Lists.newArrayList(builder.reviews);
    }
    public String getIsbn() {
        return isbn;
    }
    public int getPublicationYear() {
        return publicationYear;
    }
    public List getReviews() {
        return Lists.newArrayList(reviews);
    }
    public static class BookBuilder {
        private String isbn;
        private int publicationYear;
        private List reviews;
        public BookBuilder isbn(String isbn) {
            this.isbn = isbn;
            return this;
        }
        public BookBuilder publicationYear(int year) {
            this.publicationYear = year;
            return this;
        }
        public BookBuilder reviews(List reviews) {
            this.reviews = reviews == null ? new ArrayList() : reviews;
            return this;
        }
        public Book build() {
            return new Book(this);
        }
    }
}

We’ll go through the important points in this pretty simple class. First of all, as you’ve probably noticed, I’m using the builder pattern again. This is not just because I’m a big fan of it but also because I wanted to illustrate a few points that I didn’t want to get into my previous post without first giving you a basic understanding of the concept of immutability. Now, let’s go through the 5 steps that I mentioned you need to follow to make a class immutable and see if they hold valid for this Book example:

• Don’t provide any public methods that modify the object’s state. Notice that the only methods on the class are its private constructor and getters for its properties but no method to change the object’s state.
• Prevent the class from being extended. This one is quite tricky. I mentioned that the easiest way to ensure this was to make the class final but the Book class is clearly not final. However, notice that the only constructor available is private.  The compiler makes sure that a class without public or protected constructors cannot be  subclassed. So in this case the final keyword on the class declaration is not necessary but it might be a good idea to include it anyway just to make your intention clear to anyone who sees your code.
• Make all fields final. Pretty straightforward, all attributes on the class are declared as final.
• Never provide access to any mutable attribute. This one is actually quite interesting. Notice how the Book class has a List<String> attribute that is declared as final and whose value is set on the class constructor. However, this List is a mutable object. That is, while the reviews reference cannot change once it is set, the content of the list can. A client with a reference to the same list could add or delete an element and, as a result, change the state of the Book object after its creation. For this reason, note that on the Book constructor we don’t assign the reference directly. Instead, we use the Guava library to make a copy of the list by calling “this.reviews = Lists.newArrayList(builder.reviews);“. The same situation can be seen on the getReviews method, where we return a copy of the list instead of the direct reference. It is worth noting that this example might be a bit oversimplified, because the reviews list can only contain strings, which are immutable. If the type of the list is a mutable class then you would also have to make a copy of each object in the list and not just the list itself.

That last point illustrates why immutable classes result in cleaner designs and easier to read code. You can just share around those immutable objects without having to worry about defensive copies. In fact, you should never make any copies at all because any copy of the object would be forever equal to the original. A corollary is that immutable objects are just plain simple. They can be in only one state and they keep that state for their entire life. You can use the class constructor to check any invariants (i,e,. conditions that need to be valid on the class like range of values for one of its attributes) and then you can ensure that those invariants will remain true without any effort on your part or your clients.

Another huge benefit of immutable objects is that they are inherently thread-safe. They cannot be corrupted by multiple threads accessing the objects concurrently. This is, by far, the easiest and less error prone approach to provide thread safety in your application.

But what if you already have a Book instance and you want to change the value of one of its attributes? In other words, you want to change the state of the object. On an immutable class this is, by definition, not possible. But, as with most things in software, there’s always a workaround. In this particular case there’s actually two.

The first option is to use the Fluent Interface technique on the Book class and have setter-like methods that actually create an object with the same values for all its attributes except for the one you want to change. In our example we would have to add the following to the Book class:

    private Book(BookBuilder builder) {
        this(builder.isbn, builder.publicationYear, builder.reviews);
    }
    private Book(String isbn, int publicationYear, List reviews) {
        this.isbn = isbn;
        this.publicationYear = publicationYear;
        this.reviews = Lists.newArrayList(reviews);
    }
    public Book withIsbn(String isbn) {
        return new Book(isbn,this.publicationYear, this.reviews);
    }

Note that we added a new private constructor where we can specify the value of each attribute and modified the old constructor to use the new one. Additionally, we added a new method that returns a new Book object with the value we wanted for the isbn attribute. The same concept applies to the rest of the class’ attributes. This is known as a functional approach because methods return the result of operating on their parameters without modifying them. This is to contrast it from the procedural or imperative approach where methods apply a procedure to their operands, thus changing their state.

This approach to generate new objects shows the only real disadvantage of immutable classes: they require us to create a new object for each distinct value we need and this can produce a considerable overhead in performance and memory consumption. This problem is magnified if you want to change several attributes of the object because you are generating a new object in each step and you end up discarding all intermediate objects and keeping just the last result.

We can provide a better alternative for the case of multi-step operations like the one I described on the last paragraph with the help of the builder pattern. Basically, we add a new constructor to the builder that takes an already created instance to set all its initial values. Then, the client can use the builder in the usual way to set all the desired values and then use the build method to create the final object. That way, we avoid creating intermediate objects with only some of the values we need. In our example this technique would look something like this on the builder side:

public BookBuilder(Book book) {
    this.isbn = book.getIsbn();
    this.publicationYear = book.getPublicationYear();
    this.reviews = book.getReviews();
}

Then, on our clients, we can have:

Book originalBook = getRandomBook();

Book modifiedBook = new BookBuilder(originalBook).isbn("123456").publicationYear(2011).build();

Now, obviously the builder is not thread-safe so you have to take all the usual precautions, such as not sharing a builder with multiple threads.

I mentioned that the fact that we have to create a new object for every change in state can have an overhead in performance and this is the only real disadvantage of immutable classes. However, object creation is one of the aspects of the JVM that is under continuous improvement. In fact, except for exceptional cases, object creation is a lot more efficient than you probably think. In any case, it’s usually a good idea to come up with a simple and clear design and then, only after measuring, refactor for performance. Nine out of ten times when you try to guess where your code is taking so much time you’ll discover that you were wrong. Additionally, the fact that immutable objects can be shared freely without having to worry about the consequences gives you the chance to encourage clients to reuse existing instances wherever possible, thus reducing considerably the number of objects created. A common way to do this is to provide public static final constants for the most common values. This technique is heavily used on the JDK, for example in Boolean.FALSE or BigDecimal.ZERO.

To conclude this post, if you want to take something out of it let it be this: classes should be immutable unless there’s a very good reason to make them mutable. Don’t automatically add a setter for every class attribute. If for some reason you absolutely can’t make your class immutable, then limit its mutability as much as possible. The fewer states in which an object can be, the easier it is to think about the object and its invariants. And don’t worry about the performance overhead of immutability, chances are that you won’t have to worry about it.

The builder pattern in practice

IMPORTANT: This blog has been moved to jlordiales.me. The content here might be outdated. To see this post on the new blog go to http://jlordiales.me/2012/12/13/the-builder-pattern-in-practice

So, this is my first post (and my first blog for that matter). I can’t remember exactly where I read this (although I’m almost sure it was on Practices of an Agile Developer), but writing in a blog is supposed to help you get your thoughts together. Concretely, by taking the time to explain what you know, you get a better understanding of it yourself.

And that’s exactly what I’m going to try to do here, explain things to get a better understanding of them. And, as a bonus, it will also serve me as centralized place to go to when I want to revisit something I’ve done in the past. Hopefully, this will help some of you in the process.

With the introduction out of the way, lets jump straight into this first post which, as the title so eloquently says :), is about the builder pattern. I’m not going to dive into much details about the pattern because there’s already tons of posts and books that explain it in fine detail. Instead, I’m going to tell you why and when you should consider using it. However, it is worth mentioning that this pattern is a bit different to the one presented in the Gang of Four book. While the original pattern focuses on abstracting the steps of construction so that by varying the builder implementation used we can get a different result, the pattern explained in this post deals with removing the unnecessary complexity that stems from multiple constructors, multiple optional parameters and overuse of setters.

Imagine you have a class with a substantial amount of attributes like the User class below. Let’s assume that you want to make the class immutable (which, by the way, unless there’s a really good reason not to you should always strive to do. But we’ll get to that in a different post).

public class User {
    private final String firstName;    //required
    private final String lastName;    //required
    private final int age;    //optional
    private final String phone;    //optional
    private final String address;    //optional
...
}

Now, imagine that some of the attributes in your class are required while others are optional. How would you go about building an object of this class? All attributes are declared final so you have to set them all in the constructor, but you also want to give the clients of this class the chance of ignoring the optional attributes.

A first and valid option would be to have a constructor that only takes the required attributes as parameters, one that takes all the required attributes plus the first optional one, another one that takes two optional attributes and so on. What does that look like? Something like this:

    public User(String firstName, String lastName) {
        this(firstName, lastName, 0);
    }

    public User(String firstName, String lastName, int age) {
        this(firstName, lastName, age, "");
    }

    public User(String firstName, String lastName, int age, String phone) {
        this(firstName, lastName, age, phone, "");
    }

    public User(String firstName, String lastName, int age, String phone, String address) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
        this.phone = phone;
        this.address = address;
    }

The good thing about this way of building objects of the class is that it works. However, the problem with this approach should be pretty obvious. When you only have a couple of attributes is not such a big deal, but as that number increases the code becomes harder to read and maintain. More importantly, the code becomes increasingly harder for clients. Which constructor should I invoke as a client? The one with 2 parameters? The one with 3? What is the default value for those parameters where I don’t pass an explicit value? What if I want to set a value for address but not for age and phone? In that case I would have to call the constructor that takes all the parameters and pass default values for those that I don’t care about. Additionally, several parameters with the same type can be confusing. Was the first String the phone number or the address?

So what other choice do we have for these cases? We can always follow the JavaBeans convention, where we have a default no-arg constructor and have setters and getters for every attribute. Something like:

public class User {
	private String firstName; // required
	private String lastName; // required
	private int age; // optional
	private String phone; // optional
	private String address;  //optional

	public String getFirstName() {
		return firstName;
	}
	public void setFirstName(String firstName) {
		this.firstName = firstName;
	}
	public String getLastName() {
		return lastName;
	}
	public void setLastName(String lastName) {
		this.lastName = lastName;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	public String getPhone() {
		return phone;
	}
	public void setPhone(String phone) {
		this.phone = phone;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address = address;
	}
}

This approach seems easier to read and maintain. As a client I can just create an empty object and then set only the attributes that I’m interested in. So what’s wrong with it? There are two main problems with this solution. The first issue has to do with having an instance of this class in an inconsistent state. If you want to create an User object with values for all its 5 attributes then the object will not have a complete state until all the setX methods have been invoked. This means that some part of the client application might see this object and assume that is already constructed while that’s actually not the case.
The second disadvantage of this approach is that now the User class is mutable. You’re loosing all the benefits of immutable objects.

Fortunately there is a third choice for these cases, the builder pattern. The solution will look something like the following.

public class User {
	private final String firstName; // required
	private final String lastName; // required
	private final int age; // optional
	private final String phone; // optional
	private final String address; // optional

	private User(UserBuilder builder) {
		this.firstName = builder.firstName;
		this.lastName = builder.lastName;
		this.age = builder.age;
		this.phone = builder.phone;
		this.address = builder.address;
	}

	public String getFirstName() {
		return firstName;
	}

	public String getLastName() {
		return lastName;
	}

	public int getAge() {
		return age;
	}

	public String getPhone() {
		return phone;
	}

	public String getAddress() {
		return address;
	}

	public static class UserBuilder {
		private final String firstName;
		private final String lastName;
		private int age;
		private String phone;
		private String address;

		public UserBuilder(String firstName, String lastName) {
			this.firstName = firstName;
			this.lastName = lastName;
		}

		public UserBuilder age(int age) {
			this.age = age;
			return this;
		}

		public UserBuilder phone(String phone) {
			this.phone = phone;
			return this;
		}

		public UserBuilder address(String address) {
			this.address = address;
			return this;
		}

		public User build() {
			return new User(this);
		}

	}
}

A couple of important points worth noting:

• The User constructor is private, which means that this class can not be directly instantiated from the client code.
• The class is once again immutable. All attributes are final and they’re set on the constructor. Additionally, we only provide getters for them.
• The builder uses the Fluent Interface idiom to make the client code more readable (we’ll see an example of this in a moment).
• The builder constructor only receives the required attributes and this attributes are the only ones that are defined “final” on the builder to ensure that their values are set on the constructor.

The use of the builder pattern has all the advantages of the first two approaches I mentioned at the beginning and none of their shortcomings. The client code is easier to write and, more importantly, to read. The only critique that I’ve heard about the pattern is the fact that you have to duplicate the class’ attributes on the builder. However, given the fact that the builder class is usually a static member class of the class it builds, they can evolve together fairly easy.

Now, how does the client code trying to create a new User object looks like? Let’s see:

	public User getUser() {
		return new
				User.UserBuilder("Jhon", "Doe")
				.age(30)
				.phone("1234567")
				.address("Fake address 1234")
				.build();
	}

Pretty neat, isn’t it? You can build a User object in 1 line of code and, most importantly, is very easy to read. Moreover, you’re making sure that whenever you get an object of this class is not going to be on an incomplete state.

This pattern is really flexible. A single builder can be used to create multiple objects by varying the builder attributes between calls to the “build” method. The builder could even auto-complete some generated field between each invocation, such as an id or serial number.

An important point is that, like a constructor, a builder can impose invariants on its parameters. The build method can check these invariants and throw an IllegalStateException if they are not valid.
It is critical that they be checked after copying the parameters from the builder to the object, and that they be checked on the object fields rather than the builder fields. The reason for this is that, since the builder is not thread-safe, if we check the parameters before actually creating the object their values can be changed by another thread between the time the parameters are checked and the time they are copied. This period of time is known as the “window of vulnerability”. In our User example this could look like the following:

public User build() {
    User user = new user(this);
    if (user.getAge() 120) {
        throw new IllegalStateException(“Age out of range”); // thread-safe
    }
    return user;
}

The previous version is thread-safe because we first create the user and then we check the invariants on the immutable object. The following code looks functionally identical but it’s not thread-safe and you should avoid doing things like this:

public User build() {
    if (age 120) {
        throw new IllegalStateException(“Age out of range”); // bad, not thread-safe
    }
    // This is the window of opportunity for a second thread to modify the value of age
    return new User(this);
}

A final advantage of this pattern is that a builder could be passed to a method to enable this method to create one or more objects for the client, without the method needing to know any kind of details about how the objects are created. In order to do this you would usually have a simple interface like:

public interface Builder {
    T build();
}

In the previous User example, the UserBuilder class could implement Builder<User>. Then, we could have something like:

UserCollection buildUserCollection(Builder<? extends User> userBuilder){...}

Well, that was a pretty long first post. To sum it up, the Builder pattern is an excellent choice for classes with more than a few parameters (is not an exact science but I usually take 4 attributes to be a good indicator for using the pattern), especially if most of those parameters are optional. You get client code that is easier to read, write and maintain. Additionally, your classes can remain immutable which makes your code safer.

UPDATE: if you use Eclipse as your IDE, it turns out that you have quite a few plugins to avoid most of the boiler plate code that comes with the pattern. The three I’ve seen are:

• http://code.google.com/p/bpep/
  
• http://code.google.com/a/eclipselabs.org/p/bob-the-builder/
• http://code.google.com/p/fluent-builders-generator-eclipse-plugin/

I haven’t tried any of them personally so I can’t really give an informed decision on which one is better. I reckon that similar plugins should exist for other IDEs.