How can the builder patten use singleton in it's implement?

Question

I ran across several references to the builder patter being implemented as a singleton:

From Wikipedia: Builder Pattern: "Abstract Factory, Builder, and Prototype can use Singleton in their implementations"

How can the builder patten implement singleton? How do you maintain the state created by buildParts() when multiple threads are making calls to a builder that is a singleton?

Answer 1

I don't think the Builders should be singleton.

If you really really wanted to I guess you could do something like this:

public final class Builder{
    private static final Builder instance = new Builder();

    private Part part1= null;

    private Part part2= null;
    ...

    public static Builder getInstance() { 
         return instance;
    }

    private Builder() { /*singleton  */ }

    public Builder buildPart1(Part p){
        part1 = p;
        return this;
    }
    ...build other parts similar;

    Product getResult(){
        return new ConcreteProduct(part1, part2 ...);
    }
}

But by re-using the same singleton instance, you would have to make sure you set all the values correctly and not accidentally re-use old values from previously built products.

Even with proper synchronization, I think the potential for bugs and surprises from other threads/ previous built production values leaking into your Product would not make it worth it.