Motivation

Recently I searched for a way to initialize a complex object without passing a lot of parameter to the constructor. I tried it with the builder pattern, but I don't like the fact, that I'm not able to check at compile time if I really set all needed values.

Traditional builder pattern

When I use the builder pattern to create my Complex object, the creation is more "typesafe", because it's easier to see what an argument is used for:

new ComplexBuilder()
        .setFirst( "first" )
        .setSecond( "second" )
        .setThird( "third" )
        ...
        .build();

But now I have the problem, that I can easily miss an important parameter. I can check for it inside the build() method, but that is only at runtime. At compile time there is nothing that warns me, if I missed something.

Enhanced builder pattern

Now my idea was to create a builder, that "reminds" me if I missed a needed parameter. My first try looks like this:

public class Complex {
    private String m_first;
    private String m_second;
    private String m_third;

    private Complex() {}

    public static class ComplexBuilder {
        private Complex m_complex;

        public ComplexBuilder() {
            m_complex = new Complex();
        }

        public Builder2 setFirst( String first ) {
            m_complex.m_first = first;
            return new Builder2();
        }

        public class Builder2 {
            private Builder2() {}
            Builder3 setSecond( String second ) {
                m_complex.m_second = second;
                return new Builder3();
            }
        }

        public class Builder3 {
            private Builder3() {}
            Builder4 setThird( String third ) {
                m_complex.m_third = third;
                return new Builder4();
            }
        }

        public class Builder4 {
            private Builder4() {}
            Complex build() {
                return m_complex;
            }
        }
    }
}

As you can see, each setter of the builder class returns a different internal builder class. Each internal builder class provides exactly one setter method and the last one provides only a build() method.

Now the construction of an object again looks like this:

new ComplexBuilder()
    .setFirst( "first" )
    .setSecond( "second" )
    .setThird( "third" )
    .build();

...but there is no way to forget a needed parameter. The compiler wouldn't accept it.

Optional parameters

If I had optional parameters, I would use the last internal builder class Builder4 to set them like a "traditional" builder does, returning itself.

Questions

• Is this a well known pattern? Does it have a special name?
• Do you see any pitfalls?
• Do you have any ideas to improve the implementation - in the sense of fewer lines of code?

Solution 1:

The traditional builder pattern already handles this: simply take the mandatory parameters in the constructor. Of course, nothing prevents a caller from passing null, but neither does your method.

The big problem I see with your method is that you either have a combinatorical explosion of classes with the number of mandatory parameters, or force the user to set the parameters in one particular sqeuence, which is annoying.

Also, it is a lot of additional work.

Solution 2:

No, it's not new. What you're actually doing there is creating a sort of a DSL by extending the standard builder pattern to support branches which is among other things an excellent way to make sure the builder doesn't produce a set of conflicting settings to the actual object.

Personally I think this is a great extension to builder pattern and you can do all sorts of interesting things with it, for example at work we have DSL builders for some of our data integrity tests which allow us to do things like assertMachine().usesElectricity().and().makesGrindingNoises().whenTurnedOn();. OK, maybe not the best possible example but I think you get the point.

Solution 3:

public class Complex {
    private final String first;
    private final String second;
    private final String third;

    public static class False {}
    public static class True {}

    public static class Builder<Has1,Has2,Has3> {
        private String first;
        private String second;
        private String third;

        private Builder() {}

        public static Builder<False,False,False> create() {
            return new Builder<>();
        }

        public Builder<True,Has2,Has3> setFirst(String first) {
            this.first = first;
            return (Builder<True,Has2,Has3>)this;
        }

        public Builder<Has1,True,Has3> setSecond(String second) {
            this.second = second;
            return (Builder<Has1,True,Has3>)this;
        }

        public Builder<Has1,Has2,True> setThird(String third) {
            this.third = third;
            return (Builder<Has1,Has2,True>)this;
        }
    }

    public Complex(Builder<True,True,True> builder) {
        first = builder.first;
        second = builder.second;
        third = builder.third;
    }

    public static void test() {
        // Compile Error!
        Complex c1 = new Complex(Complex.Builder.create().setFirst("1").setSecond("2"));

        // Compile Error!
        Complex c2 = new Complex(Complex.Builder.create().setFirst("1").setThird("3"));

        // Works!, all params supplied.
        Complex c3 = new Complex(Complex.Builder.create().setFirst("1").setSecond("2").setThird("3"));
    }
}