In java, can you use the builder pattern with required and reassignable fields?

Question

This is related to the following question:

How to improve the builder pattern?

I\'m curious whether it\'s possible to implement a builder with the following

Answer 1

Your requirements are really hard, but see if this generic solution fits the bill:

public final class Foo {

    public final int a;
    public final int b;
    public final int c;

    private Foo(
            int a,
            int b,
            int c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    public static BuilderA> newBuilder() {
        return new BuilderFinal();
    }

    public static class BuilderA> {
        private volatile int a;

        @SuppressWarnings("unchecked")
        public T a(int v) {
            a = v;
            return (T) this;
        }

        public int a() {
            return a;
        }
    }

    public static class BuilderB> extends BuilderA {
        private volatile int b;

        @SuppressWarnings("unchecked")
        public T b(int v) {
            b = v;
            return (T) this;
        }

        public int b() {
            return b;
        }
    }

    public static class BuilderC extends BuilderB {
        private volatile int c;

        @SuppressWarnings("unchecked")
        public T c(int v) {
            c = v;
            return (T) this;
        }

        public int c() {
            return c;
        }
    }

    public static class BuilderFinal extends BuilderC {

        public Foo build() {
            return new Foo(
                    a(),
                    b(),
                    c());
        }
    }

    public static void main(String[] args) {
        Foo f1 = newBuilder().a(1).b(2).c(3).build();
        Foo f2 = newBuilder().a(1).b(2).c(3).a(4).build();
    }

}