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

I had a crazy idea once, and it kind of goes against some of your requirements, but I think you can have the builder constructor take the required parameters, but in a way that makes it still clear which parameters are being set. Take a look:

package myapp;

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 class Builder {
    private int a;
    private int b;
    private int c;

    public Builder(A a, B b, C c) {
      this.a = a.v;
      this.b = b.v;
      this.c = c.v;
    }

    public Builder a(int v) { a = v; return this; }
    public Builder b(int v) { b = v; return this; }
    public Builder c(int v) { c = v; return this; }

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

  private static class V {
    int v;
    V(int v) { this.v = v; }
  }
  public static class A extends V { A(int v) { super(v); } }
  public static class B extends V { B(int v) { super(v); } }
  public static class C extends V { C(int v) { super(v); } }
  public static A a(int v) { return new A(v); }
  public static B b(int v) { return new B(v); }
  public static C c(int v) { return new C(v); }

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

}

For other clients, static imports are your friends:

package myotherapp;

import myapp.Foo;
import static myapp.Foo.*;

public class Program {

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

}