Wednesday, March 02, 2016

Hand-rolling builders in Java

I showed off a hand-rolled example Java builder pattern today. It has some benefits over existing builder solutions, but is more work than I like:

  1. All parts of the builder are immutable; you can pass a partially built object for another object to complete (I'm looking at you, Lombok)
  2. It's syntactically complete; that is, code won't compile without providing all arguments for the thing to build
  3. It's easy to generalize; in fact, I'm thinking about an annotation processor to generate it for you (but not there quite yet)
@EqualsAndHashCode
@ToString
public final class CartesianPoint {
    public final int x;
    public final int y;

    public static Builder builder() {
        return new Builder();
    }

    private CartesianPoint(final int x, final int y) {
        this.x = x;
        this.y = y;
    }

    public static final class Builder {
        public WithX x(final int x) {
            return new WithX(x);
        }

        @RequiredArgsConstructor
        public static final class WithX {
            private final int x;

            public WithY y(final int y) {
                return new WithY(y);
            }

            @RequiredArgsConstructor
            public final class WithY {
                private final int y;

                public CartesianPoint build() {
                    return new CartesianPoint(x, y);
                }
            }
        }
    }
}

That was a lot to say! Which is why most times you don't hand-roll builders. Usage is obvious:

@Test
public void shouldBuild() {
    final CartesianPoint point = CartesianPoint.builder().
            x(1).
            y(2).
            build();
    assertThat(point.x).isEqualTo(1)
    assertThat(point.y).isEqualTo(2);
}

Adding caching for equivalent values is not hard:

public static final class Builder {
    private static final ConcurrentMap
            cache = new ConcurrentHashMap<>();

    public WithX x(final int x) {
        return new WithX(x);
    }

    @RequiredArgsConstructor
    public static final class WithX {
        private final int x;

        public WithY y(final int y) {
            return new WithY(y);
        }

        @RequiredArgsConstructor
        public final class WithY {
            private final int y;

            public CartesianPoint build() {
                final CartesianPoint point = new CartesianPoint(x, y);
                final CartesianPoint cached = cache.
                        putIfAbsent(point, point);
                return null == cached ? point : cached;
            }
        }
    }
}
Post a Comment