So last year I posted about using Joshua Bloch’s Builder pattern to create objects from a hierarchy of subclasses.

I realized later that while I did have something like this completely working at one point, I had it working in C#, which has deceptively different generic semantics. (It also has named parameters, which makes the Builder pattern a fair bit less necessary.) As Zdenek Henek demonstrated (several weeks ago, sorry Zdenek!), in Java, the original version I posted doesn’t allow you to call the arguments in any order.

This one does:

class Shape
{
    private final double opacity;

    public double getOpacity ()
    {
        return opacity;
    }

    public static abstract class ShapeBuilder<S extends Shape, B extends ShapeBuilder<S, B>>
    {

        private double opacity;

        @SuppressWarnings( "unchecked" )
        public B opacity ( double opacity )
        {
            this.opacity = opacity;
            return (B) this;
        }

        public abstract S build ();
    }

    private static class DefaultShapeBuilder extends ShapeBuilder<Shape, DefaultShapeBuilder>
    {
        @Override
        public Shape build ()
        {
            return new Shape( this );
        }
    }

    public static ShapeBuilder<?, ?> builder ()
    {
        return new DefaultShapeBuilder();
    }

    protected Shape ( ShapeBuilder<?, ?> builder )
    {
        this.opacity = builder.opacity;
    }
}

class Rectangle extends Shape
{

    private final double height;
    private final double width;

    public double getHeight ()
    {
        return height;
    }

    public double getWidth ()
    {
        return width;
    }

    public static abstract class RectangleBuilder<S extends Rectangle, B extends RectangleBuilder<S, B>> extends ShapeBuilder<S, B>
    {
        private double height;
        private double width;

        @SuppressWarnings( "unchecked" )
        public B height ( double height )
        {
            this.height = height;
            return (B) this;
        }

        @SuppressWarnings( "unchecked" )
        public B width ( double width )
        {
            this.width = width;
            return (B) this;
        }
    }

    public static RectangleBuilder<?, ?> builder ()
    {
        return new DefaultRectangleBuilder();
    }

    protected Rectangle ( RectangleBuilder<?, ?> builder )
    {
        super( builder );
        this.height = builder.height;
        this.width = builder.width;
    }

    private static class DefaultRectangleBuilder extends RectangleBuilder<Rectangle, DefaultRectangleBuilder>
    {
        @Override
        public Rectangle build ()
        {
            return new Rectangle( this );
        }
    }
}

class RotatedRectangle extends Rectangle
{
    private final double theta;

    public double getTheta ()
    {
        return theta;
    }

    public static abstract class RotatedRectangleBuilder<S extends RotatedRectangle, B extends RotatedRectangleBuilder<S, B>> extends Rectangle.RectangleBuilder<S, B>
    {
        private double theta;

        @SuppressWarnings( "Unchecked" )
        public B theta ( double theta )
        {
            this.theta = theta;
            return (B) this;
        }
    }

    public static RotatedRectangleBuilder<?, ?> builder ()
    {
        return new DefaultRotatedRectangleBuilder();
    }

    protected RotatedRectangle ( RotatedRectangleBuilder<?, ?> builder )
    {
        super( builder );
        this.theta = builder.theta;
    }

    private static class DefaultRotatedRectangleBuilder extends RotatedRectangleBuilder<RotatedRectangle, DefaultRotatedRectangleBuilder>
    {
        @Override
        public RotatedRectangle build ()
        {
            return new RotatedRectangle( this );
        }
    }
}

class BuilderTest
{
    public static void main ( String[] args )
    {
        RotatedRectangle rotatedRectangle = RotatedRectangle.builder()
                .theta( Math.PI / 2 )
                .width( 640 )
                .height( 400 )
                .height( 400 )
                .opacity( 0.5d )
                .width( 111 )
                .opacity( 0.5d )
                .width( 222 )
                .height( 400 )
                .width( 640 )
                .width( 640 )
                .build();
        System.out.println( rotatedRectangle.getTheta() );
        System.out.println( rotatedRectangle.getWidth() );
        System.out.println( rotatedRectangle.getHeight() );
        System.out.println( rotatedRectangle.getOpacity() );
    }
}

Note though it requires some unchecked casts, and depends on the convention that each Builder’s generics are self-referential, which could present some risk if it’s extended further. Test early, test often.