Builder: a step builder forces fields in a fixed order

Each step returns a different interface that only exposes the next method to call, so the compiler itself rejects a build() before size() and topping() were both supplied. That removes the runtime validation an ordinary builder would otherwise need.

Code

interface NeedSize { NeedTopping size(String size); }
interface NeedTopping { NeedBuild topping(String topping); }
interface NeedBuild { Sandwich build(); }
class Sandwich {
    final String size, topping;
    private Sandwich(String size, String topping) { this.size = size; this.topping = topping; }
    public String toString() { return size + " sandwich with " + topping; }
    static NeedSize builder() {
        return size -> topping -> () -> new Sandwich(size, topping);
    }
}
Sandwich s = Sandwich.builder().size("footlong").topping("turkey").build();
System.out.println(s);

Output

footlong sandwich with turkey