T val = def;
if (a != null) {
  val = a;
} else if (b != null) {
  val = b;
} ...

// abbreviated to

T val = First.of(a).or(b)...orDef(def);

// both methods (of and or) take T, Supplier<T> and Runnables.
// The first value != null is returned as Optional. If no such value is returned,
// an empty Optional is returned.

Runnable r = () -> {
  if (condition) {
    operation_a;
  } else {
    operation_b;
  }
};

// abbreviated to

Runnable r = () -> Do.on(condition, () -> operation_a, () -> operation_b);