import java.lang.invoke.VarHandle;
import java.lang.invoke.MethodHandles;

// assumes that each set function is called only once
sclass PairCollector<A, B> {
  volatile A a;
  volatile B b;
  volatile int counter;
  
  // statically get var handles
  private static final VarHandle vhA, vhB, vhCounter;
  static ctex {
    var l = MethodHandles.lookup();
    vhA = l.findVarHandle(PairCollector.class, "a", O);
    vhB = l.findVarHandle(PairCollector.class, "b", O);
    vhCounter = l.findVarHandle(PairCollector.class, "counter", int.class);
  }
  
  void setA(A a) { this.a = a; incCounter(); }
  void setB(B b) { this.b = b; incCounter(); }
  
  IVF1<A> setterForA() { ret a -> setA(a); }
  IVF1<B> setterForB() { ret b -> setB(b); }
  
  private void incCounter() {
    if (((int) vhCounter.getAndAdd(this, 1))+1 == 2)
      complete(a, b);
  }
  
  // can override or swap
  swappable void complete(A a, B b) {}
}