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) {}

}