A super simple coroutine explanation of a coroutine in java.

readme.txt

This solution is incorrect and is only meant to be a clarification 
of a problem discussed between two friends.  It will consume the 
stack until a stack overflow if the limit is set to high.  True 
coroutines are only possible in java using threads, native code or 
bytecode manipulation.  If there is a librarless way to create a 
coroutine in java please fork and contribute.

Blah.java

public class Blah {
  int limit = 500;
  int count = 0;

  int B, X, Y, Z;

  int c_state = 0, p_state = 0;

  public void resume(String function) {
    if(function == "producer") {
      producer();
    }

    if(function == "consumer") {
      consumer();
    }
  }

  public void consumer() {
    switch(c_state++) {
      case 0:
        X = B;
        resume("producer");
        break;
      case 1:
        Y = B;
        resume("producer");
        break;
      case 2:
        Z = B;
        System.out.printf("x = %d, y = %d, z = %d\n", X, Y, Z);
        c_state = 0;

        if(count++ < limit)
          resume("producer");
        
        break;
    }
  }

  public void producer() {
    switch(p_state++) {
      case 0:
        B++;
        resume("consumer");
        break;
      case 1:
        B++;
        p_state = 0;
        resume("consumer");
        break;
    }
  }

  public static void main(String[] args) {
    Blah b = new Blah();
    b.resume("producer");
  }
}

Is this little lib a solution for you:
HeinerKuecker/Coroutine-in-pure-Java