It is possible to compile this code with JDK 8, but not with JDK 9. What can happen when you run it? Vote here: https://twitter.com/heinzkabutz/status/791220763059781632

HardPlace.java

abstract class Brick {
  public Brick() {
    greet();
  }

  public abstract void greet();
}

public class HardPlace {
  public class Between extends Brick {
    public void greet() {
      System.out.println("Hello " + assert);
    }
  }

  private final String assert;

  public HardPlace(String assert) {
    this.assert = assert;
  }

  public void test() {
    new Between();
  }

  public static void main(String[] assert) {
    new HardPlace("Franz").test();
  }
}

OK, deleting any comments that contain spoilers ...