Listener.java

public interface Listener
{
  public void fire();
}

SneakyNumber.java

public class SneakyNumber
{
  private int num;
  
  // List of all listeners who want to know what happens to this object.
  private LinkedList<Listener> listeners = new LinkedList<>();
  
  public SneakyNumber(int num)
  {
    this.num = num;
  }
  
  public void setNum(int num)
  {
    this.num = num;
  
    // SOMEONE JUST CHANGED THE NUMBER, OH SH** ALERT EVERYONE THAT'S LISTENING.
      for (Listener i : listeners)
        i.fire();
  }
  
  public int getNum()
  {
    return num;
  }
  
  public void addListener(Listener var)
  {
    listeners.add(var);
  }
}

Tester.java

public abstract class Test
{
  public static void main(String[] args)
  {
    SneakyNumber x = new SneakyNumber(5);
    x.addListener(new Listener() ->
    {
      System.out.println("This number just changed, and now it's new value is " + x.getNum()); 
    }
    
    x.setNum(10);
  }
}