public class Dog {
  private String name;
  private int age;

  private Person walker;

  public Dog(String name, int age) {
    this.name = name;
    this.age = age;
  }

  public void walkWith(Person walker) {
    this.walker = walker;
  }

  public void returnFromWalk() {
    walker = null;
  }

  public boolean isOnWalk() {
    return walker != null;
  }
}